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We announce the discovery of a planetary system with 7 transiting planets around a Kepler target, a current record for transiting systems. Planets b, c, e and f are reported for the first time in this work. Planets d, g and h were previously reported in the literature (Batalha et al. 2013), although here we revise their orbital parameters and validate their planetary nature. Planets h and g are gas giants and show strong dynamical interactions. The orbit of planet g is perturbed in such way that its orbital period changes by 25.7h between two consecutive transits during the length of the observations, which is the largest such perturbation found so far. The rest of the planets also show mutual interactions: planets d, e and f are super-Earths close to a mean motion resonance chain (2:3:4), and planets b and c, with sizes below 2 Earth radii, are within 0.5% of the 4:5 mean motion resonance. This complex system presents some similarities to our Solar System, with small planets in inner orbits and gas giants in outer orbits. It is, however, more compact. The outer planet has an orbital distance around 1 AU, and the relative position of the gas giants is opposite to that of Jupiter and Saturn, which is closer to the expected result of planet formation theories. The dynamical interactions between planets are also much richer., submitted to ApJ in its original form on July, 9th 2013 and accepted in its present form on November, 8th 2013

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has observed the northern magnetospheric cusp of Mercury regularly since the probe was inserted into orbit about the innermost planet in March 2011. Observations from the Fast Imaging Plasma Spectrometer (FIPS) made at altitudes 10 cm−3) that are exceeded only by those observed in the magnetosheath. These high plasma densities are also associated with strong diamagnetic depressions observed by MESSENGER's Magnetometer. Plasma in the cusp may originate from several sources: (1) Direct inflow from the magnetosheath, (2) locally produced planetary photoions and ions sputtered off the surface from solar wind impact and then accelerated upward, and (3) flow of magnetosheath and magnetospheric plasma accelerated from dayside reconnection X-lines. We surveyed 518 cusp passes by MESSENGER, focusing on the spatial distribution, energy spectra, and pitch-angle distributions of protons and Na+-group ions. Of those, we selected 77 cusp passes during which substantial Na+-group ion populations were present for a more detailed analysis. We find that Mercury's cusp is a highly dynamic region, both in spatial extent and plasma composition and energies. From the three-dimensional plasma distributions observed by FIPS, protons with mean energies of 1 keV were found flowing down into the cusp (i.e., source (1) above). The distribution of pitch angles of these protons showed a depletion in the direction away from the surface, indicating that ions within 40° of the magnetic field direction are in the loss cone, lost to the surface rather than being reflected by the magnetic field. In contrast, Na+-group ions show two distinct behaviors depending on their energy. Low-energy (100–300 eV) ions appear to be streaming out of the cusp, showing pitch-angle distributions with a strong component antiparallel to the magnetic field (away from the surface). These ions appear to have been generated in the cusp and accelerated locally (i.e., source (2) above). Higher-energy (≥1 keV) Na+-group ions in the cusp exhibit much larger perpendicular components in their energy distributions. During active times, as judged by frequent, large-amplitude magnetic field fluctuations, many more Na+-group ions are measured at latitudes south of the cusp. In several cases, these Na+-group ions in the dayside magnetosphere are flowing northward toward the cusp. The high mean energy, pitch-angle distributions, and large number of Na+-group ions on dayside magnetospheric field lines are inconsistent with direct transport into the cusp of sputtered ions from the surface or newly photoionized particles. Furthermore, the highest densities and mean energies often occur together with high-amplitude magnetic fluctuations, attributed to flux transfer events along the magnetopause. These results indicate that high-energy Na+-group ions in the cusp are likely formed by ionization of escaping neutral Na in the outer dayside magnetosphere and the magnetosheath followed by acceleration and transport into the cusp by reconnection at the subsolar magnetopause (i.e., source 3 above).

In this study we looked to characterize the fundamental shape of the Moon using topography and gravity data and we studied a lunar rock sample in order to measure its dominant magnetic moment. Studying the shape of the Moon would give us a better understanding of the large-scale geophysics that has led to the present state of the Moon and studying lunar rock samples helps us better understand if the observed magnetic fields on the Moon are due to a past global magnetic field. Our findings suggest that the Moon does have a prominent degree-2 shape that is explained by tidal heating and a frozen-in shape from its past tidal stretching. In addition, the rock sample we studied shows signs for a prominent magnetic signature that is likely the result of the rock cooling in an environment that was dominated by a lunar magnetic field.

Extrasolar planet surveys have identified an abundant new population of highly irradiated planets with sizes that are in between that of the Earth and Neptune. Such planets are unlike anything found in our own Solar System, and many of their basic properties are not understood. As such, these planets provide a fundamental test for models of planets formation and evolution with important implications for the formation of the Earth and planet habitability. In order to understand these new classes of planets, we have developed planetary structure and evolution models that can be used both to answer questions about individual planetary systems and to study populations of planets as a whole. In brief, these models allow usto follow a planet's mass, size, internal structure, and composition as it ages; from the time it finishes formation until it is detected billions of years later.These evolution models are critical because a planet's composition can change substantially over its lifetime. Close-in planets, like most of those found so far, are bombarded by large amounts of ionizing radiation, which over time can completely strip away a planet's atmosphere; even turning a gas-rich Neptune sized planet into a barren rocky super-Earth.Using these models, we explore the structure, composition, and evolution of sub- Neptune sized extrasolar planets found by NASA's Kepler mission. We examine the relationships between planetary masses, radii, and compositions. We show how these compositionshave been sculpted by photo-evaporation, and we examine the interplay between thermal and evaporative evolution.

The large-scale dynamic behavior of Mercury’s highly compressed magnetosphere is predominantly powered by magnetic reconnection, which transfers energy and momentum from the solar wind to the magnetosphere. The contribution of flux transfer events (FTEs) at the dayside magnetopause to the redistribution of magnetic flux in Mercury’s magnetosphere is assessed with magnetic field data acquired in orbit about Mercury by the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. FTEs with core fields greater than the planetary field just inside the magnetopause are prevalent at Mercury. Fifty-eight such large-amplitude FTEs were identified during February and May 2012, when MESSENGER sampled the subsolar magnetosheath. The orientation of each FTE was determined by minimum variance analysis, and the magnetic flux content of each was estimated using a force-free flux rope model. The average flux content of the FTEs was 0.06 MWb, and their durations imply a transient increase in the cross-polar cap potential of ~25 kV. For a substorm timescale of 2–3 min, as indicated by magnetotail flux loading and unloading, the FTE repetition rate (10 s) and average flux content (assumed to be 0.03 MWb) imply that FTEs contribute at least ~30% of the flux transport required to drive the Mercury substorm cycle. At Earth, in contrast, FTEs are estimated to contribute less than 2% of the substorm flux transport. This result implies that whereas at Earth, at which steady-state dayside reconnection is prevalent, multiple X-line dayside reconnection and associated FTEs at Mercury are a dominant forcing for magnetospheric dynamics.

The identification of Meteor Crater in Arizona as an extraterrestrial impact by Eugene Shoemaker provided the first evidence of this geologic phenomenon and opened the door to a new field of research that has eventually lead to the identification of over ~150 terrestrial impact structures. Subsequently impacts have been evoked in the formation of the moon, delivery of volatiles and bio-precursors to early Earth, creation of habitats for the earliest life and, in more recent times, major mass extinction events. However, understanding the impact flux to the Earth-Moon system has been complicated by the constant weathering and erosion at Earth's surface and the complex nature of impactite samples such that only a hand full of terrestrial craters have been accurately and precisely dated. Currently 40Ar/39Ar step-heating analysis of impactite samples is commonly used to infer impact ages but can be problematic due to the presence of relic clasts, incomplete 40Ar outgassing or excess 40Ar, and recoil and shock effects. The work presented here attempts to develop zircon geochronology to probe planetary impact histories as an alternative to current methods and provides another tool by which to constrain the bolide flux to the Earth-Moon system. Zircon has become the premier geo-chronometer in earth science and geochemical investigation of Hadean zircon from Western Australia has challenged the long-standing, popular conception that the near-surface Hadean Earth was an uninhabitable and hellish world; Zircons may preserve environmental information regarding their formation and thus provide a rare window into conditions on early Earth. Isotopic and petrologic analyses of these ancient grains have been interpreted to suggest that early Earth was more habitable than previously envisioned, with water oceans, continental crust, and possibly even plate tectonics. The Hadean is also suspected to be a time of major planetary bombardment however identifying impact signatures within the Hadean population remains difficult and this study hopes to develop criteria to recognize impact zircon and possibly provide constraints on the early impactor flux. Five large terrestrial craters, Vredefort and Morokweng, South Africa, Sudbury and Manicouagan, Canada, and Popigai, Russia, are the focus of this study as smaller craters do not have the energy to produce thick melt sheets, which persist over time-scales sufficient for crystallization of zircon, permitting geochemical and geochronological analysis. Geochemical analysis of these impact-produced zircons yields similar chemical signatures to endogenic igneous zircon from crustal melts and highlights the need for well-developed criteria for discriminating impact and endogenic grains for impact geochronology. One such criterion is modeling of impact zircon crystallization temperature spectra for simulated impact events on targets of varying composition. Provided some assumptions the zircon crystallization spectra can be estimated from well established Zr systematics in crustal melts. Results for impacts into an Archean terrestrial surface (used as a proxy for the Hadean as little to no rock record exists >4.0 Ga) yields a crystallization spectra significantly higher than that reported for the Hadean zircon population and appears to rule out impacts as a dominant source for these ancient grains. When no dateable impact melt sheet exists, either due to the lack of energy of the impact itself or from subsequent erosion at Earth's surface, loss of radiogenic lead, Pb*, has been suggested as an alternative method to date the event. Pb*-loss was investigated from target rocks from Vredefort and Morokweng and suggests that Pb* diffusion, even in zircon isolated from shocked and brecciated target rocks, is remarkably slow. This may explain the seeming lack of 'reset' zircon in terrestrial impactites. Little is known about Pb* diffusion pathways associated with shock microstructures introduced during impact cratering and future diffusion studies may provide better constraints on this problem. Although little disturbance was identified in Pb* of target zircon, other low temperature geochronometers, zircon (U-Th)/He dating in this case, have been shown to be completely 'reset' and accurately date impacts. Zircon (U-Th)/He ages isolated from the target rock below ~850 m of well-dated impact melt at Morokweng yield ages consistent with the impact melt sheet and provide an alternative tool to dating events where such melts no longer exists. This geochronometer was also applied to impactites from Popigai, Russia and results in an age that is significantly younger than that reported in the literature and coincident with the Eocene-Oligocene boundary mass extinction event however the lack of any impact signatures at this boundary is puzzling. Constraining the impact flux to the Earth-Moon system not only allows for a better understanding into early Earth evolution and the formation of a habitable planet but also provides constraints on the modern impactor flux, important criteria for estimating the likelihood of future impact events. Zircon geochronology offers an exciting new tool by which to date impact events and has the potential to assist understanding of complex impactite samples from terrestrial craters and future sample return missions.

