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The sublimation of ice covered by dust is a common way of outgassing of comets. The rate of this process depends on the thermal conductivity and permeability of dust, as well as the properties of the ice itself. It can be calculated using the Hertz-Knudsen equation, corrected for temperature dependent sublimation coefficient αs(T) (Gundlach et al., 2011; Kossacki et al., 1999; Kossacki & Leliwa-Kopystynski, 2014; Kossacki et al., 2017) and the Knudsen equation for the permeability of dust. In this work we present direct experimental verification of this set of equations using sand with different granulation: coarse (grain radii rg = 0.5 − 1 mm), medium (rg = 0.25 − 0.5 mm), fine (rg = 0.125 = 0.25 mm) and unsorted (rg = 0 − 1 mm, rg, av = 0.5 mm). We have found that at T

Electron irradiation of ice is an important process in the Solar System, especially for icy satellites (e.g., Europa) whose tenuous atmospheres originate from the thermal and radiation-induced outgassing of their icy surfaces. Laboratory experiments have been performed on the electron irradiation of water ice to study the volatile gases (e.g., H2 and O2) that leave the surface and the chemical compounds (e.g., H2O2) that are produced inside the ice. Semi-empirical models have also been developed to estimate the production of the chemical species in the ice during irradiation as functions of electron energy and ice temperature. In this study, we build a chemistry-transport model to simulate chemical processes occurring in the ice during irradiation by 10 keV electrons and to describe how the chemical species of interest are formed, transported, and distributed in the ice. The simulated H2O2 mixing ratio agrees well with experiments performed by Hand and Carlson (2011). Our model can be applied to the surfaces of icy satellites (e.g., Europa) to estimate the oxidant generation in the ice and to evaluate the potential habitability of those satellites. This study calls for further experimental studies on ice under electron irradiation to constrain critical parameters such as rate coefficients and diffusion coefficients of volatile species produced by radiation in the ice.

Chemical analysis of lunar soil samples often involves thermal processing to extract their volatile constituents, such as loosely adsorbed water. For the characterization of volatiles and their bonding mechanisms it is important to determine their desorption temperature. However, due to the low thermal diffusivity of lunar regolith, it might be difficult to reach a uniform heat distribution in a sample that is larger than only a few particles. Furthermore, the mass transport through such a sample is restricted, which might lead to a significant delay between actual desorption and measurable outgassing of volatiles from the sample. The entire volatiles extraction process depends on the dynamically changing heat and mass transfer within the sample, and is influenced by physical parameters such as porosity, tortuosity, gas density, temperature and pressure. To correctly interpret measurements of the extracted volatiles, it is important to understand the interaction between heat transfer, sorption, and gas transfer through the sample. The present paper discusses the molecular kinetics and mechanisms that are involved in the thermal extraction process and presents a combined parametrical computation model to simulate this process. The influence of water content on the gas diffusivity and thermal diffusivity is discussed and the issue of possible resorption of desorbed molecules within the sample is addressed. Based on the multi-physical computation model, a case study for the ProSPA instrument for in situ analysis of lunar volatiles is presented, which predicts relevant dynamic process parameters, such as gas pressure and process duration.

The atmosphere of Jupiter's volcanic moon Io consists of mainly sulfur dioxide (SO2), and this main constituent has been studied with a variety of observing techniques across many wavelengths over the years. Here we study absorption by SO2 at the hydrogen Ly-α line (1216 A) in a large set of images taken by the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST) between 1997 and 2018. An advanced statistical analysis using a Monte-Carlo trial method is applied to derive the SO2 column density from the Ly-α intensity, which includes the uncertainties of the used variables such as solar and background flux. Our analysis produces a probability distribution function of the SO2 column density and highlights some short-comings of the observing technique. Most importantly, the HST/STIS images of the surface-reflected Ly-α flux are only sensitive to SO2 column densities between ~1015 cm−2 and ~5×1016 cm−2. Due to strong non-linearity in the relationship between the SO2 abundance and the Ly-α flux at the low and high values of detected flux, SO2 abundance directly retrieved from the STIS images will generally fall within these boundaries. This explains the relatively low equatorial column density of about 1016 cm−2 reported by previous studies using the Ly-α images (e.g., Feldman et al., 2000; Feaga et al., 2009) compared to other studies (e.g., Spencer et al., 2005; Tsang et al., 2013; Jessup and Spencer, 2015; Lellouch et al., 2015), where the obtained column density is often 1017 cm−2. By assuming a log-normal probability distribution function for the column density, a new estimate of the SO2 column density is then fitted, indirectly accounting for abundances beyond the detectability limits. This method suggests slightly higher equatorial SO2 abundances and much larger upper-limits, revealing that the Ly-α observations are in fact consistent with the higher abundances found in other studies. We then investigate the SO2 abundances at three volcanic sites (Loki, Marduk, Thor), where plumes were observed before and where the sensitivity in our images is comparably high. The observations did not reveal transient changes due to local outgassing at any of the three sites. Finally, the heliocentric distance of Io changed from 4.95 AU to 5.45 AU between the observation dates, potentially allowing us to investigate the influence of solar intensity changes on the SO2 column density via surface frost sublimation. However, the derived error bars are significantly larger than the derived variability, preventing any firm conclusion on seasonal changes and local volcanic outgassing.

