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Introduction In the research on the origin of life on our Planet, the possibility of a contribution played by organic substances of extraterrestrial origin, which may have reached our planet through micro-fragments of planetary and cometary origin, is being taken into consideration more and more seriously (Flynn et al., 2003). These particles may also provide the necessary thermal protection for thermolabile, life-related molecules, to the high temperatures reached during the first stages of the atmospheric entry processes. Particularly interesting is the size of the order of a tenth of a millimeter, which corresponds to a peak in the distribution of the material entering the atmosphere. Physical models of the entry process of such grains have been available in the literature for many years. When the composition of most promising mineral phase is considered, the most interesting candidates are carbonates (mostly of Mg, Ca, and Fe) which have been associated, in meteorites and in cometary grains, with organic molecules (McKay et. al., 1996; Flynn et al., 2000; Pizzarello et al., 2006; Matrajt et al., 2012) for several reasons. Carbonates are very common in the Solar System. Outside the Earth, they have been identified on the surface of Mars (Ehlmann et al., 2008; Palomba et al., 2009; Wray et al., 2016), on Ceres (Rivkin et al., 2006; De Sanctis et al., 2016) and tentatively on other asteroids (Rivkin, 2009) as well as in cometary comas (Fomenkova et al., 1992; Lisse et al., 2006; Wirick et al., 2007). Since these astrophysical sites are thought to be the main sources of meteorites and micrometeorites, it is not surprising to find carbonates also in these small objects which reach our planet. In particular, Mg-, Ca-, and Fe/Mn-rich carbonate globules of putative biotic origin have been found in the Martian meteorite ALH84001 (McKay et al., 1996). Carbonate decomposition All the available studies of the atmospheric entry grains assume compositions corresponding to the bulk of meteorite bodies, i.e. silicates and metals. Only recently, carbonates have been taken in account as carriers in an astrobiological perspective (Bisceglia et al., 2017; Micca Longo & Longo, 2017; 2018). Carbonates are known to undergo decomposition in vacuum at temperatures of a few hundred °C (producing metal oxides and gaseous carbon dioxide) (L’vov, 1997; 2002). Therefore, during the atmospheric entry, the grains with carbonate composition are expected to be enriched in oxides and depleted of the initial carbonate amount. This process, being endothermic, can contribute to the thermal protection of associated organic matter. However, the kinetics of the decomposition under such conditions is not well understood. The decomposition model developed in Micca Longo & Longo (2017 and 2018) is based on a well-mixed and ideal solid mixture, and it allows a first evaluation of grains behavior during their passage through the Earth’s atmosphere. The Langmuir law allows to calculate the evaporation rate, per unit time and area, as stated in Bisceglia et al. (2017), in terms of the vapor pressure for the solid mixture carbonate/oxide. Laboratory measurements The kinetics of CO2 diffusion within this kind of grains is still a subject if study. A first attempt was done in Micca Longo et al. (2019) where an important role for the evolution of the model was given to the interpretation of laboratory experiments emulating the conditions during the atmospheric entry processes. In this work, we report the laboratory measurements on Ca and Mg carbonates done to follow the diffusion process of CO2 within such materials. Powders were thermally processed under vacuum (10-4 and 10-5 mbar), for about 3.5 hours, at several temperatures by using a Carbolite furnace able to reach temperatures up to 1200°C. Infrared spectroscopy, gravimetry, Scanning Electron Microscopy, and Energy Dispersive X-ray analysis were involved to investigate spectral, morphological, and compositional modifications induced by thermal processing as done in Orofino et al. (2007), Blanco et al. (2011) and Micca Longo et al. (2019). References Bisceglia E., Micca Longo G., Longo S. (2017) International Journal of Astrobiology 16, 130. Blanco A., et al. (2011) Icarus 213, 473. De Sanctis M.C., et al. (2016) Nature 536, 54. Ehlmann B.L., et al. (2008) Science 322, 1828. Flynn G.J., et al. (2000) Bioastronomy 99, 213. Flynn G.J., et al. (2003) Geochimica et Cosmochimica Acta 67, 4791. Fomenkova M.N., et al. (1992) Science 258, 266. Lisse C.M., et al. (2006) Science 313, 635. L’vov B.V. (1997) Thermochimica Acta 303, 161. L’vov B.V. (2002) Thermochimica Acta 386, 1. Matrajt G., et al. (2012) Meteoritics & Planetary Science 47, 525. McKay D.S., et al. (1996) Science 273, 924–930. Micca Longo G., Longo S. (2017) International Journal of Astrobiology 16, 368. Micca Longo G., Longo S. (2018) Icarus. doi: 10.1016/j.icarus.2017.12.001 Micca Longo G., et al. (2019) Geoscience. doi: 10.3390/geosciences9020101 Orofino V., et al. (2007) Icarus 187, 457. Palomba E., et al. (2009) Icarus 203, 58. Pizzarello S., Cooper G.W., Flynn G.J. (2006) Meteorites & the Early Solar System 2, 625. Rivkin A.S. (2009) Division for Planetary Sciences Meeting Abstracts 41, Abstract #32.07. Wirick S., et al. (2007) Lunar and Planetary Science Conference Proceedings 38, Abstract #1534. Wray J.J., et al. (2016) Journal of Geophysical Research: Planets 121, 652.