During its interplanetary trajectory in 2007–2009, the MErcury Surface, Space ENvrionment, GEochemistry, and Ranging (MESSENGER) spacecraft passed through the gravitational focusing cone for interstellar helium multiple times at a heliocentric distance R ≈ 0.3 AU. Observations of He+ interstellar pickup ions made by the Fast Imaging Plasma Spectrometer sensor on MESSENGER during these transits provide a glimpse into the structure of newly formed inner heliospheric pickup-ion distributions. This close to the Sun, these ions are picked up in a nearly radial interplanetary magnetic field. Compared with the near-Earth environment, pickup ions observed near 0.3 AU will not have had sufficient time to be energized substantially. Such an environment results in a nearly pristine velocity distribution function that should depend only on pickup-ion injection velocities (related to the interstellar gas), pitch-angle scattering, and cooling processes. From measured energy-per-charge spectra obtained during multiple spacecraft observational geometries, we have deduced the phase-space density of He+ as a function of magnetic pitch angle. Our measurements are most consistent with a distribution that decreases nearly monotonically with increasing pitch angle, rather than the more commonly modeled isotropic or hemispherically symmetric forms. These results imply that pitch-angle scattering of He+ may not be instantaneous, as is often assumed, and instead may reflect the velocity distribution of initially injected particles. In a slow solar wind stream, we find a parallel-scattering mean free path of λ ∼ 0.1 AU and a He+ production rate of ∼0.05 m−3 s−1 within 0.3 AU.

The amount of water runoff need to evolve landscapes is rarely assessed. Empirical studies correlate erosion rate to runoff or mean annual precipitation, but rarely is the full history of a landscape known such that it is possible to assess how much water was required to produce it. While this may not seem to be of primary importance on Earth where water is commonly plentiful, on Mars the amount of water to drive landscape evolution is a key question. Here we tackle this question through a series of five chapters, one devoted to field work at Meteor Crater, another to laboratory experiments about controlling processes, and then two chapters on analysis of landforms and implications of water runoff on Mars (associated with the Mars Science Laboratory mission to Gale Crater), and then we complete this effort with a consideration of how we can reliably assign relative timing between events resulting in small depositional features. What follows below is a summary of what is found in each chapter.Meteor Crater, a 4.5 km2 impact crater that formed ~50,000 years ago in northern Arizona, has prominent gully features on its steep walls that appear similar to some gullies found on Mars. At the crater bottom, there are over 30 meters of lake sediments from a lake that disappeared ~10,000 to 11,000 years ago, indicating the transition from the Pleistocene to the current, drier climate. A combination of fieldwork, cosmogenic dating, and topographic analysis of LiDAR data show that debris flows, not seepage erosion and fluvial processes as previously suggested in the literature, drove gully incision during their formation period of ~40,000 years before the onset of the Holocene. Runoff from bare bedrock source areas high on the crater wall cut into lower debris mantled slopes, where the runoff bulked up and transformed into debris flows that carried boulders down to ~5 to 8 degree slopes, leaving distinct boulder lined levees and lobate tongues of terminal debris deposits that crisscrossed on the lower slopes. We hypothesize that the fine material, likely generated in the impact, and deposited with the coarse debris on the lower portion of the crater wall, is key to this bulking up process as flows cut across the deposits. Fluvial processes following the debris flow gullies extended alluvial deposits to the crater floor and contributed to lake infilling. Cosmogenic dating confirms that most of the modification of the crater walls occurred before the early Holocene. To account for the 75 distinct deposits currently lying on the crater floor, debris flow frequency would be about 1 event every 17 years, assuming debris flow activity terminated ~10,000 years ago. Assuming a water-to-rock ratio of 0.2 at the time of transport, it would have taken ~100,000 m3 of water to transport the ~500,000 m3 of debris flow deposits on the crater floor. Given the 4.5 km2 size of the crater, this extensive erosion would require less than 0.02 m of total runoff, or the equivalent of just 0.001 mm/year over a 40,000 year period. This insignificant amount of water was likely packaged into short-lived storm or snow-melt events when debris flows were generated. Much more runoff did occur, as evidenced by the lake and fluvial deposits, as well as the likely cool, wet conditions of the late Pleistocene. This suggests only a small fraction of the total runoff is needed to do considerable geomorphic evolution, producing strongly gully-scared crater walls. Currently, only minor fluvial modification of the gully networks occurs.To test the hypothesis that fine-grained material is a necessary component behind sustained mobilization in granular debris flows, we performed a series of experiments in a 4 m diameter vertically rotating drum. Specifically, we assessed the role of fines and coarse grain size distribution on the rate of pore fluid pressure generation and its dissipation upon cessation of movement of the granular mass. We varied the amount of fines, from no fines to amounts found in debris flow deposits, and varied the coarse grain size distribution, from a single grain size (10 mm) to a range found in a natural flow deposit. The mixtures with fines contents close to those found in actual debris flows had elevated pore pressures, indicating they were almost fully liquefied, though the highest excess pore pressures were generated with the combination of fine material and a wide coarse grain size distribution. A boulder was also placed upon the fines-rich mixtures just after cessation of motion and it was observed that the pore fluid pressure rose instantly, bearing most of the grain's weight, and then the pressure slowly declined. When the same boulder was placed on the water-gravel flows, there was no change in fluid pressure, as the mass was supported entirely by grain-grain contacts. Our observations, combined with observations of others in the literature, inform the conceptual model presented herein where we hypothesize that the dilational separation of particles during debris flow shear leads to coarse particles (in this case particles >12 mm) slowly settling through the highly viscous non-Newtonian fluid. As the fines-rich mixture has a yield strength of ~5 Pa, we calculate that particles ~12 mm and smaller remain suspended. This combined effect requires the fluid to sustain the weight of the particles, leading to pore pressures equal to the entire weight of the solid and fluid mass.As part of the MSL science team, we focused on understanding the geomorphology and its implications for the hydrologic history of the landing site for the Curiosity rover, which was located at the distal end of the Peace Vallis fan in Gale Crater. The Peace Vallis fan covers 80 km2 and is fed by a 730 km2 catchment. Valley incision into accumulated debris delivered sediment through a relatively low-density valley network to a mainstem channel to the fan. An estimated total fan volume of 0.9 km3 matches the calculated volume of removal due to valley incision (0.8 km3). The fan profile is weakly concave up with a mean slope of 1.5% for the lower portion. Runoff (discharge/watershed area) to produce the fan is estimated to be more than 600 m, perhaps as much as 6000 m, indicating a hydrologic cycle that likely lasted at least 1000's of years. Atmospheric precipitation, possibly snow, not groundwater seepage produced the runoff. Based on topographic data, Peace Vallis fan likely onlapped the rise onto which Curiosity landed, Bradbury Rise, and spilled into a topographic low to the east of the rise. This argues that the light-toned fractured terrain within this topographic low corresponds to the distal deposits of Peace Vallis fan and in such a setting lacustrine deposits were predicted (and later confirmed by observations from the rover).Previous studies had suggested that there might have been large lakes in Gale. This history is important to understanding Mars' climate history and the volumes of water that may have passed through or covered the sediments encountered by Curiosity. Here we use improved imagery, topography, and ground-based observations from the Curiosity rover to map deltas, fans and gullies, which leads to a proposed history of lakes in Gale. We report evidence for at least three distinct large lakes within Gale, all which occurred after the crater's central mound, Aeolis Mons, reached close to its current topographic form. From corresponding deltaic deposits off the southern rim of Gale crater and the southern flank of Aeolis Mons, we identified the highest lake level at -3280 meters, which would have had a mean depth of 700 meters. The larger context provided by craters near Gale suggests that this lake was derived from regionally sourced water from the south. The next lake level, which was established after a period of drying with subsequent lake level decline before rewetting and lake level rise, is defined by four deltaic features as well as the termination of gullies around the northern rim of Gale. This second lake corresponds to an elevation of -3980 m, has a mean depth of 300 meters, and was likely sourced from water more local to Gale. Lake levels then rose to the third lake level, at -3780 m, with a mean depth of 400 meters. Two deltaic deposits derived from sediment from the rim of Gale define this lake. This last lake declined sufficiently quickly to prevent incision into any of the deltaic forms around the crater, though hydrologic activity likely continued as evidenced by a time of fan building, including the Peace Vallis fan, around the crater. Quantification of the fan and delta volumes and their gully sources on the crater wall and rim, and on Aeolis Mons, suggest that these discrete deposits contain essentially all of the sediment mobilized from the upslope source areas. Importantly for the MSL mission, this suggests that most of the sediments the rover has and will encounter experienced at least a few cycles of drying and wetting.The depositional features (i.e. alluvial fans and deltas) within Gale Crater are small but they likely record a time history of lakes and other depositional events within the crater. An obvious question to ask is: can crater counting, which has been used to date much of Mars' surface, be used on such small surfaces to provide accurate age dating? Here we introduce a simple simulation model to quantify the effects of sample area size and crater obliteration effects on age estimates derived from crater size-frequency distributions. Our results show, quantitatively, that crater counting to estimate age of small surfaces (

Solar wind protons observed by the MESSENGER spacecraft in orbit about Mercury exhibit signatures of precipitation loss to Mercury’s surface. We apply proton-reflection magnetometry to sense Mercury’s surface magnetic field intensity in the planet’s northern and southern hemispheres. The results are consistent with a dipole field offset to the north and show that the technique may be used to resolve regional-scale fields at the surface. The proton loss cones indicate persistent ion precipitation to the surface in the northern magnetospheric cusp region and in the southern hemisphere at low nightside latitudes. The latter observation implies that most of the surface in Mercury’s southern hemisphere is continuously bombarded by plasma, in contrast with the premise that the global magnetic field largely protects the planetary surface from the solar wind.

Data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Neutron Spectrometer (NS) have been used to identify energetic neutrons (0.5–8 MeV energy) associated with solar events that occurred on 4 June 2011. Multiple lines of evidence, including measurements from the NS and the MESSENGER Gamma-Ray Spectrometer, indicate that the detected neutrons have a solar origin. This evidence includes a lack of time-coincident, energetic (>45 MeV) charged particles that could otherwise create local neutrons from nearby spacecraft material and a lack of proton-induced gamma rays that should be seen if energetic protons were present. NS data cannot rule out the presence of lower-energy ions (