During accretion, the young rocky planets are so hot that they become endowed with a magma ocean. From that moment, the mantle convective thermal flux control the cooling of the planet and an atmosphere is created by outgassing. This atmosphere will then play a key role during this cooling phase. Studying this cooling phase in details is a necessary step to explain the great diversity of the observed telluric planets and especially to assess the presence of surface liquid water. We used here a radiative-convective 1D atmospheric model (H2O, CO2) to study the impact of the Bond albedo on the evolution of magma ocean planets. We derived from this model the thermal emission spectrum and the spectral reflectance of these planets, from which we calculated their Bond albedos. Compared to Marcq et al. (2017), the model now includes a new module to compute the Rayleigh scattering, and state of the art CO2 and H2O gaseous opacities data in the visible and infrared spectral ranges. We show that the Bond albedo of these planets depends on their surface temperature and results from a competition between Rayleigh scattering from the gases and Mie scattering from the clouds. The colder the surface temperature is, the thicker the clouds are, and the higher the Bond albedo is. We also evidence that the relative abundances of CO2 and H2O in the atmosphere strongly impact the Bond albedo. The Bond albedo is higher for atmospheres dominated by the CO2, better Rayleigh scatterer than H2O. Finally, we provide the community with an empirical formula for the Bond albedo that could be useful for future studies of magma ocean planets., 16 pages, 10 figures, 5 tables. Accepted for publication in the Journal Icarus the 27 August 2018

Because of observational biases and outgassing, comets are notoriously challenging in terms of trajectory estimation and ephemeris predictions. We used tracking data collected by ESA's Rosetta mission at 67P/Churyumov-Gerasimenko to derive 10-m level constraints on the distance between the Earth and 67P in 2014 and 2016, which, together with high-quality ground-based astrometry, provide unprecedented information on a comet's trajectory. Based on these data, we reconstruct the trajectory of 67P from 2012 to 2018 by modeling nongravitational perturbations with a rotating jet model, informed by measured physical properties of the comet. Using this trajectory reconstruction as reference, we characterize the astrometric error of publicly available optical observations of 67P and show how their quality significantly degrades with brightness and proximity to the Sun. Moreover, we assessed the accuracy of predictions made with different nongravitational perturbation models and data treatments. While each comet is unique and it is difficult to provide a general recipe, the reliability of an ephemeris can benefit from the availability of high-quality data with quantified uncertainties and can be gauged by comparing different ephemeris solutions and tracking a solution's stability and predictive performance as new observations are added to the fit.

Io’s atmosphere is predominately SO 2 that is sustained by a combination of volcanic outgassing and sublimation. The loss from the atmosphere is the main mass source for Jupiter’s large magnetosphere. Numerous previous studies attributed various transient phenomena in Io’s environment and Jupiter’s magnetosphere to a sudden change in the mass loss from the atmosphere supposedly triggered by a change in volcanic activity. Since the gas in volcanic plumes does not escape directly, such causal correlation would require a transient volcano-induced change in atmospheric abundance, which has never been observed so far. Here we report four observations of atmospheric SO 2 and NaCl from the same hemisphere of Io, obtained with the IRAM NOEMA interferometer on 11 December 2016, 14 March, 6 and 29 April 2017. These observations are compared to measurements of volcanic hot spots and Io’s neutral and plasma environment. We find a stable NaCl column density in Io’s atmosphere on the four dates. The SO 2 column density derived for December 2016 is about 30% lower compared to the SO 2 column density found in the period of March to April 2017. This increase in SO 2 from December 2016 to March 2017 might be related to increasing volcanic activity observed at several sites in spring 2017, but the stability of the volcanic trace gas NaCl and resulting decrease in NaCl/SO 2 ratio do not support this interpretation. Observed dimmings in both the sulfur ion torus and Na neutral cloud suggest rather a decrease in mass loading in the period of increasing SO 2 abundance. The dimming Na brightness and stable atmospheric NaCl furthermore dispute an earlier suggested positive correlation of the sodium cloud and the hot spot activity at Loki Patara, which considerably increased in this period. The environment of Io overall appears to be in a rather quiescent state, preventing further conclusions. Only Jupiter’s aurora morphology underwent several short-term changes, which are apparently unrelated to Io’s quiescent environment or the relatively stable atmosphere.