The presence of ancient fluvial systems on Mars, known as valley networks, is considered one of the most compelling evidence that liquid water was once stable on the planet's surface. To understand the formation mechanisms of these valleys and to acquire information on the ancient climatic conditions of the planet, we determined the formation time of a sample group of 63 Martian valley networks. Our sample group was divided into two subgroups: valleys with an interior channel (13 valleys); and valleys without visible interior channels (50 valleys). For the first subgroup we estimated the formation time using a method based on the calculation of water and sediment discharges. We assumed four different possibilities: continuous flow; 5% of intermittence, typical of terrestrial humid or sub-humid conditions; 1% as for semiarid or arid environments; and finally, 0.1% for hyper-arid conditions. Once obtained the formation times for each valley we evaluated the erosion rates. Subsequently, the mean erosion rates obtained for the first subgroup of valleys were used to calculate the formation times for the remaining 50 valleys using the ratio between the eroded volume and the erosion rate. For the whole sample group of the fluvial systems, we found formation timescales ranging from 5 × 102 yr to 8 × 106 yr (with a median of 3 × 104 yr) for a continuous sediment flow, while the range is from 1 × 104 yr to 2 × 108 yr (median 5 × 105 yr) with an intermittence of 5%, from 5 × 104 yr to 8 × 108 yr (median 3 × 106 yr) with an intermittence of 1%, and from 5 × 105 yr and to 8 × 109yr (median 3 × 107yr) with a 0.1% intermittence. However, based on our results, a continuous sediment flow as well as an intermittence of 0.1% seem unlikely. Plausible values of formation timescales are instead obtained with intermittencies of 5% and 1% corresponding to humid and semiarid/arid environment, respectively. Our results do not allow to discriminate between the two scenarios; on the contrary the scenario of a permanently cold and icy Noachian Mars can be ruled out by our findings.

The existence of a wet period lasting long enough to allow the development of elementary forms of life on Mars has always been a very interesting issue. Given this perspective, the research for geological markers of such occurrences has been continually pursued. Once a favorable site is detected, effort should be spent to get as much information as possible aimed at a precise assessment of the genesis and evolution of the areas showing the selected markers. In this work, we discuss the recent finding of possible deposits pointing to the past existence of liquid water in Tikhonravov crater located in Arabia Terra. Comparison of CRISM spectra and those of laboratory minerals formed by aqueous alteration has led us to the conclusion that the studied areas within the impact crater host phyllosilicates deposits. In addition, analysis of the CRISM spectra has resulted in the tentative identification of carbonates mixed with phyllosilicates.

Candidate closed- and open-basin lakes have been widely observed on the Martian surface. Many of them show the presence of exposed sediments of possible lacustrine origin. Understanding the mineralogy of these structures provide insights on the nature of processes that originated and shaped their interior and on the role of liquid water on Mars. Spectral data obtained from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) have been so far analyzed with particular attention to the possible detection of aqueous alteration minerals. In this work, we mapped 16 closed-basins and 39 open-basins, not present in the past published Martian basins maps. We looked for recent CRISM observations of these and other previously mapped open- and closed-basin lakes. We further confirmed the presence of water alteration minerals in two previously analyzed structures and discovered other deposits of such minerals in two of the newly mapped sites. Light-toned bedrock exposures that show the presence of hydrated minerals (such as Fe/Mg smectites) along with unaltered mafic material (olivine) have been observed. Based on our observations and the comparison with the literature, it can be deduced that the observed deposits are most probably of lacustrine origin. Furthermore, the location of these deposits of altered minerals suggests that, at least in one case (where altered material has been detected on the floor of the outlet valley), the observed minerals have likely been transported within the lacustrine deposits rather than altered in situ. To the best of our knowledge, this is the first time that this occurrence has been observed and it is an indication of presence of such kind of minerals even in small valleys.

Using the Secular Light Curve (SLC) formalism (Ferr\'in, 2010), we have catalogued 88 probable members of the Taurid Complex (TC). 51 of them have useful SLCs and 34 of these (67%) exhibit cometary activity. This high percentage of active asteroids gives support to the hypothesis of a catastrophe that took place during the Upper Paleolithic (Clube and Napier, 1984), when a large short-period comet, arriving in the inner Solar System from the Kuiper Belt, experienced, starting from 20 thousand years ago, a series of fragmentations that produced the present 2P/Encke comet, together with a large number of other members of the TC. The fragmentation of the progenitor body was facilitated by its heterogeneous structure (very similar to a rubble pile) and this also explains the current coexistence in the complex of fragments of different composition and origin. We have found that (2212) Hephaistos and 169P/NEAT are active and members of the TC with their own sub-group. Other components of the complex are groups of meteoroids, that often give rise to meteor showers when they enter the terrestrial atmosphere, and very probably also the two small asteroids that in 1908 and 2013 exploded in the terrestrial atmosphere over Tunguska and Chelyabinsk, respectively. What we see today of the TC are the remnants of a very varied and numerous complex of objects, characterized by an intense past of collisions with the Earth which may continue to represent a danger for our planet., Comment: 45 pages, 16 Figures, 9 Tables