Ο άνθρωπος, παρά τα τεράστια άλματα τεχνολογικής ανάπτυξης που έχει κάνει, δε διαθέτει ακόμα τη δυνατότητα για να πραγματοποιήσει ταξίδια ρουτίνας σε κοντινά ουράνια σώματα. Ωστόσο, η θεμελίωση της Διαστημικής Επιστήμης είναι προ-ετών μία πραγματικότητα. Οι Αποστολές Apollo, που έλαβαν χώρα κατά το χρονικό διάστημα 1969-1972, περιελάμβαναν την προσελήνωση, τον περίπατο των αστροναυτών και τη συλλογή πολύτιμων δειγμάτων. Έκτοτε, δεν έχει πραγματοποιηθεί άλλο όμοιο ταξίδι του ανθρώπινου είδους. Η πιθανότητα πραγματοποίησης μελλοντικών αποστολών όπως η επιστροφή στη Σελήνη, ή στον πλανήτη Άρη, ή σε κάποιο αστεροειδή (π.χ. Εστία), φαντάζει αρκετά μικρή ώστε να υλοποιηθεί ακόμα και τις επόμενες δεκαετίες. Παρολαυτά, η φύση, έχει το μηχανισμό και τις διαδικασίες επίλυσης αυτού του προβλήματος, καθώς «στέλνει» εξω-γήϊνα κομμάτια πετρώματος με τη μορφή μετεωρίτη, στη Γη. Η πτώση μετεωριτών στη Γη αποτελεί ένα σημαντικό γεγονός, καθώς αποκαλύπτουν σημαντικές πληροφορίες σχετικά με τα αρχέγονα στάδια σχηματισμού του ηλιακού μας συστήματος, την ηλικία και τη διαδικασία δημιουργίας άλλων πλανητών. Ωστόσο, το μεγάλο ερώτημα, αν φιλοξενούν ή μπορούν να φιλοξενήσουν ίχνη που υποδηλώνουν ζωή απασχολεί τους ερευνητές τα τελευταία είκοσι χρόνια. Η επιστημονική ομάδα υπό την αιγίδα του καθηγητή McKay το 1996 στα πλαίσα μελέτης του μετεωρίτη ALH 84001 με προέλευση τον πλανήτη Άρη, υποστήριξε ότι ανακάλυψε δομές που αντιστοιχούν σε νανοβακτήρια. Στην παρούσα εργασία εστιάζουμε στους μετεωρίτες με προέλευση τον πλανήτη Άρη, παρουσιάζοντας την ιστορία από τη γέννεση των πρωτολίθων τους, μέχρι και τη στιγμή που προσκρούουν στη γη. Γίνεται προσπάθεια να κατανοηθεί ο τρόπος δημιουργίας ενός μετεωρίτη χρησιμοποιώντας ορυκτολογικά-πετρολογικά-γεωχημικά στοιχεία, και η τοποθέτηση του κάθε γεγονότος στο χρόνο με βάση σύγχρονα γεωχρονολογικά δεδομένα. Σε πρόσφατες μελέτες του μετεωρίτη Tissint, έχοντας επίσης προέλευση από τον πλανήτη Άρη, υποστηρίζεται ότι ανακαλύφθηκαν δομές πλούσιες σε άνθρακα και οξυγόνο. Επίσης πρόσφατες επιτόπιες παρατηρήσεις από το διαστημικό όχημα Curiosity, υποδεικνύουν την ύπαρξη επιφανειακού νερού που έρεε κατά το παρελθόν στην επιφάνεα του πλανήτη Άρη. Με βάση τα προηγούμενα, καταλήγουμε ότι ο πλανήτης Άρης ίσως να μην αποτελεί ένα «νεκρό» πλανήτη. Αποδεικνύουμε όμως ότι πολλά από τα πετρώματα που φθάνουν στη Γη, έχουν υποστεί μία πολύπλοκη ιστορία η οποία συνδέεται με την ανάπτυξη εξαιρετικά μεγάλων πιέσεων και θερμοκρασιών στην επιφάνεια του πλανήτη από τον οποίο προέρχονται, ικανών να καταστρέψουν οποιοδήποτε ίχνος ζωής πριν από αυτές. Αποτέλεσμα αυτού είναι η αξιολόγηση των μελετών να γίνεται με σύνεση πριν αναντίρρητα αποδειχθεί το γεγονός ύπαρξης ζωής., The humankind, despite the recent technological achievements, does not yet have the ability to carry out routine trips to nearby celestial bodies. However, space science is a reality. The “Apollo” missions, that took place during the period 1969-1972, included the moon landing, the walk of astronauts and collection of valuable samples. Since then, no similar space journey has been carried out. The possibility for future missions such as the return to the Moon or Mars, or to an asteroid (e.g., Vesta), seems small enough to be implemented in the next decades. Nevertheless, nature has the mechanism and procedures to resolve this problem by sending extra-terrestrial rocks in earth in the form of meteorites. Meteorite fall on Earth is a major event, as it reveals important information about the primordial stages of formation of our solar system, or the creation processes of other planets. However, the big question still remains; whether these rocks host or have traces of past life in turn employs researchers in the last twenty years. McKay et al. (1996) studied the meteorite ALH 84001 originating from Mars, claimed for important discoveries such as structures corresponding to nanobacteria. In the current paper, we focus on the origin of Martian meteorites, presenting their complete geological history; from the genesis of their protoliths till their falling to the earth. We attempt to shade light in the understanding of meteorite formation using mineralogical-petrological-geochemical data, and the assignment of timing for each event based upon contemporary geochronological data. Recently, studies of the Martian meteorite Tissint, allegedly discovered structures rich in carbon and oxygen. Furthermore, recent field observations from Curiosity rover, indicates the existence of surface water that flowed once in the past at the Martian surface. We conclude that the planet Mars might not be a "dead" planet. But it turns out that many of the meteorites that reach the Earth, have undergone a complex history which is associated with the development of very high pressures and temperatures on the surface of the planet (e.g., Mars) from which they originate, able to destroy any trace of life before them. After all, we should be very sceptic and evaluate all the possibilities before the acceptance for the existence of life out there.

We owe a major part of our knowledge about surface composition and structure of solid planetary surfaces to infrared imaging and Fourier transform spectroscopy. Based on these methods, it succeeds to observe single planetary objects in a global geo-scientific content. The topics of infrared surface studies are mineralogical composition analyses, measurement of surface temperature, thermal inertia, and photometric observation of surface regolith texture. The comparison of infrared with photogeologic data forms the essential basis for comparative studies in planetology. The present paper summarizes outstanding results by examples of ESA experiments like VIRTIS on Venus Express and Rosetta, PFS on Mars Express, MERTIS on Bepi Colombo, and TIRVIM on ExoMars, and provides an outlook for future plans. The instruments are described, and the interplay of disciplines like planetology, IR measuring techniques, and space flight engineering is demonstrated. Infrared remote sensing provides essential knowledge about the current state of solid planetary surfaces. This allows studying fundamental questions in comparative planetology.

During the MESSENGER spacecraft's interplanetary trajectory to Mercury, the Fast Imaging Plasma Spectrometer (FIPS) measured the first helium pickup ion distributions at a heliocentric distance (R) ranging between 0.3 and 0.7 AU. From several transits of MESSENGER through the interstellar helium gravitational focusing cone, we map the cone structure in the ecliptic at R = 0.3 AU and compare it with observations of He+ at R = 1 AU made with the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE). Average downwind enhancements of near 8 and average cone widths of plus or minus 30 degrees measured by each sensor match known models of neutral helium during this most recent, unusually quiet, solar minimum. The average cone center direction is calculated to be at J2000 ecliptic longitude L = 76.0 degrees (plus or minus 6.0 degrees) and 77.0 degrees (plus or minus1.5 degrees) from FIPS and SWICS observations, respectively. These parameters are also in agreement with previous determinations of the downwind direction of interstellar flow and demonstrate the effectiveness of using pickup ion observations inside 1 AU to map the structure of the neutral helium distribution in the heliosphere, as well as to diagnose the conditions of the interstellar medium.

MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) orbital observations of Mercury have revealed elevated S abundances, Ca-S and Mg-S correlations, and a low upper limit for ferrous iron in surface silicates. These data indicate the presence of Ca and/or Mg sulfides in volcanic rocks and a low oxygen fugacity (fO2) in their parental magmas. We have evaluated coupled fO2 and fS2 values and FeO contents in Mercury's magmas from silicate-sulfide equilibria and empirical models for silicate melts and metallurgical slags. The evaluated fO2 at 1700-1800 K is 4.5 to 7.3 log10 units below the iron-wüstite buffer. These values correspond to 0.028-0.79 wt % FeO, implying that Fe must be also present in sulfides and metal and are also consistent with the composition of the partial melt of an enstatite chondrite. This derived upper limit for FeO is substantially lower than the limits obtained from reflectance measurements of Mercury's surface materials. The low fO2 and FeO values provide new constraints for igneous processes on Mercury as well as the formation, evolution, and internal structure of the innermost planet.

Radar-bright features near Mercury's poles were discovered in Earth-based radar images and proposed to be water ice present in permanently shadowed areas. Images from MESSENGER's one-year primary orbital mission provide the first nearly complete view of Mercury’s north polar region, as well as multiple images of the surface under a range of illumination conditions. We find that radar-bright features near Mercury's north pole are associated with locations persistently shadowed in MESSENGER images. Within 10 degrees of the pole, almost all craters larger than 10 km in diameter host radar-bright deposits. There are several craters located near Mercury's north pole with sufficiently large diameters to enable long-lived water ice to be thermally stable at the surface within regions of permanent shadow. Craters located farther south also host radar-bright deposits and show a preference for cold-pole longitudes; thermal models suggest that a thin insulating layer is required to cover these deposits if the radar-bright material consists predominantly of longlived water ice. Many small (less than 10 km diameter) and low-latitude (extending southward to 66 degrees N) craters host radar-bright material, and water ice may not be thermally stable in these craters for ~1 Gy, even beneath an insulating layer. The correlation of radar-bright features with persistently shadowed areas is consistent with the deposits being composed of water ice, and future thermal modeling of small and low-latitude craters has the potential to further constrain the nature, source, and timing of emplacement of the radar-bright material.

[1] We here document with magnetic field observations a passage of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft through Mercury's magnetosphere under conditions of a quasi-parallel bow shock, i.e., when the direction of the upstream interplanetary magnetic field was within 45° of the bow shock normal. The spacecraft's fast transition of the magnetosheath and the steady solar wind conditions during the period analyzed allow both spatial and temporal properties of the shock crossing to be investigated. The observations show that the shock reformation process can be nearly periodic under stable solar wind conditions. Throughout the 25-min-long observation period, the pulsation duration deviated by at most ~10% from the average 10 s period measured. This quasiperiodicity allows us to study all aspects of the shock reconfiguration, including ultra-low-frequency waves in the upstream region and large-amplitude magnetic structures observed in the vicinity of the magnetosheath-solar wind transition region and inside the magnetosheath. We also show that bow shock reformation can be a substantial source of wave activity in the magnetosphere, on this occasion having given rise to oscillations in the magnetic field with peak-to-peak amplitudes of 40–50 nT over large parts of the dayside magnetosphere. The clean and cyclic behavior observed throughout the magnetosphere, the magnetosheath, and the upstream region indicates that the subsolar region was primarily influenced by a cyclic reformation of the shock front, rather than by a spatial and temporal patchwork of short large-amplitude magnetic structures, as is generally the case at the terrestrial bow shock under quasi-parallel conditions.

On 18 March 2011, MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) became the first spacecraft to orbit Mercury, providing a new opportunity to study the outer boundary of the planet's magnetosphere--the magnetopause. Here we characterize Mercury's magnetopause using measurements collected by MESSENGER's Magnetometer and Fast Imaging Plasma Spectrometer. Analysis of measurements from two of MESSENGER's "hot seasons," when the orbital periapsis is on Mercury's dayside and the magnetopause crossing takes place in the subsolar region, resulted in 43 events with well-determined boundary normals. The typical duration of a magnetopause traversal was ~5 s. The average normal magnetic field component was ~20 nT, and the dimensionless reconnection rate, i.e., the ratio of the normal magnetic field component to the total field magnitude just inside the magnetopause, was 0.15 plus or minus 0.02. This rate is a factor of ~3 larger than values found during the most extensive surveys at Earth. The ratio of the reconnection rate at Mercury to that of the Earth is comparable to the ratio of the solar wind Alfvén speeds at their respective orbits. We also find that the magnetopause reconnection rate at Mercury is independent of magnetic field shear angle, but it varies inversely with plasma b, the ratio of total thermal pressure to magnetic pressure, in the magnetosheath. These results suggest that reconnection at Mercury is not only more intense than at Earth but also that it occurs for nearly all orientations of the interplanetary magnetic field due to the low-b nature of the solar wind in the inner heliosphere.