Recent measurements of the present-day Ar abundance and isotopic ratios in the martian atmosphere by the SAM instrument suite onboard the Curiosity rover can be used to constrain the atmospheric and volatile evolution. We have examined the role of volcanic outgassing, escape to space via sputtering, crustal erosion, impact delivery, and impact erosion in reproducing the Ar isotope ratios from an initial state 4.4 billion years ago. To investigate the effects of each of these processes, their timing, and their intensity we have modeled exchanges of Ar isotopes between various reservoirs (mantle, crust, atmosphere, etc.) throughout Mars’ history. Furthermore, we use present-day atmospheric measurements to determine the parameter space consistent with observations. We find that significant loss to space (at least 48% of atmospheric 36 Ar) is required to match the observed 36 Ar/ 38 Ar ratio. Our estimates of volcanic outgassing do not supply sufficient 40 Ar to the atmosphere to match observations, so in our model at least 31% of 40 Ar produced in the crust must have also been released to the atmosphere. Of the total 40 Ar introduced into the atmosphere about 25% must have been lost to space. By adding the present-day isotopic abundances with our results of total integrated Ar loss we find a “restored” value of atmospheric 40 Ar/ 36 Ar, which represents what that ratio would be if the total integrated Ar loss had remained in the atmosphere. We determine the restored value to be ∼900–1500. This is below the present martian atmospheric value (1900 ± 300), but 3–5 times greater than the terrestrial value.

Explaining the evidence for surface liquid water on early Mars has been a challenge for climate modelers, as the sun was ∼ 30% less luminous during the late-Noachian. We propose that the additional greenhouse forcing of C O 2 - H 2 collision-induced absorption is capable of bringing the surface temperature above freezing and can put early Mars into a limit-cycling regime. Limit cycles occur when insolation is low and C O 2 outgassing rates are unable to balance with the rapid drawdown of C O 2 during warm weathering periods. Planets in this regime will alternate between global glaciation and transient warm climate phases. This mechanism is capable of explaining the geomorphological evidence for transient warm periods in the martian record. Previous work has shown that collision-induced absorption of C O 2 - H 2 was capable of deglaciating early Mars, but only with high H 2 outgassing rates (greater than ∼ 600 Tmol/yr) and at high surface pressures (between 3 to 4 bars). We used new theoretically derived collision-induced absorption coefficients for C O 2 - H 2 to reevaluate the climate limit cycling hypothesis for early Mars. Using the new and stronger absorption coefficients in our 1-dimensional radiative convective model as well as our energy balance model, we find that limit cycling can occur with an H 2 outgassing rate as low as ∼ 300 Tmol/yr at surface pressures below 3 bars. Our results agree more closely with paleoparameters for early martian surface pressure and hydrogen abundance.

A recent study by Ramirez et al. (2014) demonstrated that an atmosphere with 1.3-4 bar of CO2 and H2O, in addition to 5-20% H2, could have raised the mean annual and global surface temperature of early Mars above the freezing point of water. Such warm temperatures appear necessary to generate the rainfall (or snowfall) amounts required to carve the ancient martian valleys. Here, we use our best estimates for early martian outgassing rates, along with a 1-D photochemical model, to assess the conversion efficiency of CO, CH4, and H2S to CO2, SO2, and H2. Our outgassing estimates assume that Mars was actively recycling volatiles between its crust and interior, as Earth does today. H2 production from serpentinization and deposition of banded iron-formations is also considered. Under these assumptions, maintaining an H2 concentration of ~1-2% by volume is achievable, but reaching 5% H2 requires additional H2 sources or a slowing of the hydrogen escape rate below the diffusion limit. If the early martian atmosphere was indeed H2-rich, we might be able to see evidence of this in the rock record. The hypothesis proposed here is consistent with new data from the Curiosity Rover, which show evidence for a long-lived lake in Gale Crater near Mt. Sharp. It is also consistent with measured oxygen fugacities of martian meteorites, which show evidence for progressive mantle oxidation over time., Comment: 23 pages, 9 figures, published Icarus

The mass and composition of Titan’s massive atmosphere, which is dominated by N2 and CH4 at present, have probably varied all along its history owing to a combination of exogenous and endogenous processes. In the present study, we investigate its fate during the Late Heavy Bombardment (LHB) by modeling the competitive loss and supply of volatiles by cometary impacts and their consequences on the atmospheric balance. For surface albedos ranging between 0.1 and 0.7, we examine the emergence of an atmosphere during the LHB as well as the evolution of a primitive atmosphere with various masses and compositions prior to this event, accounting for impact-induced crustal NH3–N2 conversion and subsequent outgassing as well as impact-induced atmospheric erosion. By considering an impactor population characteristic of the LHB, we show that the generation of a N2-rich atmosphere with a mass equivalent to the present-day one requires ammonia mass fraction of 2–5%, depending on surface albedos, in an icy layer of at least 50 km below the surface, implying an undifferentiated interior at the time of LHB. Except for high surface albedos ( A S ⩾ 0.7 ) where most of the released N2 remain frozen at the surface, our calculations indicate that the high-velocity impacts led to a strong atmospheric erosion. For a differentiated Titan with a thin ammonia-enriched crust (⩽5 km) and A S 0.6 , any atmosphere preexisting before the LHB should be more than 5 times more massive than at present, in order to sustain an atmosphere equivalent to the present-day one. This implies that either a massive atmosphere was formed on Titan during its accretion or that the nitrogen-rich atmosphere was generated after the LHB.