International audience; K nowledge of the surface temperature distribution of a comet's nucleus proves to be of fundamental importance for a number of reasons: the evaluation of the thermophysical properties (thermal inertia and roughness, at several spatial scales), the characterization of the thermal behaviour of peculiar surface units (for example, local ice exposures, or inherently cold material units not to be confused with shaded areas), and for the understanding of the physical processes affecting the surface and the shallow subsur-face layers (for example, sublimation of volatile compounds). The distribution of surface temperatures of a cometary nucleus, closely measured by a spacecraft, can be compared with theoretical models meant to predict the thermophysical properties of the nucleus at various depths and with previous thermal observations carried out by both Earth-based and space-based telescopes, with the goal of validating and/or improving those models 1. Furthermore, the temporal evolution of the surface temperatures, primarily driven by insolation, is critical in triggering the activity of a comet, allowing the migration of volatile compounds from the interior of the nucleus and the production of gas and dust observable with different techniques 2. In the past, thermal surveys of minor bodies were carried out using ground-based and space-based facilities. However, due to the limited spatial resolution of those observations, in most cases the derived thermal properties were only global averages, and the determined temperatures were highly model dependent. Before Rosetta, direct measurements of cometary nuclei surface temperatures were obtained during short periods for a handful of comets, namely 1P/ Halley 3 , 19P/Borrelly 4 , 9P/Tempel 1 1 and 103P/Hartley 2 5 , with a maximum spatial resolution of approximately 30 m per pixel. Here we study the surface temperature distribution of the nucleus of comet 67P/Churyumov-Gerasimenko as derived by the Visible InfraRed and Thermal Imaging Spectrometer, Mapping channel (VIRTIS-M, hereafter VIRTIS) 6 in Rosetta's early global mapping phase after comet encounter. These data cover the pre-perihelion period from 1 August to 23 September 2014, when the heliocentric distance decreased from 3.62 to 3.31 au and the spacecraft was in the altitude range 61-13 km above the surface, resulting in a spatial resolution from approximately 15 to 3 m per pixel (most data showing a resolution of 13 to 15 m per pixel). In this period, the solar phase angle ranged from 17° to 93°, which Knowledge of the surface temperature distribution on a comet's nucleus and its temporal evolution at different timescales is key to constraining its thermophysical properties and understanding the physical processes that take place at and below the surface. Here we report on time-resolved maps of comet 67P/Churyumov-Gerasimenko retrieved on the basis of infra-red data acquired by the Visible InfraRed and Thermal Imaging Spectrometer (VIRTIS) onboard the Rosetta orbiter in 2014, over a roughly two-month period in the pre-perihelion phase at heliocentric distances between 3.62 and 3.31 au from the Sun. We find that at a spatial resolution ≤15 m per pixel, the measured temperatures point out the major effect that self-heating, due to the complex shape of the nucleus, has on the diurnal temperature variation. The bilobate nucleus of comet 67P also induces daytime shadowing effects, which result in large thermal gradients. Over longer periods, VIRTIS-derived temperature values reveal seasonal changes driven by decreasing heliocentric distance combined with an increasing abundance of ice within the uppermost centimetre-thick layer, which implies the possibility of having a largely pristine nucleus interior already in the shallow subsurface.

Next space missions will investigate the possibility of extinct or extant life on Mars. Studying the infrared spectral modifications, induced by thermal processing on different carbonate samples (recent shells and fossils of different ages), we developed a method able to discriminate biogenic carbonates from their abiogenic counterparts. The method has been successfully applied to microbialites, i.e. bio-induced carbonates deposits, and particularly to stromatolites, the laminated fabric of microbialites, some of which can be ascribed to among the oldest traces of biological activity known on Earth. These results are of valuable importance since such carbonates are linked to primitive living organisms that can be considered as good analogues for putative Martian life forms. Considering that the microstructures of biogenic carbonate are different from those of abiogenic origin, we investigated the micromorphology of shells, skeletal grains and microbialites at different scale with a scanning electron microscope. The results show that this line of research may provide an alternative and complementary approach to other techniques developed in the past by our group to distinguish biotic from abiotic carbonates. In this paper, we present some results that can be of valuable interest since they demonstrate the utility for a database of images concerning the structures and textures of relevant carbonate minerals. Such data may be useful for the analysis of Martian samples, coming from sample return missions or investigated by futurein situexplorations, aimed to characterize the near-subsurface of Mars in search for past or present life.

Reflectance spectroscopy is a very helpful tool for remote sensing investigations and has been widely used in terrestrial as well as planetary observations to study the surface composition. From this perspective, the visible (Vis) and near-infrared (NIR) regions of the spectrum, where several diagnostic absorption features of minerals are located, are well suited for the identification of such materials, in particular of rock-forming silicates. Among them, pyroxenes, which have been discovered on the surface of a number of different solar system bodies, play an important role. Up to now, both laboratory and remote sensing spectroscopic studies have been focused mainly on the two major bands at about 1 and 2 µm (the so-called Band I and Band II, respectively), while little attention has been paid to the minor bands falling in the visible range. One of the most important of them, present in many pyroxenes as well as in olivines, is the weak feature (reflectance minimum) near 670 nm, generally characterized by its variable wings (reflectance maxima) at about 570 nm and 720 nm. The intensity and the exact position of this feature depend on the type of pyroxene as well as on the grain size of the particles under consideration. In this work we present the Vis/NIR experimental reflectance spectra concerning enstatite and diopside, which are excellent representative of Low Calcium Pyroxenes (typically orthopyroxenes), and High Calcium Pyroxenes (typically clinopyroxenes) respectively. The results are very interesting and show a good correlation between the grain size of our samples and the relative intensities of the reflectance maxima occurring on both sides of the 670 nm feature. A similar study performed on Acfer 353, a pyroxene-rich eucritic meteorite of the Howardite–Eucrite–Diogenite family, of putative Vestan origin, shows that also in this case the variability in the Vis region of the spectra is linked to the grain size of the meteoritic particles. The connection between grain size and the behavior of the 670 nm feature can potentially be used to derive the average dimension of the regolith existing on the surface of planets and minor bodies. Both laboratory and observational study of the minor features, frequently ignored in the interpretation of the spectra acquired in remote sensing, can give complementary and useful information and thus can definitely contribute to a better knowledge of the mineralogy of planetary and asteroidal surfaces.

Gobekli Tepe is the oldest and one of the most important among the megalithic sites in the world. Its archaeoastronomical relevance has been recently evidenced by Collins (2013), according to whom the central pillars in four of the enclosures discovered in the site are oriented toward the setting point of the star Deneb (α Cyg), as this point moves in the course of the time, due to the equinox precession and the proper motion of the star. Taking into account these effects, Collins (2013) obtained an astronomical dating for the various enclosures which agrees rather well with the one obtained by Dietrich (2011) with the technique of carbon-14. In the present paper the careful evaluation of the effects caused by atmospheric extinction has enabled us to verify that the central pillars of the studied enclosures are in fact turned to face the setting point of Deneb, but these alignments occurred in epochs, still in agreement with the ones obtained by Dietrich (2011), but different from those proposed by Collins (2013). We have also individuated, for the first time, the probable astronomic alignments of two other enclosures at Gobekli Tepe, i.e. enclosures F and A. In particular, the first one seems to be oriented towards the rising point of the Sun on the day of the Harvest Festival, a day approximately halfway between the summer solstice and the autumn equinox. The second one, instead, shows an orientation towards the rising point of the Moon at its minor standstill. The positions of both celestial bodies have been obtained by extrapolating their declination to the date of the presumed construction reported by Dietrich (2011). A short discussion about the putative cultural motivations of these alignments is also presented.