Orbital images acquired by the Mercury, Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft reveal a distinctive low-reflectance material on the surface of Mercury. Such material occurs in small, isolated, and thin surficial units. We term these features “dark spots.” Dark spots have the lowest average reflectance yet documented on the planet. In every case observed at sufficiently high resolution, dark spots feature hollows at their centers. Not all hollows, however, are surrounded by a dark spot. Dark spots have been found on low-reflectance smooth plains, intercrater plains, heavily cratered terrain, and impact craters at almost all longitudes on Mercury, but they have not been documented on high-reflectance smooth plains material. Dark spots are one of the youngest endogenic features on Mercury, and some postdate craters with distinctive rays. Sulfides may be the phase responsible for the low albedo of dark spot material. We propose that dark spots form during the initial stages of hollow formation, perhaps in a manner similar to intense outgassing events that feature exit velocities in excess of 100 m/s. Such outgassing could contemporaneously produce a depression that constitutes an embryonic hollow. Under this scenario, dark spot material is subsequently removed or modified by regolith gardening or other surface processes on time scales shorter than the lifetime of the central hollow.

Measurements from the Fast Imaging Plasma Spectrometer (FIPS) and Magnetometer (MAG) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft during 40 orbits about Mercury are used to characterize the plasma depletion layer just exterior to the planet’s dayside magnetopause. A plasma depletion layer forms at Mercury as a result of piled-up magnetic flux that is draped around the magnetosphere. The low average upstream Alfvénic Mach number (MA ~3–5) in the solar wind at Mercury often results in large-scale plasma depletion in the magnetosheath between the subsolar magnetopause and the bow shock. Flux pileup is observed to occur downstream under both quasi-perpendicular and quasi-parallel shock geometries for all orientations of the interplanetary magnetic field (IMF). Furthermore, little to no plasma depletion is seen during some periods with stable northward IMF. The consistently low value of plasma β, the ratio of plasma pressure to magnetic pressure, at the magnetopause associated with the low average upstream MA is believed to be the cause for the high average reconnection rate at Mercury, reported to be nearly 3 times that observed at Earth. Finally, a characteristic depletion length outward from the subsolar magnetopause of ~300 km is found for Mercury. This value scales among planetary bodies as the average standoff distance of the magnetopause.

Three advances of the previous half-century fundamentally altered petrology, along with the rest of the Earth sciences. Planetary exploration, plate tectonics, and a plethora of new tools all changed the way we understand, and the way we explore, our natural world. And yet the same large questions in petrology remain the same large questions. We now have more information and understanding, but we still wish to know the following. How do we account for the variety of rock types that are found? What does the variety and distribution of these materials in time and space tell us? Have there been secular changes to these patterns, and are there future implications? This review examines these bigger questions in the context of our new understandings and suggests the extent to which these questions have been answered. We now do know how the early evolution of planets can proceed from examples other than Earth, how the broad rock cycle of the present plate tectonic regime of Earth works, how the lithosphere atmosphere hydrosphere and biosphere have some connections to each other, and how our resources depend on all these things. We have learned that small planets, whose early histories have not been erased, go through a wholesale igneous processing essentially coeval with their formation. By inference, this also happened to Earth. The early differentiation on a small planet produces observable basaltic rock types—and produces little else besides a residue and a planetary core. In contrast, the larger Earth’s preservation of its original differentiation products has been eroded by continued activity which still involves extensive basaltic volcanism with further reprocessing through plate tectonic cycles to form continents and cratons. We also now have a good understanding of the pressure-induced phase changes that are responsible for the Earth’s mantle’s seismic layered structure. It is unclear the extent to which this layered seismic structure corresponds to chemical layering as well as to mineralogical layering. Earth’s transition zone, lower, and upper mantles may not have the same composition. It is possible that still larger exoplanets might be expected to develop additional modes of activity with emphasis on additional phase changes producing more internal layering and differentiation.

Landforms unique to Mercury, hollows are shallow, flat-floored irregular depressions notable for their relatively high reflectance and characteristic color. Here we document the range of geological settings in which hollows occur. Most are associated with impact structures (simple bowl-shaped craters to multiring basins, and ranging from Kuiperian to Calorian in age). Hollows are found in the low-reflectance material global color unit and in low-reflectance blue plains, but they appear to be absent from high-reflectance red plains. Hollows may occur preferentially on equator- or hot-pole-facing slopes, implying that their formation is linked to solar heating. Evidence suggests that hollows form because of loss of volatile material. We describe hypotheses for the origin of the volatiles and for how such loss proceeds. Intense space weathering and solar heating are likely contributors to the loss of volatiles; contact heating by melts could promote the formation of hollows in some locations. Lunar Ina-type depressions differ from hollows on Mercury in a number of characteristics, so it is unclear if they represent a good analog. We also use MESSENGER multispectral images to characterize a variety of surfaces on Mercury, including hollows, within a framework defined by laboratory spectra for analog minerals and lunar samples. Data from MESSENGER's X-Ray Spectrometer indicate that the planet’s surface contains up to 4% sulfur. We conclude that nanophase or microphase sulfide minerals could contribute to the low reflectance of the low-reflectance material relative to average surface material. Hollows may owe their relatively high reflectance to destruction of the darkening agent (sulfides), the presence of alteration minerals, and/or physical differences in particle size, texture, or scattering behavior.

The recent determination of the gravity field of Mercury and new Earth-based radar observations of the planet's spin state afford the opportunity to explore Mercury's internal structure. These observations provide estimates of two measures of the radial mass distribution of Mercury: the normalized polar moment of inertia and the fractional polar moment of inertia of the solid portion of the planet overlying the liquid core. Employing Monte Carlo techniques, we calculate several million models of the radial density structure of Mercury consistent with its radius and bulk density and constrained by these moment of inertia parameters. We estimate that the top of the liquid core is at a radius of 2020 ± 30 km, the mean density above this boundary is 3380 ± 200 kg m-3, and the density below the boundary is 6980 ± 280 kg m-3. We find that these internal structure parameters are robust across a broad range of compositional models for the core and planet as a whole. Geochemical observations of Mercury's surface by MESSENGER indicate a chemically reducing environment that would favor the partitioning of silicon or both silicon and sulfur into the metallic core during core-mantle differentiation. For a core composed of Fe-S-Si materials, the thermodynamic properties at elevated pressures and temperatures suggest that an FeS-rich layer could form at the top of the core and that a portion of it may be presently solid. Key PointsNew MESSENGER and Earth-based radar data provide Mercury's moments of inertiaMercury's core-mantle boundary is 420 +/- 30 km below the planet's surfaceThe core may be compositionally segregated ©2013. American Geophysical Union. All Rights Reserved.

Owing to its very dense and extended rings as well as the diversity of its satellites, the Saturnian system is a peculiar object in the solar system. However, although Cassini spacecraft, in orbit around Saturn since July 2004, has provided an impressive amount of data, the origin of the entire system (planet-rings-satellites) still remains largely unknown. The lack of constraints on ring's mass and age, coupled with the poor knowledge of rheological properties of satellites ground, in particular Enceladus, do not allow to disentangle between formation models.Mains remote sensing instruments on board the Cassini spacecraft give access to the photometrical behavior of observed bodies, mainly controlled by the regolith that blanket them as well as their macroscopic structure (e.g. ring's thickness and density, satellite's surface topography…). Hence, during my PhD I have developed planetary surfacephotometrical models, which take into account multi-scale effects. Comparison between these models and Cassini spectro-photometrical data leads to the first determination of Saturn's rings structure as a function of its distance to Saturn as well as their chemical composition. A similar work based on shadow measurements of Enceladus's craters gave access to its ground rheological properties.After a brief introduction on the general context of Saturn's formation models and the photometrical multi-scale models I built, I will detail the final results on these two objects and will discuss how this knew knowledge helps us to better understand how this unique system has formed., Le système de Saturne se singularise dans le système solaire de par l'importance de son système d'anneau, extrêmement massif et étalé ainsi que par la grande diversité de ses satellites naturels. Néanmoins, malgré la grande quantité de données recueillies par la sonde Cassini, en orbite autour de Saturne depuis Juillet 2004, le mode de formation du système dans son ensemble (planète-satellites-anneaux) reste une énigme. Le manque de contraintes sur la masse et l'âge des anneaux, couplé à la mauvaise connaissance des propriétés rhéologiques de la surface des satellites, en particulier d'Encelade, ne permettent pas à l'heure actuelle de trancher entre les différents scénarios de formations.Les principaux instruments de la sonde Cassini donnent accès au comportement photométrique des corps observés, contrôlé aussi bien par le régolithe microscopique les recouvrant que par leur structure macroscopique (épaisseur et densité des anneaux, topographie des surfaces des satellites par exemple). C'est pourquoi, au cours de ma thèse je me suis intéressé à modéliser la photométrie des surfaces planétaires en distinguant ces effets multi-échelles. La comparaison de ces modèles aux données spectro-photométriques de la sonde Cassini a permis, après inversion, de remonter à la structure et la composition des anneaux de Saturne d'une part, ainsi qu'aux propriétés rhéologiques du satellite Encelade d'autre part, ce qui, dans les deux cas, est une première à ce jour.Après avoir introduit le contexte général de formation du système de Saturne et les différents modèles photométriques que j'ai construits, je détaillerais les résultats obtenus sur ces deux objets et discuterai l'impact de ces nouvelles données sur notre compréhension des mécanismes de formation d'un système si unique.

The Rachmaninoff, Raditladi, and Mozart peak-ring impact basins on Mercury display a distinctive pattern of tectonic features consisting of a central zone that is either devoid of tectonic landforms or contains small ridges, a medial annulus of prominent and predominantly circumferentially oriented graben, and a distal zone displaying graben that occur in a mix of orientations and that are less evident toward the peak ring. Here we use finite element models to explore three candidate scenarios for the formation of these tectonic features: (1) thermal contraction of the interior smooth plains, (2) isostatic uplift of the basin floor, and (3) subsidence following volcanic loading. Our results suggest that only thermal contraction can account for the observed pattern of graben, whereas some combination of subsidence and global contraction is the most likely explanation for the central ridges in Rachmaninoff and Mozart. Thermal contraction models, however, predict the formation of graben in the centermost region of each basin, where no graben are observed. We hypothesize that graben in this region were buried by a thin, late-stage flow of plains material, and images of partially filled graben provide evidence of such late-stage plains emplacement. These results suggest that the smooth plains units in these three basins are volcanic in origin. The thermal contraction models also imply a cooling unit ~1 km thick near the basin center, further supporting the view that plains-forming lavas on Mercury were often of sufficiently high volume and low viscosity to pool to substantial thicknesses within basins and craters.