We conducted a qualitative study to simulate the flux of volatile gases expected to occur at the lunar surface due to cometary impact or lunar outgassing events. A small sample cell containing 8.8 g of JSC-1A lunar soil simulant in a vacuum system with a base pressure of 1.5 × 10−8 Torr was exposed to various gases using dynamic pressure dosing at room temperature to observe any retention of those gases as a function of the exposure times, temperatures and pressures used. Gases included pure argon, a five-component gas mixture (H2, He, Ne, N2, Ar), a simulated Mars atmospheric mixture (CO2, N2, Ar, CO, O2), and a simulated Titan mixture (N2, CH4). Results at exposure pressures of approximately 1.5 × 10−8 Torr above background showed no observable retention of rare gases, slight retention of molecular gases, but surface retention of the triatomic gas CO2 occurred at room temperature with a time to reach equilibrium of greater than 10 min, which was an unanticipated result. Despite several bakeouts and months under ultrahigh vacuum (UHV) conditions, trace levels of atmospheric gases continued to evolve from the simulant. Mechanical and optical probing of the simulant surface increased this latent gas evolution, particularly for CO2 and CO, with some evidence also for the release of CH4. We assert our results are, by analogy, applicable to protocols and instrumentation needed for conducting analytical chemistry aboard future landed lunar missions.

We surveyed the Tharsis and Syrtis volcanic regions on Mars during 23 November 2011 to 13 May 2012 which corresponded to its mid Northern Spring and early Northern Summer seasons (Ls = 34–110°). Strong submillimeter rotational transitions of sulfur dioxide (SO2), sulfur monoxide (SO) and hydrogen sulfide (H2S) were targeted. No active release was detected, and we infer 2 σ upper limits across the disk of the planet of 1.1 ppb, 0.7 ppb and 1.3 ppb for SO2, SO and H2S, respectively. Our derived upper limit for SO2 is comparable to previously reported limits, whereas for H2S we set a more stringent upper limit than previously measured, and we establish a limit for SO. Among the targeted molecules, SO2 is the strongest indicator for volcanic outgassing. Assuming a photochemical lifetime of 2 years for SO2, our upper limit of 1.1 ppb implies an outgassing rate less than 55 metric tons/day. This rate limits the daily amount of degassing magma to less than 12,000 m 3 . Our sensitivity is sufficient to detect a volcanic release on Mars that is 4 % the SO2 released continuously from Kilauea volcano in Hawaii or 5 % that of the Masaya volcano in Nicaragua. The non-detection of the sulfur compounds in the atmosphere of Mars indicates the absence of major volcanic outgassing.

We present initial data from our campaign to monitor Io in the near-infrared, beginning in August 2013, using 3.8-μm adaptive optics imaging at Gemini N and 2–5 μm disk-integrated spectroscopy at NASA’s IRTF. Conducted during 2013–2014, these observations are coincident with the ISAS/JAXA EXCEED mission’s continuous monitoring of the Io plasma torus and will enable the speculated effects of volcanic outgassing on the torus to be observed directly, in addition to enabling an assessment of the frequency and properties of large-scale outbursts. On 29 August 2013 we detected a powerful eruption (designated 201308C) on Io at 223.5 ± 2.6°W, 29.1 ± 1.8°N. Emitting between 15 and >25 TW, this event is one of the most powerful eruptions ever seen on Io and falls into the rare “outburst” class. This was the third eruption of this type seen on Io in August 2013, an unprecedented occurrence. Also unprecedented was the charting of the decay in thermal emission over the subsequent days and weeks. Modeling of the outburst spectrum places a lower bound of 1200–1300 K on the eruption temperature, and is suggestive of temperatures 1900 K or higher, typically associated with ultramafic lava composition. The eruption is likely a highly energetic, high-volume lava fountain event.

Comets are expected to be the most primitive objects in the Solar System. The chemical composition of these objects is frequently assumed to be directly provided by the observations of the abundances of volatile molecules in the coma. The present work aims to determine the relationship between the chemical composition of the coma, the outgassing profile of volatile molecules and the internal chemical composition, and water ice structure of the nucleus, and physical assumptions on comets. To do this, we have developed a quasi 3D model of a cometary nucleus which takes into account all phase changes and water ice structures (amorphous, crystalline, clathrate, and a mixture of them); we have applied this model to the Comet 67P/Churyumov-Gerasimenko, the target of the Rosetta mission. We find that the outgassing profile of volatile molecules is a strong indicator of the physical and thermal properties (water ice structure, thermal inertia, abundances, distribution, physical differentiation) of the solid nucleus. Day/night variations of the rate of production of species helps to distinguish the clathrate structure from other water ice structures in nuclei, implying different thermodynamic conditions of cometary ice formation in the protoplanetary disc. The relative abundance (to H 2 O) of volatile molecules released from the nucleus interior varies by some orders of magnitude as a function of the distance to the Sun, the volatility of species, their abundance and distribution between the ”trapped” and ”condensed” states, the structure of water ice, and the thermal inertia and other physical assumptions (dust mantle, …) on the nucleus. For the less volatile molecules such as CO 2 and H 2 S, the relative (to H 2 O) abundance of species in coma remain similar to the primitive composition of the nucleus (relative deviation less than 25%) only around the perihelion passage (in the range −3 to −2 to +2–3 AU), whatever is the water ice structure and chemical composition, and under the conditions that the nucleus is not fully covered by a dust mantle. The relative (to H 2 O) abundance of highly volatile molecules such as CO and CH 4 in the coma remain approximately equal to the primitive nucleus composition only for nuclei made of clathrates. The nucleus releases systematically lower relative abundances of highly volatile species (up to one order of magnitude) around perihelion (in the range −3 to −2 to +2–3 AU) in the cases of the crystalline and amorphous water ice structures in the nuclei. The rate of production, the outgassing profile and the relative abundances (to H 2 O) of volatile molecules are the key parameters allowing one to retrieve the chemical composition and thermodynamic conditions of cometary ice formation in the early Solar System. The coming observations of the coma and nucleus by the Rosetta mission instruments (VIRTIS, MIRO, …) should provide the necessary constraints to the model to allow it to infer the primordial ice structure and composition of the comet.