A number of evidences, as the past presence of water, a denser atmosphere and a mild climate, suggest that on early Mars environmental conditions should have been favourable to the emergence of life. Therefore, if life actually developed on Mars a few hundreds of million years ago, or even more recently, some biomarkers may still be present. In several previous papers, based on laboratory analysis, we have investigated the modifications induced by thermal processing in the mid-infrared spectra of carbonate samples, in the form of fresh shells and fossils of different ages, skeletal remains of already complex terrestrial life, whose biotic origin is not questionable. In this way we have been able to develop a spectroscopic method for discriminating biotic samples from their abiotic counterparts. In this work we show that our method can be successfully applied also to microbialites, i.e. bio-induced carbonates deposits, and particularly to stromatolites, the laminated fabric of microbialites, some of which are among the most primitive and oldest traces of biological activity on Earth. This result is of valuable importance since such primitive living organisms can be considered good analogues for putative martian life forms.

Hematite is an iron oxide that is very important for the study of climatic evolution of Mars. It can occur in three forms: nanophase (dark purple), fine-grained (red) and coarse-grained (gray). In a previous work, we studied the influence of particle size and shape on the infrared spectra (in the wavelength range 6.25–50 μm) of submicron red hematite particles and found that bulk optical constants did not fit the spectra of very fine particles with several classes of models. In the present paper, we derive bulk optical constants of a sample of the same parent material of hematite already used in a previous work in order to determine the particulate optical constants. As a first result we find that, also in this case, bulk and particulate optical constants are different from each other. Furthermore, we show that these bulk optical constants, although derived starting from the same parent material of hematite and used with a model adopting the laboratory measured grain size distribution of the sample, cannot be used to reproduce the spectra of submicron particles. Our results can help the scientific community to appropriately model the contribution of hematite submicron grains to the martian dust for a better understanding of the geologic evolution of the planet.

Although water vapour is the main species observed in the coma of comet 67P/Churyumov–Gerasimenko1, 2 and water is the major constituent of cometary nuclei3, 4, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far5, 6. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far7, 8, 9. The nucleus of 67P/Churyumov–Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material10. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations6, 7, 8, 9. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers6, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust10 is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov–Gerasimenko11, 12 is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet’s formation.

Since long time various qualitative speculations have been proposed about the link between the three major Giza pyramids and the stars. In particular, according to a popular and controversial hypothesis (the so-called Orion Correlation Theory), a perfect coincidence would exist between the mutual positions of the three stars of the Orion Belt and those of the main Giza pyramids. In the present paper, this apparent coincidence has been subjected to some statistical verifications, in order to assess the probability that the correlation between stars and pyramids, both in relative position and in luminosity/height, can be merely due to the case. These statistical analyses have been performed by means of Monte Carlo simulations and have been coupled with previous astronomical/astrophysical tests of the presumed correlation, finding that the coincidence does not seem to be fortuitous and that it is compatible with the naked-eye astrometry and photometry of the Orion Belt stars. On the contrary, unlike what stated by another popular and controversial theory (the so-called Cygnus-Giza Correlation), we have found no coincidence between the mutual positions of the three pyramids and those of the three stars of the short arm of the asterism of Northern Cross, in the Cygnus constellation.

We perform an optical characterization of UV laser ablated silicates (olivine, pyroxene), starting from their reflectance spectra in the 0.3–2.5 μm spectral range. The goal is to provide useful tools to model space weathering effects on surfaces of asteroids and TNOs (trans-neptunian objects). We determine that the reddening and darkening spectral trend is compatible with the Hapke's space weathering model, using the optical constants of metallic iron in a silicate matrix. This result is supported by new magnetic susceptibility measurements on laser ablated orthopyroxene. We also investigate the potential contribution of formation of amorphous silicates in the process. Applying our results to silicate-rich surfaces in the Solar System, we investigate the possibility of a weathered olivine component on the surface of Centaur 5145 Pholus. Inclusion of this component slightly decreases the amount of complex organics and water ice from those previously estimated. Thus, the current Pholus spectrum is consistent with the presence of either unweathered or weathered olivine, or potentially both materials.

Because it is possible that Mars experienced an ancient climate warmer and wetter than the present one, it is reasonable to suppose that simple forms of life developed there, in the underground or at the surface, as in the terrestrial primitive biosphere. In this work we have performed infrared (IR) transmission spectroscopic analyses directed to examine the reaction to heat treatments of biotic (fresh and fossil materials) and abiotic particulate samples composed of calcium carbonate (aragonite or calcite). The aim is to check if IR spectroscopy can be a useful tool for discriminating between abiotic and biotic (fossil) carbonate samples collected on the martian surface, during in situ or sample-return missions. An important result we found is that, after thermal processing, it is possible to distinguish, by means of IR spectroscopy, abiotic aragonite from aragonite of recent biological origin. In addition we have found that the spectral behaviour of all the analysed fossil samples after heat treatment is the same as that of fresh biotic material irrespective of the original structure (whether aragonite or calcite).

We present a laboratory study of crystallisation in three annealed amorphous Mg-silicates with decreasing Mg:Si, using IR spectroscopy and X-ray diffraction. Crystallisation of high Mg:Si silicate was accompanied by the development of features in both 10 and 20 μm bands. Crystallisation was not observed for low Mg:Si, but features developed at 10 μm while the 20 μm band remained largely amorphous. At intermediate Mg:Si, amorphous structural rearrangement occurred prior to crystallisation, with features in the 10 μm band developing ahead of the 20 μm band. The spectroscopy results imply short-range ordering precedes long-range ordering, however X-ray diffraction on annealed samples showed crystallisation occurring first over long-ranges, with each silicate retaining shorter-range disorder. The discrepancy is resolved if the notion of 10 μm band features being solely attributable to macroscopic crystallinity is abandoned. We argue such features, although present in crystalline silicates, arise initially from Si–O bond ordering, which precedes the formation of bulk crystalline structure. Astronomical observations at 10 μm, in isolation, may therefore not be reliable indicators of the physical state of grains.