The characterization of the surface of planetary objects, through space observations, gives key clues to the past and present geological, geochemical and climate processes. Near-infrared hyperspectral microscopy, through its capability to identify the molecular and mineralogical composition of a sample at its grain size, is an innovative technique that will efficiently complement both remote sensing and in situ measurements. Recent technical achievements in near-infrared detectors, space cryo-coolers and dispersive systems, has enabled us to design MicrOmega, a highly miniaturized near-infrared hyperspectral microscope, to be implemented on landers/rovers: it has been selected within the Pasteur payload of the ESA ExoMars rover, with launch scheduled for 2018.My thesis activity started with the study of the extension of the spectral range beyond 2.5 µm, driven by the goal of identifying and characterizing potential organic compounds, and with the analysis of the impact on the instrument design of such an extension. The outcomes were used to set the MicrOmega / ExoMars instrument baseline. My involvement in this program included the development of algorithms enabling, in an automated way, the identification and the location, within the analyzed samples, of compounds with specific composition; it will be used both to limit the amount of information to be downloaded, and to indicate key targets for point analyzers, such as the Raman spectrometer RLS and the laser desorption spectrometer MOMA, thus increasing the synergy between the suite of ExoMars laboratory instruments. During my thesis, the shift of the launch of the Phobos Grunt mission opened the possibility to develop and deliver a flight model of MicrOmega, in less than two years; I thus have been involved in all steps of its development, from its design to its final calibration. As a follow-up, another mission of opportunity emerged, to which I have also been associated: MicrOmega has been selected as part of the Hayabusa-2 mission, which will in situ analyze a C-type asteroid.; L’analyse de la surface par des moyens spatiaux des objets du Système Solaire permet de remonter aux processus géologiques, géochimiques et climatiques qui s’y sont déroulés. La microscopie hyperspectrale dans le proche infrarouge, de par sa faculté à analyser la composition moléculaire et minéralogique d’un échantillon à l’échelle des grains, est une technique novatrice dans le cadre de la planétologie, amenée à compléter les mesures effectuées depuis l’orbite et celles des autres instruments d'analyse in situ. Les développements techniques récents liés aux détecteurs matriciels dans le proche infrarouge, aux machines cryogéniques de dimension et masse réduites, ainsi qu’aux systèmes dispersifs nous donnent désormais la capacité de développer des microscopes hyperspectraux compatibles en termes de masse, volume, puissance et télémétrie avec les contraintes fortes liées à un atterrisseur/rover. Le concept développé a ainsi donné naissance à l’instrument MicrOmega, sélectionné pour faire partie de la charge utile Pasteur du rover ExoMars de l'ESA. Mon travail de thèse s'est tout d'abord fixé pour objectif d'étudier l'extension de la gamme spectrale de l'instrument vers l'infrarouge au-delà de 2.5 µm afin d'identifier et de caractériser d'éventuels composés carbonés ; j'ai procédé à l'analyse des conséquences de cette extension sur la conception de l'instrument. Les résultats de ces études ont permis de faire évoluer le design et les spécifications de l'instrument MicrOmega pour ExoMars. Mon implication dans la préparation de cette mission m'ont conduit à développer des algorithmes de détection automatiques de composés spécifiques au sein d'un échantillon, de manière à coupler les mesures de MicrOmega avec celles du spectromètre RAMAN RLS et du laser à désorption de MOMA, permettant ainsi d'accroître la synergie entre les instruments de la charge utile.Au cours de ma thèse, le décalage du lancement de la mission Phobos Grunt a permis de proposer d'y adjoindre un modèle de MicrOmega ; j'ai ainsi pu participer à l'ensemble des phases de développement d'un modèle de vol de MicrOmega, de sa conception initiale à l'étalonnage final. Suite à ce développement, une autre mission d'opportunité est apparue, à laquelle j'ai également été associé : l'instrument MicrOmega a été sélectionné pour la mission Hayabusa-2, destinée à l'analyse in situ d'un astéroïde-C.

The characterization of the surface of planetary objects, through space observations, gives key clues to the past and present geological, geochemical and climate processes. Near-infrared hyperspectral microscopy, through its capability to identify the molecular and mineralogical composition of a sample at its grain size, is an innovative technique that will efficiently complement both remote sensing and in situ measurements. Recent technical achievements in near-infrared detectors, space cryo-coolers and dispersive systems, has enabled us to design MicrOmega, a highly miniaturized near-infrared hyperspectral microscope, to be implemented on landers/rovers: it has been selected within the Pasteur payload of the ESA ExoMars rover, with launch scheduled for 2018.My thesis activity started with the study of the extension of the spectral range beyond 2.5 µm, driven by the goal of identifying and characterizing potential organic compounds, and with the analysis of the impact on the instrument design of such an extension. The outcomes were used to set the MicrOmega / ExoMars instrument baseline. My involvement in this program included the development of algorithms enabling, in an automated way, the identification and the location, within the analyzed samples, of compounds with specific composition; it will be used both to limit the amount of information to be downloaded, and to indicate key targets for point analyzers, such as the Raman spectrometer RLS and the laser desorption spectrometer MOMA, thus increasing the synergy between the suite of ExoMars laboratory instruments. During my thesis, the shift of the launch of the Phobos Grunt mission opened the possibility to develop and deliver a flight model of MicrOmega, in less than two years; I thus have been involved in all steps of its development, from its design to its final calibration. As a follow-up, another mission of opportunity emerged, to which I have also been associated: MicrOmega has been selected as part of the Hayabusa-2 mission, which will in situ analyze a C-type asteroid.; L’analyse de la surface par des moyens spatiaux des objets du Système Solaire permet de remonter aux processus géologiques, géochimiques et climatiques qui s’y sont déroulés. La microscopie hyperspectrale dans le proche infrarouge, de par sa faculté à analyser la composition moléculaire et minéralogique d’un échantillon à l’échelle des grains, est une technique novatrice dans le cadre de la planétologie, amenée à compléter les mesures effectuées depuis l’orbite et celles des autres instruments d'analyse in situ. Les développements techniques récents liés aux détecteurs matriciels dans le proche infrarouge, aux machines cryogéniques de dimension et masse réduites, ainsi qu’aux systèmes dispersifs nous donnent désormais la capacité de développer des microscopes hyperspectraux compatibles en termes de masse, volume, puissance et télémétrie avec les contraintes fortes liées à un atterrisseur/rover. Le concept développé a ainsi donné naissance à l’instrument MicrOmega, sélectionné pour faire partie de la charge utile Pasteur du rover ExoMars de l'ESA. Mon travail de thèse s'est tout d'abord fixé pour objectif d'étudier l'extension de la gamme spectrale de l'instrument vers l'infrarouge au-delà de 2.5 µm afin d'identifier et de caractériser d'éventuels composés carbonés ; j'ai procédé à l'analyse des conséquences de cette extension sur la conception de l'instrument. Les résultats de ces études ont permis de faire évoluer le design et les spécifications de l'instrument MicrOmega pour ExoMars. Mon implication dans la préparation de cette mission m'ont conduit à développer des algorithmes de détection automatiques de composés spécifiques au sein d'un échantillon, de manière à coupler les mesures de MicrOmega avec celles du spectromètre RAMAN RLS et du laser à désorption de MOMA, permettant ainsi d'accroître la synergie entre les instruments de la charge utile.Au cours de ma thèse, le décalage du lancement de la mission Phobos Grunt a permis de proposer d'y adjoindre un modèle de MicrOmega ; j'ai ainsi pu participer à l'ensemble des phases de développement d'un modèle de vol de MicrOmega, de sa conception initiale à l'étalonnage final. Suite à ce développement, une autre mission d'opportunité est apparue, à laquelle j'ai également été associé : l'instrument MicrOmega a été sélectionné pour la mission Hayabusa-2, destinée à l'analyse in situ d'un astéroïde-C.

Clouds on Venus are formed of sulfuric acid aerosols. The clouds are optically very thick in the visible, inhibiting visual observation of the surface. The clouds are organized in three quite distinct layers with differing properties in terms of particle size distributions and possibly composition. Some early observations hint to the existence of other species in the clouds, but recent missions have not been able to verify their composition. The lowest cloud layers are very dynamic and the reason for the bi- and trimodal size distributions could be partly explained by mixing and distinct altitudes of nucleation. However, the possibility of mixed phase clouds and different chemical compositions of the droplets have not yet been evaluated. Both the clouds of Venus and aerosols found in the Earth's stratosphere are composed of sulfuric acid droplets. On the Earth, the sulfur in the stratosphere originates from OCS, a long-lived sulfur species emitted at the surface and transported by the general circulation, and from volcanic eruptions that can inject material above the tropopause. It is thought that the water - sulfuric acid droplets form by binary homogeneous nucleation on both planets. However, the existence of condensation nuclei on Venus has not been ruled out. We use a model developed for the sulfuric acid aerosols and polar stratospheric clouds (PSCs) in the terrestrial stratosphere (Jumelet et al., 2009; Larsen et al., 2000). The model describes several key microphysical processes, including nucleation via a parameterization for two-component nucleation (Vehkamäki et al., 2002) of water and sulfuric acid. This parameterization is in principle quite applicable for the Venusian atmosphere, but we discovered that the lower limit of relative humidity is still too high considering the extremely dry Venusian atmosphere. We are extending the parameterization of (Vehkamäki et al., 2002) to lower relative humidity (see another abstract by Määttänen et al., this conference). In addition to formation of droplets via nucleation, the model includes their subsequent growth by condensation, evaporation of droplets, coagulation, and sedimentation.

The growth or evaporation of droplets or crystals in a gas mixture is controlled by the balance of mass and heat transfer. The nature of the condensing vapor and above all its relative abundance in the atmosphere plays a major role in the mass transfer processes. We investigate the influence of molecular diffusion, thermal diffusion or Soret effect (mass transfer), and Dufour effect (heat transfer), with previous work done by Kulmala and Vesala (1991) as a starting point (hereafter KV91) and extending it to the case of high relative abundances of the condensing vapor.

The first unambiguous spectroscopic observation of CO2 ice clouds on Mars came from Montmessin et al. (2007), but observations have been reported for a decade now (Clancy et al. 2003). The CO2 ice clouds are a rare phenomenon in the Solar System, since 95% of the martian atmosphere consist of CO2 gas, and thus on Mars we are dealing with the condensation of the main component of the atmosphere. The condensation is moreover occuring in a rarefied atmosphere (large Knudsen numbers) that can have dramatic consequences on the crystal growth through the limiting effect of the heat transfer. CO2 ice cloud modeling has turned out to be challenging : recent efforts (e.g. Colaprete et al. 2008) fail in explaining typical small sizes (80 nm-130 nm) observed in equatorial mesospheric ice clouds (Montmessin et al. 2006). Recent modelling studies suggest that the effect of the thermal tide in cooling the mesosphere is a prerequisite for the cloud formation (Gonzalez-Galindo et al. 2011), but other perturbations are required to attain the CO2 condensation temperatures. A recent study has shown a strong correlation between mesospheric ice cloud observations and the filtering of gravity waves through the atmosphere (Spiga et al. 2012). Those waves could create cold pockets in wich T ≤ Tcond, and thus provoke a supersaturated environment in which the clouds can form. However, the nature of the key microphysical processes in the formation of CO2 ice clouds remains unclear, especially in the mesosphere. We have adapted a microphysical model previously developed for water ice clouds formation on Mars (Montmessin et al. 2004) for modeling these CO2 ice clouds.