Jupiter Trojans appear to be a key population of small bodies to study and test the models of the Solar System formation and evolution. Because understanding the evolution of Trojans can bring strong and unique constraints on the origins of our planetary system, a significant observational effort has been undertaken to unveil their physical characteristics. The data gathered so far are consistent with Trojans having volatile-rich interiors (possibly water ice) and volatile-poor surfaces (fine grained silicates). Since water ice is not thermodynamically stable against sublimation at the surface of an object located at ∼5 AU, such layering seems consistent with past outgassing. In this work, we study the thermal history of Trojans after the formation of a dust mantle by possible past outgassing, so as to constrain the depth at which water ice could be stable. We find that it could have survived 100 m below the surface, even if Trojans orbited close to the Sun for ∼10,000 years, as suggested by the most recent dynamical models. Water ice should be found ∼10 m below the surface in most cases, and below 10 cm in the polar regions in some cases.

The near nucleus coma of Comet 9P/Tempel 1 has been simulated with the 3D Direct Simulation Monte Carlo (DSMC) code PDSC++ (Su C. C. [2013]. Parallel Direct Simulation Monte Carlo (DSMC) Methods for Modeling Rarefied Gas Dynamics. PhD Thesis National Chiao Tung University Taiwan) and the derived column densities have been compared to observations of the water vapour distribution found by using infrared imaging spectrometer on the Deep Impact spacecraft (Feaga L.M. A'Hearn M.F. Sunshine J.M. Groussin O. Farnham T.L. [2007]. Icarus 191(2) 134 145. http://dx.doi.org/10.1016/j.icarus.2007.04.038). Modelled total production rates are also compared to various observations made at the time of the Deep Impact encounter. Three different models were tested. For all models the shape model constructed from the Deep Impact observations by Thomas et al. (Thomas P.C. Veverka J. Belton M.J.S. Hidy A. A'Hearn M.F. Farnham T.L. et al. [2007]. Icarus 187(1) 4 15. http://dx.doi.org/10.1016/j.icarus.2006.12.013) was used. Outgassing depending only on the cosine of the solar insolation angle on each shape model facet is shown to provide an unsatisfactory model. Models constructed on the basis of active areas suggested by Kossacki and Szutowicz (Kossacki K Szutowicz S. [2008]. Icarus 195(2) 705 724. http://dx.doi.org/ 10.1016/j.icarus.2007.12.014) are shown to be superior. The Kossacki and Szutowicz model however also shows deficits which we have sought to improve upon. For the best model we investigate the properties of the outflow. (C) 2014 Elsevier Inc. All rights reserved.

The sublimation rate of ice is commonly calculated using simple Hertz–Knudsen formula. This formula is derived from the kinetic theory of gases and ignores microphysical processes determining the actual sublimation rate. The microphysical processes can be accounted for by including in the Herz–Knudsen equation a temperature dependent sublimation coefficient (Kossacki, K.J., Markiewicz, W.J., Skorov, Y., Koemle, N.I. [1999]. Planet. Space Sci. 47, 1521–1530; Gundlach, B., Skorov, Y.V., Blum, J. [2011]. Icarus, 213, 710–719). Here we address the question to what extent inaccuracy of the simple Hertz–Knudsen equation affects the calculated temperature of a cometary nucleus and the emission rate of water vapor to space. We performed numerical simulations dealing with evolution of a model comet of the orbit the same as Comet 67P/Churyumov-Gerasimenko, target comet of the Rosetta mission (Glassmeier, K.H., Boehnardt, H., Koshny, D., Kuhrt, E., Richter, I. [2007]. Space Sci. Rev. 128, 1–21). We have found, that the temperature below dust mantle is most sensitive to the value of the sublimation coefficient when the mantle is coarse grained, while the sublimation rate is most affected when the mantle is fine grained. We also conclude that derivation of the temperature below the mantle from the measured water production rate ignoring temperature dependence of the sublimation coefficient leads to an underestimate of the temperature by more than 10 K when the nucleus is fine grained.