Laser irradiation experiments have been performed on powdered silicates (othopyroxene, clinopyroxene, and olivine) using a nanosecond pulse UV excimer laser (193 and 248 nm) to simulate the effects of space weathering induced on minor bodies of the Solar System by micrometeorite bombardment. We have used different fluences (from 0.05 to 2 J/cm 2 ) to weather the samples, experimenting below and above the ablation threshold. All the irradiated materials have shown reddening and darkening of their UV–vis-NIR reflectance spectra. In addition we have found that: (1) below ablation threshold, weathering effects increase with increasing number of laser pulses, and with increasing fluence, confirming that a thermal process is active; (2) above ablation threshold, weathering is much stronger and efficient than in the previous case, and is independent on the number of pulses. We show that astrophysical time-scales, i.e. times necessary to obtain similar effects on planetary objects, are of about 10 8 yr for both olivine and pyroxene in the case of ablation. The time grows up to 10 10 yr in the case of thermal effects. We infer that micrometeorite bombardment can be rightly simulated by laser irradiation only considering congruent laser ablation.

International audience; We present results obtained for Epinal (H5), an ordinary chondrite meteorite, irradiated with 60 keV Ar ++ ions, simulating solar wind heavy particle irradiation. Bidirectional reflectance spectra (0.3-2.67 mum) measured after irradiating Epinal samples with different ion fluences exhibit a progressive reddening that is similar to the spread of spectra observed for S-type near-Earth asteroids. The timescales for inducing the same effects in space as those obtained in laboratory are estimated to be 10-10 yr. These results suggest irradiation by heavy ions may be a very efficient weathering process in near-Earth space.

Hematite is an iron oxide very important for the study of climatic evolution of Mars. It can occur in two forms: red and grey, mainly depending on the granulometry of the samples. Spectra of bright regions of Mars suggest the presence of red hematite particles. Moreover the Thermal Emission Spectrometer (TES), on board the Mars Global Surveyor mission, has discovered a deposit of crystalline grey hematite in Sinus Meridiani. TES spectra of that Martian region exhibit features at about 18, 23 and 33 μm that are consistent with hematite. Coarse grey hematite is considered strong evidence for longstanding water, while it is unknown whether the formation of fine-grained red hematite requires abundant water. Studies are needed in order to further characterize the spectral properties of the two kinds of hematite. For this reason we have analyzed a sample of submicron hematite particles in the 6.25–50 μm range in order to study the influence of particles size and shape on the infrared spectra. The optical constants of a particulate sample have been derived and compared with published data concerning bulk samples of hematite. Our results seem to indicate that particle shape is an important factor to take into account for optical constants derivation.

Spectra for certain comets show the presence of crystalline silicate dust grains believed to have been incorporated during comet formation. While grain crystallization is widely assumed to result from the thermal annealing of precursor amorphous grains, the physical processes behind the silicate amorphous-to-crystalline transition are poorly understood. This makes it difficult to place constraints on the evolutionary histories of both grains and comets, and consequently, on the nebular conditions in which they formed. It has, therefore, become necessary to study this process in the laboratory using simulated grain materials. In this paper, we discuss recent results from laboratory investigations into a basic amorphous MgSiO3 silicate annealed in the region of 1000 K. Our object is not to model the behavior of dust grains per se, but to study the underlying process of crystallization and separate the physics of the material from the astrophysics of dust grains. In our experiments, we bring together spectroscopic measurements made in the infrared with the high resolution structural probing capabilities of synchrotron X-ray powder diffraction. The combined use of these complementary techniques provides insights into the crystallization process that would not be easily obtained if each was used in isolation. In particular, we focus on the extent to which the identification of certain spectral features attributed to crystalline phases extends to the physical structure of the grain material itself. Specifically, we have identified several key features in the way amorphous MgSiO3 behaves when annealed. Rather than crystallize directly to enstatite (MgSiO3) structures, in crystallographic terms, amorphous MgSiO3 can enter a mixed phase of crystalline forsterite (Mg2SiO4) and SiO2-rich amorphous silicate where structural evolution appears to stall. Spectroscopically, the evolution of the 10 m band does not appear to correlate directly with structural evolution, and therefore, may be a poor indicator of the degree of crystallinity. Indeed, certain features in this band may not be indicators of crystal type. However, the 20 μm band is found to be a good indicator of crystal structure. We suggest that forsterite forms from the ordering of pre-existing regions rich in SiO4 and that this phase separation is aided by a dehydrogenation processes that results in the evolutionary stall. The implications of this work regarding future observations of comets are discussed.

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ −1 ), and the broad absorption feature in the 2.9-to-3.6–micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

The messages we receive from Mars about the presence of carbonates are quite contradictory. On the one hand, images of the planet clearly show the signatures of past bodies of standing water, where the accumulation of sedimentary deposits should have occurred. On the other hand, the apparent absence of carbonates in the homogeneous fines covering the Martian surface raises the question: where could carbonates hide now? In this paper we discuss the subject and demonstrate that a proposed destruction mechanism, the photodecomposition caused by UV radiation, cannot account alone for the “missing” carbonates. By means of simulations we show that the difficulties in detecting the carbonates can be ascribed to the low temperatures of the emitting zones, observed up to now. Finally, we derive the minimum amount of such minerals that could possibly be detected in future observations of Mars, and we compare the results of simulations with observed spectra.