My Ph.D. work consists in the investigation of satellite magnetic data to infer the deep internal conductivity distribution. I developed a new global electromagnetic induction method applied to planetary magnetic datasets without strong a priori hypothesis on the external inducing source field. My method is based on a spectral correction of gapped data magnetic time series to restore the time spectral content of the source field. This external source depends on the planetary environment and is therefore different for each planetary bodies. The method aims at recovering with a maximum accuracy internal and external spherical harmonic coefficients of transients fields, whose ratio is used as a transfer function to retrieve the internal distribution of electrical conductivity. While for the Earth, a good proxy of the source field activity is the Dst index, no such proxy exists for other planets. Hence, for our study of Mars transient magnetic field from MGS, one of the major part of my work is the determination of an appropriate continuous proxy for the external variability. On Earth the external electromagnetic source is well known, and may be described by a spherical harmonic geometry dominated by the dipole term. This source field may be characterized using a magnetic activity index named the Dst index. The method has been tested on synthetic data generated within the framework of SWARM mission. This mission consists of a 3 satellites constellation. One of the main objectives is to infer the 3D electrical distribution in the deep Earth. SWARM synthetic data consist in a time series of spherical harmonic (SH) coefficients, external and internal, generated from a simple non-realistic 3D model. In this model, several regional and local conductors, in a radially symmetric 3 layers model have been embedded. Using this dataset, our method give satisfactory results. We have been able to obtain the external and internal SH coefficients - for the first SH degree, which is known to be the most energetic degree of the external source - using only one of the 3 synthetic time series. Then, the method has been used on real data from Ørsted. In this case, we had to pre-process the data to correct from ionospheric and aligned currents contributions. We developed a statistical analysis to remove the ionospheric field using 2 geomagnetic indices : AL and Kp. Hence, we have enlarged data toward higher and lower latitudinal zones than what has been done in previous works. Finally, we have been able to obtain 1D conductivity models, which fits reasonably with existing conductivity data in the deep Earth. Finally, we worked on Mars Global Surveyor (MGS) data. One of the most time consuming parts of this work was the determination of an appropriate continuous proxy for the external variability in the vicinity of Mars. Without any measurements of the IMF (Interplanetary Magnetic Field) during MGS sciences acquisition, we have used ACE (Advanced Composition Explorer) data. This satellite orbits around the L1 point of the Sun-Earth system, measuring solar wind magnetic characteristics. We have time-shifted ACE data to Mars position for 4 temporal windows where Mars and Earth were closed to the same Parker's spiral's arm, and finally determined a proxy explaining the major part of the variability observed in Mars data. Despite numerous gaps in MGS data, we have been able to establish the 1D conductivity distribution, fitting reasonably existing geochemical models. Although the method may be unstable for some cases, we obtained satisfactory results for in depth conductivity of the planet.; Les méthodes d'induction électromagnétique permettent de caractériser la conductivité électrique des matériaux, dont les corps planétaires telluriques, depuis les couches superficielles de la croûte jusqu'aux zones les plus internes, dans le manteau inférieur. Pour une source de champ électromagnétique donnée, des courants sont induits dans les matériaux qui y sont soumis. Avec l'essor des données magnétiques satellitaires, de nouvelles méthodes d'analyse des données magnétiques permettent d'obtenir des images unidimensionnelles de la structure électrique de ces corps car la structure spatio-temporelle de la source électromagnétique en est bien connue. Les travaux de mon doctorat ont eu pour but de mettre en place une nouvelle méthode d'analyse permettant de déterminer des modèles de structure interne globaux pour n'importe quel corps du système solaire pour lequel on dispose de longues séries temporelles magnétiques satellitaires. Après avoir testé cette méthode sur des modèles synthétiques et l'avoir appliqué au cas de données réelles terrestre pour lesquelles des études d'induction électromagnétiques antérieurs permettent d'avoir un a priori sur le modèle de conductivité électrique attendu, nous avons obtenu les premiers modèles de conductivité électrique martien en utilisant les données magnétiques du satellite Mars Global Surveyor. Ces résultats nous ont permis de valider des modèles de structure interne antérieurs établis à partir d'analyses géochimiques et minéralogiques des météorites martiennes. Cette méthode innovante est aujourd'hui la seule capable d'obtenir une image électrique des manteaux telluriques à partir de données magnétiques satellitaires pour des corps autres que la Terre ou la Lune et pour lesquels aucun a priori sur la structure spatio-temporelle du champ électromagnétique inducteur externe n'est nécessaire.

Earth-based radar observations of the spin state of Mercury at 35 epochs between 2002 and 2012 reveal that its spin axis is tilted by (2.04 plus or minus 0.08) arc min with respect to the orbit normal. The direction of the tilt suggests that Mercury is in or near a Cassini state. Observed rotation rate variations clearly exhibit an 88-day libration pattern which is due to solar gravitational torques acting on the asymmetrically shaped planet. The amplitude of the forced libration, (38.5 plus or minus 1.6) arc sec, corresponds to a longitudinal displacement of ∼450 m at the equator. Combining these measurements of the spin properties with second-degree gravitational harmonics (Smith et al., 2012) provides an estimate of the polar moment of inertia of MercuryC/MR2 = 0.346 plus or minus 0.014, where M and R are Mercury's mass and radius. The fraction of the moment that corresponds to the outer librating shell, which can be used to estimate the size of the core, is Cm/C = 0.431 plus or minus 0.025.

We present the analysis of 205 spatially resolved measurements of the surface composition of Mercury from MESSENGER's X-Ray Spectrometer. The surface footprints of these measurements are categorized according to geological terrain. Northern smooth plains deposits and the plains interior to the Caloris basin differ compositionally from older terrain on Mercury. The older terrain generally has higher Mg/Si, S/Si, and Ca/Si ratios, and a lower Al/Si ratio than the smooth plains. Mercury's surface mineralogy is likely dominated by high-Mg mafic minerals (e.g., enstatite), plagioclase feldspar, and lesser amounts of Ca, Mg, and/or Fe sulfides (e.g., oldhamite). The compositional difference between the volcanic smooth plains and the older terrain reflects different abundances of these minerals and points to the crystallization of the smooth plains from a more chemically evolved magma source. High-degree partial melts of enstatite chondrite material provide a generally good compositional and mineralogical match for much of the surface of Mercury. An exception is Fe, for which the low surface abundance on Mercury is still higher than that of melts from enstatite chondrites and may indicate an exogenous contribution from meteoroid impacts.

The structure of Mercury’s internal magnetic field has been determined from analysis of orbital Magnetometer measurements by the MESSENGER spacecraft. We identified the magnetic equator on 531 low-altitude and 120 high-altitude equator crossings from the zero in the radial cylindrical magnetic field component, Bp. The low-altitude crossings are offset 479 plus or minus 6 km northward, indicating an offset of the planetary dipole. The tilt of the magnetic pole relative to the planetary spin axis is less than 0.8 degrees. The high-altitude crossings yield a northward offset of the magnetic equator of 486 plus or minus 74 km. A field with only nonzero dipole and octupole coefficients also matches the low-altitude observations but cannot yield off-equatorial Bp = 0 at radial distances greater than 3520 km. We compared offset dipole and other descriptions of the field with vector field observations below 600 km for 13 longitudinally distributed, magnetically quiet orbits. An offset dipole with southward directed moment of 190 nT-R3M yields root-mean-square (RMS) residuals below 14 nT, whereas a field with only dipole and octupole terms tuned to match the polar field and the low-altitude magnetic equator crossings yields RMS residuals up to 68 nT. Attributing the residuals from the offset-dipole field to axial degree 3 and 4 contributions we estimate that the Gauss coefficient magnitudes for the additional terms are less than 4% and 7%, respectively, relative to the dipole. The axial alignment and prominent quadrupole are consistent with a non-convecting layer above a deep dynamo in Mercury’s fluid outer core.

Images of Mercury's northern volcanic plains taken by the MESSENGER spacecraft reveal a large number of buried impact craters and basins discernible by wrinkle-ridge rings that overlie their rims. Many of these "ghost" craters and basins contain interior graben of diverse widths and orientations. Here we use finite element models to test a variety of mechanisms for the formation of these graben and ridges. Results show that graben are best explained by cooling of large thicknesses of flood lavas within the craters and basins; conservation of surface area during cooling induces the required extensional stress state. In contrast, the development of wrinkle-ridge rings is best explained as the result of cooling and contraction of Mercury's interior, during which a reduction in Mercury's surface area led to a compressional state of stress. The critical factor in determining where large graben form is the thickness of the youngest cooling unit, the topmost sequence of lavas that cooled coevally. A thicker cooling unit leads to a deeper initiation of normal faulting (wider graben floors). Consistent with observations, the widest graben are predicted to occur where pooled lavas were thickest, and no graben are predicted within generally thinner plains outside of major craters. Observed concentrically oriented graben can be explained by variations in the thickness of the youngest cooling unit. In contrast, none of the basin uplift mechanisms considered, including isostatic response to crater topography, inward flow of the lower crust, or exterior loading by volcanic plains, can account for concentrically oriented graben.

Several series of large dipolarization events are documented from magnetic field observations in Mercury's magnetotail made by the MESSENGER spacecraft. The dipolarizations are identified by a rapid (∼1 s) increase in the northward component of the magnetic field, followed by a slower return (∼10 s) to pre-onset values. The changes in field strength during an event frequently reach 40 nT or higher, equivalent to an increase in the total magnetic field magnitude by a factor of ∼4 or more. The presence of spatially constrained dipolarizations at Mercury provides a key to understanding the magnetic substorm process in a new parameter regime: the dipolarization timescale, which is shorter than at Earth, is suspected to lead to efficient non-adiabatic heating of the plasma sheet proton population, and the high recurrence rate of the structures is similar to that frequently observed for flux ropes and traveling compression regions in Mercury's magnetotail. The relatively short lifetime of the events is attributed to the lack of steady field-aligned current systems at Mercury.

The formation of large impact basins (diameter D greater than or equal to 300 km) was an important process in the early geological evolution of Mercury and influenced the planet’s topography, stratigraphy, and crustal structure. We catalog and characterize this basin population on Mercury from global observations by the MESSENGER spacecraft, and we use the new data to evaluate basins suggested on the basis of the Mariner 10 flybys. Forty-six certain or probable impact basins are recognized; a few additional basins that may have been degraded to the point of ambiguity are plausible on the basis of new data but are classified as uncertain. The spatial density of large basins (D greater than or equal to 500 km) on Mercury is lower than that on the Moon. Morphological characteristics of basins on Mercury suggest that on average they are more degraded than lunar basins. These observations are consistent with more efficient modification, degradation, and obliteration of the largest basins on Mercury than on the Moon. This distinction may be a result of differences in the basin formation process (producing fewer rings), relaxation of topography after basin formation (subduing relief), or rates of volcanism (burying basin rings and interiors) during the period of heavy bombardment on Mercury from those on the Moon.