The model is intended to respond to the recent findings in the Venus atmosphere from the Venus Express and ground-based submillimeter and infrared observations. It extends down to 47 km for comparison with the kinetic model for the lower atmosphere (Krasnopolsky, V.A. [2007]. Icarus 191, 25–37) and to use its results as the boundary conditions. The model numerical accuracy is significantly improved by reduction of the altitude step from 2 km in the previous models to 0.5 km. Effects of the NUV absorber are approximated using the detailed photometric observations at 365 nm from Venera 14. The H2O profile is not fixed but calculated in the model. The model involves odd nitrogen and OCS chemistries based on the detected NO and OCS abundances. The number of the reactions is significantly reduced by removing of unimportant processes. Column rates for all reactions are given, and balances of production and loss may be analyzed in detail for each species. The calculated vertical profiles of CO, H2O, HCl, SO2, SO, OCS and of the O2 dayglow at 1.27 μm generally agree with the existing observational data; some differences are briefly discussed. The OH dayglow is ∼30 kR, brighter than the OH nightglow by a factor of 4. The H + O3 process dominates in the nightglow excitation and O + HO2 in the dayglow, because of the reduction of ozone by photolysis. A key feature of Venus’ photochemistry is the formation of sulfuric acid in a narrow layer near the cloud tops that greatly reduces abundances of SO2 and H2O above the clouds. Delivery of SO2 and H2O through this bottleneck determines the chemistry and its variations above the clouds. Small variations of eddy diffusion near 60 km result in variations of SO2, SO, and OCS at and above 70 km within a factor of ∼30. Variations of the SO2/H2O ratio at the lower boundary have similar but weaker effect: the variations within a factor of ∼4 are induced by changes of SO2/H2O by ±5%. Therefore the observed variations of the mesospheric composition originate from minor variations of the atmospheric dynamics near the cloud layer and do not require volcanism. NO cycles are responsible for production of a quarter of O2, SO2, and Cl2 in the atmosphere. A net effect of photochemistry in the middle atmosphere is the consumption of CO2, SO2, and HCl from and return of CO, H2SO4, and SO2Cl2 to the lower atmosphere. These processes may be balanced by thermochemistry in the lower atmosphere even without outgassing from the interior, though the latter is not ruled out by our models. Some differences between the model and observations and the previous models are briefly discussed.

We calculated 2D and 3D mantle convection models for Venus using digitized atmosphere temperatures from the model of Bullock and Grinspoon (Bullock, M.A., Grinspoon, D.H. [2001]. Icarus 150, 19–37) to study the interaction between interior dynamics and atmosphere thermal evolution. The coupling between atmosphere and interior occurs through mantle degassing and the effect of varying concentrations of the greenhouse gas H 2 O on the surface temperature. Exospheric loss of hydrogen to space is accounted for as a H 2 O sink. The surface temperature enters the mantle convection model as a boundary condition. Our results suggest a self-consistent feedback mechanism between the interior and the atmosphere resulting in spatial–temporal surface renewal. Greenhouse warming of the atmosphere results in an increase in the surface temperature. Whenever the surface temperature reaches a critical value, the viscosity difference across the lithosphere becomes smaller than about 10 5 and the surface becomes locally mobile. The critical surface temperature depends on the activation energy for mantle creep, the stress exponent in the non-Newtonian mantle rheology law, and the mantle temperature. Surface renewal together with surface lava flow may explain why the surface of Venus is young on average, i.e. not older than a few hundred million years. The mobilization of the near-surface lithosphere increases the rate of heat removal from the mantle and thereby the interior cooling rate. The enhanced cooling results in a reduction of the water outgassing rates. As a consequence of decreasing water concentrations in the atmosphere, the surface temperature decreases. Our model calculations suggest that Venus should have been geologically active until recently. This is in agreement with several lines of observational evidence from thermal emissivity measurements and crater distribution analyses.

The IRTF/CSHELL observations in February 2006 at L S = 10° and 63–93°W show ∼10 ppb of methane at 45°S to 7°N and ∼3 ppb outside this region that covers the deepest canyon Valles Marineris. Observations in December 2009 at L S = 20° and 0–30°W included spectra of the Moon at a similar airmass as a telluric calibrator. A technique for extraction of the martian methane line from a combination of the Mars and Moon spectra has been developed. The observations reveal no methane with an upper limit of 8 ppb. The results of both sessions agree with the observations by Mumma et al. (Mumma, M.J. et al. [2009]. Science 323, 1041–1045) at the same season in February 2006 and are smaller than those in the PFS and TES maps. Production and removal of the biological methane on Mars do not significantly change the redox state of the atmosphere and the balance of hydrogen. A search for ethane at 2977 cm −1 results in an upper limit of 0.2 ppb. However, this limit does not help to establish the origin of methane on Mars. Reanalysis of our search for SO 2 using TEXES confirms the recently established upper limit of 0.3 ppb. Along with the lack of hot spots and gas vents with endogenic heat sources in the THEMIS observations, the very low upper limit to SO 2 on Mars does not favor geological methane that is less abundant than SO 2 in the outgassing from the terrestrial planets.