The absence of carbonate features in the infrared spectra of the Martian surface is a wondering issue. Mars is sending us contradictory messages in this respect. On the one hand, images of the Martian surface show structures strikingly reminding of possible bodies of standing water, which could favour the accumulation of sedimentary deposits in the past. On the other, the apparent absence of carbonates in the surprisingly homogeneous fines covering the Martian surface and suspended in the atmosphere, raises the question, where the carbonates could hide now, had they ever formed on Mars. All this makes the essence of the so-called “missing carbonates” quest. We discuss here the photolysis of carbonates by ultraviolet solar radiation, and show that the process should be effectively balanced by readsorption of the atmospheric carbon dioxide on the remaining calcium oxide surface. Hence, photolysis could not account for the failure of searches for carbonates on the Martian surface. Other solutions to the problem of “missing carbonates” should then be investigated.

The identification of carbonate deposits on Mars may be extremely useful for understanding the paleoclimate of the planet and may have very important implications on the search for extinct Martian life, owing to the high potential of carbonates to record traces of fossils. In this work, we present some results on the possible presence and detectability of coated particles produced by the photodecomposition of the outer layer of carbonate grains. Our results suggest that the photodecomposition process, if really effective in the Martian environment, should not necessarily prevent the detection of relatively large carbonate particles on the surface and in the atmospheric aerosol of Mars. The detection of carbonates is obviously more likely in particular sites of the Martian surface where geomorphological indications seem to suggest the past existence of bodies of water in which carbonate precipitation could have occurred. A list of these areas is also given.

In this paper we present the basic concept of an instrument with variable input optics, which takes into account the severe constraints imposed by space applications. Up to now such kind of device has never been proposed for any of the major space infrared instruments. In the infrared spectral region, in fact, the use of lenses can severely affect the efficiency of the system and variable optics, using only mirrors, should be the baseline option. In order to show the possible applications of this optical device we describe in some detail the implementation of two different designs of optical zoom on two infrared instruments having different conceptions and objectives, namely a Fourier non-imaging spectrometer and a grating imaging spectrometer. The use of this instrument in many space experiments could have several interesting applications.

Recent laboratory experiments, compared with observations, have strongly suggested that the amount of hydrogen, present in the grain formation regions, around C-rich stars, can play a major role in the definition of the spectral characteristics of the freshly formed carbon dust particles. Such spectral characteristics could also be modified, during the thermal processing, due to the shock waves and/or the strong radiation field, that the grains undergo after their formation. In this work we report about a series of laboratory tests, during which submicronic dust particles of amorphous carbon, condensed in an hydrogen-rich atmosphere, have been processed, by means of two different methods. Some samples have been annealed into an oven at different temperatures, while others have been exposed to strong ultraviolet radiation. Their spectra have been obtained before and after the processing in the full range from ultraviolet (200 nm) to far infrared (0.1 mm). The results of the experiment are presented and discussed, together with their astrophysical implications.

Theoretical studies and numerical models of the atmosphere and surface of Mars need the knowledge of the optical constants of candidate materials for Martian dust. Limestone, as a carbonate bearing material, is commonly considered a likely component particularly important for its links with the climate evolution and water resources on Mars. In this work we present the complex indices of refraction of submicron particles of limestone, derived from laboratory transmission spectra in the wavelength range 3.5–20 μm by means of the dispersion theory. The results are discussed and compared with the bulk optical constants of the same material already available in the literature. Since a comparison between the infrared spectrum of our dust sample of limestone (composed of more than 98% of calcium carbonate) and that of pure calcite particles shows no differences, we can consider the optical properties presented in this paper valid also for a collection of randomly oriented submicron calcite grains. The extinction and scattering efficiencies of limestone grains, calculated using our particulate optical constants, have been used as input parameters in a simple model of radiative transfer to compute synthetic spectra of the surface and the aerosol of Mars. The results clearly show that, for quantitative analyses concerning the abundance and composition of the different solid materials present on the surface and atmosphere, the physical status of the various dust components has to be taken into account. This means that any extrapolation in the use of the bulk optical constants to describe the optical behaviour of small particles and vice versa has to be checked a priori.

In upcoming years several space missions will investigate the habitability of Mars and the possibility of extinct or extant life on the planet. In previous laboratory works we have investigated the infrared spectral modifications induced by thermal processing on different carbonate samples, in the form of recent shells and fossils of different ages, whose biogenic origin is indisputable. The goal was to develop a method able to discriminate biogenic carbonate samples from their abiogenic counterparts. The method has been successfully applied to microbialites, i.e. bio-induced microcrystalline carbonate deposits, and particularly to stromatolites, the laminated fabric of microbialites, some of which can be ascribed among the oldest traces of biological activity known on Earth. In this work we show that, by applying our method to different parts of the same carbonate rock, we are able to discriminate the presence, nature and biogenicity of various micrite types (i.e. detrital vs autochthonous) and to distinguish them from the skeletal grains. To test our methodology we preliminarily used the epifluorescence technique to select on polished samples, skeletal grains, autochthonous and allochthonous micrites, each one characterized by different organic matter content. The results on the various components show that, applying the infrared spectral modifications induced by thermal processing, it is possible to determine the degree of biogenicity of the different carbonate samples. The results are of valuable importance since such carbonates are linked to primitive living organisms that can be considered as good analogues for putative Martian life forms.

Currently developed studies of radiance of the surface and the atmosphere of Mars are presented. In our modelling we have taken into consideration slant path geometry, various states of the atmosphere and the surface of Mars. Preliminary laboratory measurements of spectral emissivity are used in calculations in order to account for real variations as a function of wavelength. The work presented here is directly connected with the measurements to be carried out by the Infrared Planetary Fourier Spectrometer (PFS) during the Martian mission planned to be launched in November 1996. This instrument works in the wavelength range from 1.2 to 45 μm with a resolution of 2 cm −1 and is optimised for atmospheric and surface studies. At present we limit our simulation to the spectral range from 5 to 13 μm, where some of the most relevant surface features are present.