Localized reductions in the magnetic field associated with plasma pressure in Mercury's magnetospheric cusp and nightside plasma sheet have been routinely observed by the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft. We present a statistical analysis of near-equatorial magnetic depressions to derive the structure of Mercury's plasma sheet pressure. Because the plasma pressure in the magnetosphere correlates with solar wind density, the pressures were normalized to a Mercury heliocentric distance of 0.39 AU. A model magnetic field was used to map observations obtained on the ascending and descending orbit nodes to the magnetic equator and revealed the presence of plasma in a toroidal section extending on the nightside from dusk to dawn. Mapping the data to invariant magnetic latitude shows that the pressure is symmetric about the magnetic equator. The average pressure normalized for heliocentric distance is 1.45 nPa and exhibits a weak, 0.05 nPa/h, dusk-to-dawn gradient with local time. The plasma sheet pressure can vary between successive orbits by an order of magnitude. Unlike the predictions of some global simulations of Mercury's magnetosphere, the plasma enhancements do not form a closed distribution around the planet. This difference may arise from the idealized solar wind and interplanetary magnetic field conditions used in the simulations, which maximize the size and stability of the magnetosphere, thus promoting the formation of drift paths that close around the planet. For typical plasma sheet energies, 5 keV, the first adiabatic invariant for protons fails to be conserved even within 500 km altitude at midnight, implying that stochastic processes must be considered in plasma sheet transport.

We present a baseline, time-averaged model for Mercury's magnetosphere, derived from MESSENGER Magnetometer data from 24 March to 12 December 2011, comprising the spacecraft's first three Mercury years in orbit around the innermost planet. The model, constructed under the approximation that the magnetospheric shape can be represented as a paraboloid of revolution, includes two external (magnetopause and magnetotail) current systems and an internal (dipole) field and allows for reconnection. We take advantage of the geometry of the orbital Magnetometer data to estimate all but one of the model parameters, and their ranges, directly from the observations. These parameters are then used as a priori constraints in the paraboloid magnetospheric model, and the sole remaining parameter, the dipole moment, is estimated as 190 nTRM3from a grid search. We verify that the best fit dipole moment is insensitive to changes in the other parameters within their determined ranges. The model provides an excellent first-order fit to the MESSENGER observations, with a root-mean-square misfit of less than 20 nT globally. The results show that the magnetopause field strength ranges from 10% to 50% of the dipole field strength at observation locations on the dayside and at nightside latitudes north of 60°N. Globally, the residual signatures observed to date are dominated by the results of magnetospheric processes, confirming the dynamic nature of Mercury's magnetosphere.

Earth-based radar images of Mercury show radar-bright material inside impact craters near the planet's poles. A previous study indicated that the polar-deposit-hosting craters (PDCs) at Mercury's north pole are shallower than craters that lack such deposits. We use data acquired by the Mercury Laser Altimeter on the MESSENGER spacecraft during 11 months of orbital observations to revisit the depths of craters at high northern latitudes on Mercury. We measured the depth and diameter of 537 craters located poleward of 45 degrees N, evaluated the slopes of the northern and southern walls of 30 PDCs, and assessed the floor roughness of 94 craters, including nine PDCs. We find that the PDCs appear to have a fresher crater morphology than the non-PDCs and that the radar-bright material has no detectable influence on crater depths, wall slopes, or floor roughness. The statistical similarity of crater depth-diameter relations for the PDC and non-PDC populations places an upper limit on the thickness of the radar-bright material (less than 170 m for a crater 11 km in diameter) that can be refined by future detailed analysis. Results of the current study are consistent with the view that the radar-bright material constitutes a relatively thin layer emplaced preferentially in comparatively young craters.

The Fast Imaging Plasma Spectrometer (FIPS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has made the first in situ measurements of solar wind plasma in the inner heliosphere since the Helios 1 and 2 spacecraft in the 1980s. Although the core of the solar wind velocity distribution is obstructed by the spacecraft sunshade, a data analysis technique has been developed that recovers both bulk and thermal speeds to 10% accuracy and provides the first measurements of solar wind heavy ions (mass per charge >2 amu/e) at heliocentric distances within 0.5 AU. Solar wind alpha particles and heavy ions appear to have similar mean flow speeds at values greater than that of the protons by approximately 70% of the Alfvén speed. From an examination of the thermal properties of alpha particles and heavier solar wind ions, we find a ratio of the temperature of alpha particles to that of protons nearly twice that of previously reported Helios observations, though still within the limits of excessive heating of heavy ions observed spectroscopically close to the Sun. Furthermore, examination of typical magnetic power spectra at the orbits of MESSENGER and at 1 AU reveals the lack of a strong signature of local resonant ion heating, implying that a majority of heavy ion heating could occur close to the Sun. These results demonstrate that the solar wind at plus or minus 0.3 AU is a blend of the effects of wave–particle interactions occurring in both the solar corona and the heliosphere.

The Energetic Particle Spectrometer (EPS) on the MESSENGER spacecraft, in orbit about Mercury since March 2011, has detected bursts of low- and moderate-energy (tens to hundreds of keV) electrons during portions of most orbits. There have been periods when such bursts were observed regularly on every orbit over a span of several weeks, and other periods when electrons were not observed for several days at a time. We have systematically characterized these energetic events on the basis of particle intensity over the 12-month period since MESSENGER began orbital operations. Now that MESSENGER has sampled most Mercury longitudes and local times, it is evident that the largest burst events were either at high northern latitudes or near local midnight. Lower-energy events were also seen near the equator but were mostly absent in both the dawn and dusk local time sectors. The high-latitude and nightside events are similar in particle intensity, spectra, and pitch angle and are interpreted to be the result of acceleration by the same mechanism. Another group of events occurred upstream of Mercury's bow shock. For two examples of this group of upstream events with good pitch angle coverage, the particles were field-aligned and traveling away from the bow shock. This group of events is interpreted to be similar to upstream events found at Earth during which particles are accelerated at the bow shock and subsequently travel upstream into the solar wind.

A new technique has been developed for characterizing gamma-ray emission from a planetary surface in the presence of large background signals generated in a spacecraft. This technique is applied to the analysis of Al gamma rays measured by the MESSENGER Gamma-Ray Spectrometer to determine the abundance of Al on the surface of Mercury. The result (Al/Si = 0.29-0.13+0.05) is consistent with Al/Si ratios derived from the MESSENGER X-Ray Spectrometer and confirms the finding of low Al abundances. The measured abundance rules out a global, lunar-like feldspar-rich crust and is consistent with previously suggested analogs for surface material on Mercury, including terrestrial komatiites, low-iron basalts, partial melts of CB chondrites, and partial melts of enstatite chondrites. Additional applications of this technique include the measurement of other elements on Mercury's surface as well as the analysis of data from other planetary gamma-ray spectrometer experiments.

The X-Ray Spectrometer (XRS) on the MESSENGER spacecraft measures elemental abundances on the surface of Mercury by detecting fluorescent X-ray emissions induced on the planet's surface by the incident solar X-ray flux. The XRS began orbital observations on 23 March 2011 and has observed X-ray fluorescence (XRF) from the surface of the planet whenever a sunlit portion of Mercury has been within the XRS field of view. Solar flares are generally required to provide sufficient signal to detect elements that fluoresce at energies above ∼2 keV, but XRF up to the calcium line (3.69 keV) has been detected from Mercury's surface at times when the XRS field of view included only unlit portions of the planet. Many such events have been detected and are identified as electron-induced X-ray emission produced by the interaction of ∼1-10 keV electrons with Mercury's surface. Electrons in this energy range were detected by the XRS during the three Mercury flybys and have also been observed regularly in orbit about Mercury. Knowledge of the energy spectrum of the electrons precipitating at the planet's surface makes it possible to infer surface composition from the measured fluorescent spectra, providing additional measurement opportunities for the XRS. Abundance results for Mg, Al, and Si are in good agreement with those derived from solar-induced XRF data, providing independent validation of the analysis methodologies. Derived S and Ca abundances are somewhat higher than derived from the solar-induced fluorescence data, possibly reflecting incomplete knowledge of the energy spectra of electrons impacting the planet.

We have investigated the Na distributions in Semarkona Type II chondrules by electron microprobe, analyzing olivine and melt inclusions in it, mesostasis and bulk chondrule, to see whether they indicate interactions with an ambient gas during chondrule formation. Sodium concentrations of bulk chondrule liquids, melt inclusions and mesostases can be explained to a first approximation by fractional crystallization of olivine ± pyroxene. The most primitive olivine cores in each chondrule are mostly between Fa8 and Fa13, with 0.0022–0.0069 ± 0.0013 wt.% Na2O. Type IIA chondrule olivines have consistently higher Na contents than olivines in Type IIAB chondrules. We used the dependence of olivine–liquid Na partitioning on FeO in olivine as a measure of equilibration. Extreme olivine rim compositions are ∼Fa35 and 0.03 wt.% Na2O and are close to being in equilibrium with the mesostasis glass. Olivine cores compared with the bulk chondrule compositions, particularly in IIA chondrules, show very high apparent DNa, indicating disequilibrium and suggesting that chondrule initial melts were more Na-rich than present chondrule bulk compositions. The apparent DNa values correlate with the Na concentrations of the olivine, but not with concentrations in the bulk melt. We use equilibrium DNa to find the Na content of the true parent liquid and estimate that Type IIA chondrules lost more than half their Na and recondensation was incomplete, whereas Type IIAB chondrules recovered most of theirs in their mesostases. Glass inclusions in olivine have lower Na than expected from fractionation of bulk composition liquids, and mesostases have higher Na than expected in calculated daughter liquids formed by fractional crystallization alone. These observations also require open system behavior of chondrules, specifically evaporation of Na before formation of melt inclusions followed by recondensation of Na in mesostases. Within this record of evaporation followed by recondensation, there is no indication of a stage with zero Na in the chondrules, which is predicted by models for shock wave cooling at canonical nebular pressures, suggesting high PT. The high Na concentrations in olivine and mesostases indicate very high PNa while chondrules were molten. This may be explained by local, very high particle densities where Type II chondrules formed. The high PT, PNa and number densities of chondrules implied suggest formation in debris clouds after protoplanetary collisions as an alternative to formation after passage of shock waves through large particle-rich clumps in the disk. Encounters of partially molten chondrules should have been frequent in these dense swarms. However, in many ordinary chondrites like Semarkona, "cluster chondrites", compound chondrules are not abundant but instead chondrules aggregated into clusters. Chondrule melting, cooling and clustering in dense swarms contributed to rapid accretion, possibly after collision, by fallback on the grandparent body and by reaccretion as a new body downrange.

Analysis of MESSENGER magnetic field observations taken in the southern lobe of Mercury's magnetotail and the adjacent magnetosheath on 11 April 2011 indicates that a total of 163 flux transfer events (FTEs) occurred within a 25 min interval. Each FTE had a duration of ∼2-3 s and was separated in time from the next by ∼8-10 s. A range of values have been reported at Earth, with mean values near ∼1-2 min and ∼8 min, respectively. We term these intervals of quasiperiodic flux transfer events "FTE showers." The northward and sunward orientation of the interplanetary magnetic field during this shower strongly suggests that the FTEs observed during this event formed just tailward of Mercury's southern magnetic cusp. The point of origin for the shower was confirmed with the Cooling model of FTE motion. Modeling of the individual FTE-type flux ropes in the magnetosheath indicates that these flux ropes had elliptical cross sections, a mean semimajor axis of 0.15RM (where RM is Mercury's radius, or 2440 km), and a mean axial magnetic flux of 1.25 MWb. The lobe magnetic field was relatively constant until the onset of the FTE shower, but thereafter the field magnitude decreased steadily until the spacecraft crossed the magnetopause. This decrease in magnetic field intensity is frequently observed during FTE showers. Such a decrease may be due to the diamagnetism of the new magnetosheath plasma being injected into the tail by the FTEs.