We measured the chemical composition of Comet C/2007 W1 (Boattini) using the long-slit echelle grating spectrograph at Keck-2 (NIRSPEC) on 2008 July 9 and 10. We sampled 11 volatile species (H2O, OH*, C2H6, CH3OH, H2CO, CH4, HCN, C2H2, NH3, NH2, and CO), and retrieved three important cosmogonic indicators: the ortho-para ratios of H2O and CH4, and an upper-limit for the D/H ratio in water. The abundance ratios of almost all trace volatiles (relative to water) are among the highest ever observed in a comet. The comet also revealed a complex outgassing pattern, with some volatiles (the polar species H2O and CH3OH) presenting very asymmetric spatial profiles (extended in the anti-sunward hemisphere), while others (e.g., C2H6 and HCN) showed particularly symmetric profiles. We present emission profiles measured along the Sun-comet line for all observed volatiles, and discuss different production scenarios needed to explain them. We interpret the emission profiles in terms of release from two distinct moieties of ice, the first being clumps of mixed ice and dust released from the nucleus into the sunward hemisphere. The second moiety considered is very small grains of nearly pure polar ice (water and methanol, without dark material or apolar volatiles). Such grains would sublimate only very slowly, and could be swept into the anti-sunward hemisphere by radiation pressure and solar-actuated non-gravitational jet forces, thus providing an extended source in the anti-sunward hemisphere.

The European Space Agency (ESA) Rosetta spacecraft (Schulz, R., Alexander, C., Boehnhardt, H., Glassmeier, K.H. (Eds.) [2009]. “ROSETTA – ESA”) will encounter Comet 67P/Churyumov-Gerasimenko in 2014 and spend the next 18 months in the vicinity of the comet, permitting very high spatial and spectral resolution observations of the coma and nucleus. During this time, the heliocentric distance of the comet will change from ∼3.5 AU to ∼1.3 AU, accompanied by an increasing temperature of the nucleus and the development of the coma. The Microwave Instrument for the Rosetta Orbiter (MIRO) will observe the ground-state rotational transition (110–101) of H216O at 556.936 GHz, the two isotopologues H217O and H218O and other molecular transitions in the coma during this time (Gulkis, S. et al., [2007]. MIRO: Microwave Instrument for Rosetta Orbiter. Space Sci. Rev. 128, 561–597). The aim of this study is to simulate the water line spectra that could be obtained with the MIRO instrument and to understand how the observed line spectra with various viewing geometries can be used to study the physical conditions of the coma and the water excitation processes throughout the coma. We applied an accelerated Monte Carlo method to compute the excitations of the seven lowest rotational levels (101, 110, 212, 221, 303, 312, and 321) of ortho-water using a comet model with spherically symmetric water outgassing, density, temperature and expansion velocity at three different heliocentric distances 1.3 AU, 2.5 AU, and 3.5 AU. Mechanisms for the water excitation include water–water collisions, water–electron collisions, and infrared pumping by solar radiation. Synthetic line spectra are calculated at various observational locations and directions using the MIRO instrument parameters. We show that observations at varying viewing distances from the nucleus and directions have the potential to give diagnostic information on the continuum temperature and water outgassing rates at the surface of the nucleus, and the gas density, expansion velocity, and temperature of the coma as a function of distance from the nucleus. The gas expansion velocity and temperature affect the spectral line width and frequency shift of the line from the rest frequency, while the gas density (which is directly related to the outgassing rate) and the line excitation temperature determine the antenna temperature of the absorption and emission signal in the line profile.

Axisymmetric models of the outgassing of a cometary nucleus have been constructed. Such models can be used to describe a nucleus with a single active region. The models may include a solar zenith angle dependence of the outgassing. They retrieve the outgassing flux at distances from the nucleus where collisions between molecules are unimportant, as function of the angle with respect to the outgassing axis. The observed emissions must be optically thin. Furthermore the models assume that the outflow speed at large distance from the nucleus does not depend on direction. The value of the outflow speed is retrieved. The models are applied to CN images and HCN spectra of Comet 2P/Encke, obtained nearly simultaneously in November 2003 with the 2 m optical telescope on Mount Rozhen, Bulgaria, and with the 10 m Heinrich Hertz Submillimeter Telescope on Mount Graham, Arizona, USA. According to Sekanina (1988) , Astron. J. 95, 911–924, at that time a single outgassing source was active. Input parameters to the models like the rotation period of the nucleus and a small correction to Sekanina’s rotation axis are determined from a simpler jet position angle model. The rotation is prograde with a sideric period of 11.056 ± 0.024 h, in agreement with literature values. The best fit model has an outflow speed of 0.95 ± 0.04 km s−1. The same value has been derived from the corkscrew appearing in the CN images. The location of the outgassing axis is at colatitude δa = 7.4° ± 2.9° and longitude λa = 235° ± 17° (a definition of zero longitude is provided). Comet Encke’s outgassing corresponds approximately to the longitudinally averaged solar input on a spherical nucleus (i.e. very likely comes from deeper layers) but with some deficiency of outgassing at mid-latitudes and non-zero outgassing from the dark polar cap. The presence of gas flow from the dark polar cap is explained as evidence of gas flow across the terminator. The models rely mostly on the CN images. The HCN spectra are more noisy. They provide information how to determine the best fit outflow velocity and the sense of rotation. The model HCN spectra are distinctly non-Gaussian. Within error limits they are consistent with the observations. Models based solely on the HCN spectra are also presented but, because of the lower quality of the data and the unfavorable observing geometry, yield inferior results. As a by-product we determine the CN parent life time from our CN observations. The solar EUV and Lyα radiation field at the time of our observations is taken into account.