We present here infrared laboratory spectra of two particulate materials, calcite and palagonite, commonly considered as possible components of martian dust. The extinction properties have been measured both for the single materials and for mixtures of the two components as a function of the size range. We find that, while our experimental results for palagonite are in agreement with the extinction data calculated using the optical constants available in literature, for calcite this is not the case. Such behaviour is discussed in terms of the different nature, particulate or bulk, of the sample material. As far as mixtures are concerned, our results seem to suggest that it may be possible to detect calcite if its mass percentage is at least a few percent.

The presence of carbonaceous particles, both in circumstellar regions and the interstellar medium, now seems quite well accepted. The exact nature of the carbon grains, however, is still controversial and the subject of a wide and live debate from a theoretical as well as an experimental point of view. Recently in a series of works we have shown that the carbon grains produced and studied in our laboratory may be good candidates as laboratory dust analogs. In particular, by producing and thermally processing fully hydrogenated amorphous carbon grains, we have shown that the heat treatment of the particles can be a reasonable simulation of the grain processing active in space as modeled by other authors. The heat treatment decreases the hydrogen content of the grains which, for this reason, change their internal structure and their optical properties in the 190–260 nm range. An ultraviolet extinction bump, absent in the fully hydrogenated carbon grains, appears and becomes more and more pronounced as the annealing temperature increases, while at the same time the position of the peak shifts toward longer wavelengths. In this work, we show that similar results can be obtained if carbon grains are produced by direct condensation of the carbon vapors in partially hydrogenated atmospheres. By changing the contribution of the hydrogen gas to the total pressure inside the condensation chamber, the resulting grains exhibit an extinction peak whose wavelength position shifts toward longer wavelengths as the hydrogen abundance decreases. The results presented in this paper may explain the spectral differences in the ultraviolet extinction observed toward some circumstellar regions without invoking any kind of dust evolution and, at the same time, confirm the importance of the hydrogen abundance during the dust formation processes. This means that our findings may have important consequences for the nucleation/condensation theories and evolution of dust grains as well as for the study of dust formation regions.

Synthetic amorphous silicate dust samples were produced with various metal to silicon and iron to magnesium ratios to serve as analogues to cosmic dust. The IR spectra of the dusts were measured in the 10 and 20 μ m bands to determine the effect of composition on these characteristic features. Silicate dusts were produced with only Mg and Fe cations, and also with Mg, Fe. Na, Ca and Al cations in their relative cosmic ratio to study the spectral effects of varying silicate composition. The spectral results are compatible with the condensation of cosmic silicate grains as amorphous grains containing the major metals in their relative cosmic proportions. i.e. without chemical fractionation.

Searching for traces of extinct and/or extant life on Mars is one of the major objectives for remote-sensing and in situ exploration of the planet. In previous laboratory works we have investigated the infrared spectral modifications induced by thermal processing on different carbonate samples, in the form of fresh shells and fossils of different ages, whose biotic origin is easily recognizable. The goal was to discriminate them from their abiotic counterparts. In general, it is difficult to identify biotic signatures, especially when the organisms inducing the carbonate precipitation have low fossilization potential (i.e. microbes, bacteria, archaea). A wide variety of microorganisms are implicated in carbonate genesis, and their direct characterization is very difficult to evaluate by traditional methods, both in ancient sedimentary systems and even in recent environments. In the present work we apply our analysis to problematic carbonate samples, in which there is no clear evidence of controlled or induced biomineralization. This analysis indicates a very likely biotic origin of the aragonite samples under study, in agreement with the conclusion previously reported by Guido et al. (2007) who followed a completely different approach based on a complex set of sedimentary, petrographic, geochemical and biochemical analyses. We show that our method is reliable for discriminating between biotic and abiotic carbonates, and therefore it is a powerful tool in the search for life on Mars in the next generation of space missions to the planet.

The Visible, InfraRed, and Thermal Imaging Spectrometer (VIRTIS) on Rosetta obtained hyperspectral images, spectral reflectance maps, and temperature maps of the asteroid 21 Lutetia. No absorption features, of either silicates or hydrated minerals, have been detected across the observed area in the spectral range from 0.4 to 3.5 micrometers. The surface temperature reaches a maximum value of 245 kelvin and correlates well with topographic features. The thermal inertia is in the range from 20 to 30 joules meter(-2) kelvin(-1) second(-0.5), comparable to a lunarlike powdery regolith. Spectral signatures of surface alteration, resulting from space weathering, seem to be missing. Lutetia is likely a remnant of the primordial planetesimal population, unaltered by differentiation processes and composed of chondritic materials of enstatitic or carbonaceous origin, dominated by iron-poor minerals that have not suffered aqueous alteration.

Using theoretical models and our laboratory data on different dust mixtures containing silicon carbide grains, we obtained the best fit of several infrared emission spectra of carbon stars and comets. The results suggest that silicon carbide formed in the envelopes around carbon stars and also present in our solar system may provide an important link between interstellar and interplanetary solid material.

By using a detailed radiative transfer model, we show that the observed far-infrared spectrum of the Bok globule Barnard 335 can be explained by dust emission whose only energy source is the interstellar radiation field. The anomalous excess radiation observed around 60 μm in B335 and other globules can be accounted for by a «non-conventional» dust component, the very small grains already proposed for other celestial objects

[1] In this work we report the main results of a laboratory test program aimed to check the spectral behavior of several mixtures of particulate samples. The chosen spectral interval is from 2.5 to 25.0 μm since it is the range where many minerals have their characteristic features and it is important for the interpretation of remote sensing spectra of planetary surfaces. The present study uses directional hemispherical reflectance spectra of three particulate minerals (dolomite, olivine, and quartz), with different grain sizes (ranging from 1 to 300 μm), and of their mixtures. It was found that, in the spectral range between 2.5 μm and the Christiansen feature, the reflectance of the mixtures is a nonlinear combination of the reflectance of the single pure materials, in agreement with the results found at shorter wavelength and reported in the literature. Our experimental results also confirm that, for longer wavelength, the reflectance of the mixtures is a linear combination of the reflectance of the single pure minerals. In this work, after checking some important simplifying hypotheses, we show that the mixture spectra cannot be always accurately reproduced using the volume percentage of each component, but it has a complicate behavior requiring a careful evaluation of all the parameters involved in the process. In particular we have determined experimentally the penetration depth of the radiation at different wavelengths in some of the mixtures. In this way we have been able to address the important issue of the influence of surface composition on the observed spectra.