We summarize observations by the MESSENGER spacecraft of highly coherent waves at frequencies between 0.4 and 5 Hz in Mercury's inner magnetosphere. This survey covers the time period from 24 March to 25 September 2011, or 2.1 Mercury years. These waves typically exhibit banded harmonic structure that drifts in frequency as the spacecraft traverses the magnetic equator. The waves are seen at all magnetic local times, but their observed rate of occurrence is much less on the dayside, at least in part the result of MESSENGER's orbit. On the nightside, on average, wave power is maximum near the equator and decreases with increasing magnetic latitude, consistent with an equatorial source. When the spacecraft traverses the plasma sheet during its equatorial crossings, wave power is a factor of 2 larger than for equatorial crossings that do not cross the plasma sheet. The waves are highly transverse at large magnetic latitudes but are more compressional near the equator. However, at the equator the transverse component of these waves increases relative to the compressional component as the degree of polarization decreases. Also, there is a substantial minority of events that are transverse at all magnetic latitudes, including the equator. A few of these latter events could be interpreted as ion cyclotron waves. In general, the waves tend to be strongly linear and characterized by values of the ellipticity less than 0.3 and wave-normal angles peaked near 90 degrees. Their maxima in wave power at the equator coupled with their narrow-band character suggests that these waves might be generated locally in loss cone plasma characterized by high values of the ratio b of plasma pressure to magnetic pressure. Presumably both electromagnetic ion cyclotron waves and electromagnetic ion Bernstein waves can be generated by ion loss cone distributions. If proton b decreases with increasing magnetic latitude along a field line, then electromagnetic ion Bernstein waves are predicted to transition from compressional to transverse, a pattern consistent with our observations. We hypothesize that these local instabilities can lead to enhanced ion precipitation and directly feed fieldline resonances.

This thesis has been prepared in the frame of the time of flight secondary ions mass spectrometry experiment called COSIMA on board the ESA's Rosetta cometary mission. This instrument will collect and measure the chemical nature of the grains of the comet 67P/Churyumov-Gerasimenko. The aim of my thesis is to prepare and facilitate the interpretation of mass spectra of COSIMA, especially their refractory organic component. Therefore, the mass spectra of two families of pure compounds (nitrogen heterocycles and carboxylic acids) have been measured with a laboratory instrument tailor-made to have instrumental characteristics similar to those of COSIMA.. We have established that those data can be considered as reference mass spectra to be used by the COSIMA team. This database has been a starting point for the search for markers to distinguish molecules composed of specific atoms. Two markers have been found. Nevertheless they should be handled with caution because of bias that can be induced by the presence of minerals in grains or contaminants on the targets. Cometary organic matter analogs have also been analyzed. These measurements show that the spectra collected with COSIMA can have significant biases. Identification of HCN polymers is compromised. However, the analysis of a grain from the Murchison meteorite shows that the distinction between inorganic and organic compounds is feasible. In addition, the signatures of key compounds such as hexamethylenetetramine or polyoxymethylene (POM) are clearly identifiable in the positive mode of the instrument. Since the POM is thermally unstable, a kinetic study of its thermal degradation has been carried out to constrain the operational strategy to be adopted for its detection. To maximize the chances to detect it in the cometary grains, analyses by COSIMA must be made within fifteen days after the collection of grains.; Cette thèse s'inscrit dans le cadre de l'expérience de spectrométrie de masse d'ions secondaires à temps de vol, COSIMA, se trouvant à bord de la mission européenne cométaire Rosetta. Cet instrument a pour vocation de collecter puis de mesurer la nature chimique des grains de la comète 67P/Churyumov-Gerasimenko. L'objectif de ma thèse est de préparer et de faciliter l'interprétation des spectres de masse de COSIMA, et plus particulièrement de leur composante organique réfractaire. Pour cela, à partir d'un instrument de laboratoire conçu spécialement pour présenter des caractéristiques instrumentales similaires à COSIMA, les spectres de masse de deux familles de composés purs (hétérocycles azotés et acides carboxyliques) ont été mesurés. Ces données ont été qualifiées en tant que spectres de masse de référence utilisables par l'équipe COSIMA. Cette base de données est le point de départ de la recherche de marqueurs permettant la distinction des molécules constituées d'atomes particuliers. Deux marqueurs ont été trouvés. Néanmoins ils sont à manipuler avec prudence compte tenu des biais pouvant être induits par la présence de minéraux au sein des grains ou de contaminants sur les cibles d'analyse. L'analyse d'analogues de la matière organique cométaire a également été effectuée. Il en ressort que les mesures réalisées avec COSIMA peuvent présenter des biais importants. L'identification des polymères de HCN en est d'ailleurs compromise. Toutefois, l'analyse d'un grain de la météorite de Murchison montre que la distinction entre les composantes minérale et organique est facilement réalisable. De plus, les signatures de composés clés comme l'hexaméthylènetétramine ou le polyoxyméthylène (POM) sont clairement identifiables dans le mode positif de l'instrument. Le POM étant thermiquement instable, une étude cinétique de sa dégradation thermique a été réalisée en vue de contraindre la stratégie opérationnelle à adopter pour sa détection. Pour maximiser les chances de le détecter dans les grains cométaires, les analyses par COSIMA doivent être effectuées dans les quinze jours après la collecte des grains.

On Earth, periglacial regions where an ice-rich permafrost is present provide a record of global climate changes. For example, the ice-rich permafrost (50-80 % of ice by volume) that occurs in Central Yakutia (Siberia) and in the Mackenzie River Delta (Canada) was formed during the glacial periods of the Pleistocene. This permafrost was subsequently degraded during global warming at the early Holocene interglacial period.Global and possibly ice-rich permafrost occurs on Mars as well. It is thought to be the product of obliquity-driven and relatively recent global climate change (i.e. dozens of Ma). Utopia Planitia, situated in the northern mid-latitudes, is dotted with possible periglacial landforms (scalloped depressions, polygons and polygon-junction pits) that could indicate the presence of an ice-rich permafrost. Similarly to Earth, this permafrost could be marker of recent global climate changes.This thesis focuses on the impact of global climate changes on the periglacial regions of Earth and Mars. With this aim in view, we conducted (i) field studies of the periglacial processes and landforms in the Central Yakutia (Siberia) and in the Mackenzie River Delta (Canada) and, (ii) a geomorphological study (based on high-resolution images) of the putative-periglacial landforms of Utopia Planitia.Our study shows that the assemblage of landforms in Utopia Planitia share traits of form, scale and spatial association with the landforms of the Central Yakutia and of the Mackenzie Delta (thermokarst lakes, polygons and polygon-junction ponds) indicating that Utopia Planitia has an ice-rich permafrost. The permafrost is composed of stratified sediments ~ 70 m thick with a high ice-content (possibly ≥ 50 % by volume).The permafrost appears to have a syngenetic origin: it was formed by an accumulation of sediments in the basin of Utopia Planitia under cold climate conditions that leaded to the in-situ freezing of the sediments. The sediments could have been deposited by outflow valleys from Elysium Mons and/or by an eolian activity. With regard to the latter, the synchronous formation of a possible regional ice-sheet near Utopia Planitia during medium-obliquity (~ 35°) periods of Mars could have induced a preferential eolian deposition in Utopia Planitia.Subsequently, the ice-rich permafrost was regionally degraded between ~ 5 and 10 Ma. The thermokarst was triggered by an increase of insolation during high-obliquity (~ 45°) periods of Mars. The increase of temperature caused the thermal destabilization of the permafrost inducing the sublimation of ground-ice, deeply modifying the landscape.Thus, important obliquity variations of Mars caused global climate changes that could have induced the formation and the degradation of the ice-rich permafrost of Utopia Planitia between ~ 5 and 10 Ma., Sur Terre, les régions périglaciaires ayant un pergélisol riche en glace peuvent enregistrer les changements climatiques globaux. Ce pergélisol contenant 50-80 % de glace en volume s'est formé lors des grandes périodes glaciaires du Pléistocène. Par la suite, ce pergélisol riche en glace a subi une dégradation intense lors de réchauffements climatiques globaux au début de la période interglaciaire de l'Holocène.La planète Mars comporte un pergélisol à l'échelle planétaire dont la formation serait associée à des changements climatiques globaux provoqués par des variations chaotiques de son orbite durant les derniers millions d'années. La région d'Utopia Planitia située dans les moyennes latitudes nord de Mars présente différents modelés de surface (“ scalloped depressions ”, polygones, cavités à la jonction des polygones) interprétés comme s'étant formés à partir d'un pergélisol contenant potentiellement une grande quantité de glace. De la même manière que sur Terre, ce pergélisol a pu enregistrer les derniers changements climatiques globaux survenus sur Mars.Cette thèse propose d'étudier comparativement l'impact des changements climatiques sur le paysage des régions périglaciaires sur Terre et sur Mars. Dans ce but, nous avons conduit des études sur le terrain des processus et des modelés périglaciaires en Yakoutie Centrale (Sibérie) et dans le delta du Mackenzie (Canada) associées à une étude géomorphologique à haute résolution des modelés d'Utopia Planitia.Notre étude montre que l'ensemble des modelés d'Utopia Planitia est similaire en morphologie, taille et association spatiale à celui de la Yakoutie Centrale et du delta du Mackenzie (lacs thermokarstiques, polygones, mares à la jonction des polygones) indiquant que la région présenterait un pergélisol riche en glace. Le pergélisol serait composé de sédiments stratifiés et dont l'âge de formation minimale est estimé entre ~ 5 et 100 Ma. Le pergélisol contiendrait un volume de glace important (≥ 50 % en volume) sur une épaisseur de ~ 70 m.De part ses caractéristiques, ce pergélisol aurait une origine syngénétique : sa formation serait le résultat d'une accumulation importante de sédiments au sein du bassin d'Utopia Planitia sous des conditions froides permettant le gel in-situ des sédiments. Les sédiments peuvent avoir été déposés par des vallées de débâcles provenant d'Elysium Mons et/ou par une activité éolienne importante. Par ailleurs, la formation synchrone d'une calotte de glace régionale près d'Utopia Planitia lors de périodes de moyenne obliquité (~ 35°) de Mars pourrait avoir induit un dépôt éolien préférentiel dans Utopia Planitia.Par la suite, le pergélisol riche en glace aurait subi une dégradation régionale importante entre ~ 5 et 10 Ma. Ce thermokarst aurait été déclenché par une insolation accrue lors de périodes de haute obliquité (~ 45°) de Mars. L'augmentation des températures aurait provoqué une déstabilisation thermique du pergélisol entraînant une sublimation de la glace, modifiant profondément le paysage de la région.Ainsi, les variations importantes de l'obliquité de Mars ont généré des changements climatiques globaux qui ont permis la formation et la dégradation du pergélisol riche en glace d'Utopia Planitia entre ~ 5 et 10 Ma.