International audience; The Rosetta spacecraft accomplished a flyby of Mars on its way to 67P/Churyumov-Gerasimenko on 25 February 2007. In this paper we describe the measurements obtained by the M channel of the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS-M) and the first scientific results derived from their analysis. The broad spectral coverage of the VIRTIS-M in the IR permitted the study of various phenomena occurring in the Martian atmosphere; observations were further exploited to achieve accurate absolute radiometric calibration. Nighttime data from the VIRTIS-M constrain the air temperature profile in the lower atmosphere (5-30 km), using variations in CO2 opacity at 4.3 mum. A comparison of this data with the global circulation model (GCM) by Forget et al. (1999) shows a trend of slightly higher air temperature in the VIRTIS-M retrievals; this is accompanied by the presence of moderate decreases (˜5 K) in large sections of the equatorial region. This is potentially related to the occurrence of water ice clouds. Daytime data from the VIRTIS-M reveal CO2 non-local thermodynamic equilibrium emission in the high atmosphere. A mapping of emission intensity confirms its strict dependence on solar zenith angle. Additionally, devoted limb observations allowed the retrieval of vertical emission intensity profiles, indicating a peak around 105 km in southern tropical regions. Ozone content can be effectively monitored by the emission of O2 (a1Deltag) at 1.27 mum. Retrieved emission intensity shows that polar regions are particularly rich in ozone. Aerosol scattering was observed in the 1-2.5 mum region above the night region above the night disk, suggesting the occurrence of very high noctilucent clouds.

The search for biosignatures in the soil of Mars is a major objective of the planet exploration, but the detection of such structures is not straightforward due to the degradation of the organic material. In a previous work our group has analyzed the spectral reaction to thermal processing of biomineral carbonate samples including fresh and fossil shells. We found that two terrestrial fossil shells collected from clay deposits preserve their biomineral characteristics much better than coeval fossils not embedded in clay layers. In the present paper we extend our analysis to a set of fossils found in three different terrestrial clay deposits. Our results confirm that the level of degradation may be much lower than the standard values if fossils are surrounded by clay minerals. As a result these fossils have a spectroscopic response to thermal treatment which make them much more distinguishable from their abiotic counterparts than coeval fossils not collected from clay deposits. This result implies that the phyllosilicates regions recently discovered on Mars may represent very interesting environments that can provide conditions favorable to preserve evidence of biomarkers, and hence can be regarded as good candidate locations for their detection.

The mass absorption coefficients of some glassy and crystalline silicates of astrophysical relevance have been measured. We produced and characterized also synthetic silicates of olivine type since this material is considered one of the most likely components of cometary dust grains. The experimental data are used to fit the emission feature observed in the Comet Halley spectrum between 8 and 13 mum. Satisfactory results are obtained by using synthetic olivine mixed with a small amount (5%) of crystalline grains.

Carbonate deposits have not been found so far on Mars, although there appears to have been sufficient water to have supported their formation. Many hypotheses have been proposed in order to explain this. In the present work we explore the possibility that the missed detection of carbonate deposits on the martian surface could be simply due to the fact that the concentration of carbonates, when mixed with other materials present in the sedimentary deposits, may be below the detection limit of the various instruments used so far in this search. In the present study we consider 21 putative paleolacustrine basins and use a sediment transport model to estimate the abundance of carbonates which could be present in the sediments deposited on the basin floor. In this way we find that for all the selected basins the estimated carbonate abundances are in general less than a few percent, and such values are below (or at best comparable to) the detection limits of the spectrometers flown around Mars during the recent space missions. Furthermore, applying the sediment transport model to the well studied Eberswalde crater, we conclude that the fluvio-lacustrine activity in this basin should have lasted for a period on the order of 10 3 –10 4 years, in good agreement with previous work. Our results suggest that a hydrological cycle, able to move large volumes of water and to create relatively stable lakes, could have been active intermittently on Mars in the past, producing carbonate deposits that could escape detection by the instruments that have flown to date.

Searching for traces of extinct and/or extant life on the surface of Mars is one of the major objectives for remote-sensing and in-situ exploration of the planet. In the present paper we study the infrared (IR) spectral modifications induced by thermal processing on differently preserved calcium carbonate fossils, in order to discriminate them from their abiotic counterparts. The main conclusion of this study is that the degree of alteration of the fossils, derived from IR spectral analysis, seems to be well correlated with the sample age, and that terrestrial fossils after a billion years are so altered that it becomes impossible to trace their biotic origin. Since it is reasonable to assume that the putative Martian fossils should be at least 3.5 billion years old, this would imply that our spectroscopic method could not be able to detect them, if their degradation rate were the same as that we have found in usual conditions for the terrestrial fossils. However, due to the different climate evolution of the two planets, there is the possibility of having two different degradation rates, much lower for Mars than for Earth, especially if the fossils are embedded in a protective layer, such as a clay deposit. In this case IR spectroscopy, coupled with thermal processing, can be a useful tool for discriminating between abiotic and biotic (fossil) carbonate samples collected on the Martian surface.

[1] Modern planetary scientific experiments are providing a huge amount of information that requires extensive analysis and evaluation in order to understand its scientific content. In this work we evaluate the ability of a previously developed and evaluated technique to explore a large data set in order to identify the cluster representatives of the spectral variability of a selected Martian region and obtain a reduction of the original data volume ∼2 × 104. We also demonstrate that the cluster centroids are able to quantitatively reproduce previously interpreted atmospheric, topographic, and compositional properties. The results build additional confidence in the application of the statistical techniques.