The lunar surface is bathed in a variety of impacting particles originating from the solar wind, solar flares, and galactic cosmic rays. These particles can become embedded in the regolith and/or produce a range of other molecules as they pass through the target material. The Moon therefore contains a record of the variability of the solar and galactic particle fluxes through time. To obtain useful temporal snapshots of these processes, discrete regolith units must be shielded from continued bombardment that would rewrite the record over time. One mechanism for achieving this preservation is the burial of a regolith deposit by a later lava flow. The archival value of such deposits sandwiched between lava layers is enhanced by the fact that both the under- and over-lying lava can be dated by radiometric techniques, thereby precisely defining the age of the regolith layer and the geologic record contained therein. The implanted volatile species would be vulnerable to outgassing by the heat of the over-lying flow, at temperatures exceeding 300–700 °C. However, the insulating properties of the finely particulate regolith would restrict significant heating to shallow depths. We have therefore modeled the heat transfer between lunar mare basalt lavas and the regolith in order to establish the range of depths below which implanted volatiles would be preserved. We find that the full suite of solar wind volatiles, consisting predominantly of H and He, would survive at depths of not, vert, similar13–290 cm (for 1–10 m thick lava flows, respectively). A substantial amount of CO, CO2, N2 and Xe would be preserved at depths as shallow as 3.7 cm beneath meter-thick flows. Given typical regolith accumulation rates during mare volcanism, the optimal localities for collecting viable solar wind samples would involve stacks of thin mare lava flows emplaced a few tens to a few hundred Ma apart, in order for sufficient regolith to develop between burial events. Obtaining useful archives of Solar System processes would therefore require extraction of regolith deposits buried at quite shallow depths beneath radiometrically-dated mare lava flows. These results provide a basis for possible lunar exploration activities.

More than 490 elliptical aerobraking and science phasing orbits made by Mars Global Surveyor (MGS) in 1997 and 1998 provide unprecedented coverage of the solar wind in the vicinity of the orbits of the martian moons Phobos and Deimos. We have performed a comprehensive survey of magnetic field perturbations in the solar wind to search for possible signatures of solar wind interaction with dust or gas escaping from the moons. A total of 1246 solar wind disturbance events were identified and their distribution was examined relative to Phobos, the Phobos orbit, and the Deimos orbit. We find that the spatial distribution of solar wind perturbations does not increase near or downstream of Phobos, Phobos’ orbit, or Deimos’ orbit, which would have been expected if there is significant outgassing or dust escape from the martian moons. Of the 1246 magnetic field perturbation events found in the MGS data set, 11 events were found within 2000 km of the Phobos orbit, while three events were found within 2000 km of the Deimos orbit. These events were analyzed in detail and found to likely have other causes than outgassing/dust escape from the martian moons. Thus we conclude that the amount of gas/dust escaping the martian moons is not significant enough to induce detectable magnetic field perturbations in the solar wind. In essence we have not found any clear evidence in the MGS magnetic field data for outgassing or dust escape from the martian moons.

We have conducted high-pressure experiments in the H2O–CH4 and H2O–CH4–NH3 systems in order to investigate the stability of methane clathrate hydrates, with an optical sapphire-anvil cell coupled to a Raman spectrometer for sample characterization. The results obtained confirm that three factors determine the stability of methane clathrate hydrates: (1) the bulk methane content of the samples; (2) the presence of additional gas compounds such as nitrogen; (3) the concentration of ammonia in the aqueous solution. We show that ammonia has a strong effect on the stability of methane clathrates. For example, a 10 wt.% NH3 solution decreases the dissociation temperature of methane clathrates by 14–25 K at pressures above 5 MPa. Then, we apply these new results to Titan’s conditions. Dissociation of methane clathrate hydrates and subsequent outgassing can only occur in Titan’s icy crust, in presence of locally large amounts of ammonia and in a warm context. We propose a model of cryomagma chamber within the crust that provides the required conditions for methane outgassing: emplacement of an ice plume triggers the melting (if solid) or heating (if liquid) of large ammonia–water pockets trapped at shallow depth, and the generated cryomagmas dissociate surrounding methane clathrate hydrates. We show that this model may allow for the outgassing of significant amounts of methane, which would be sufficient to maintain the presence of methane in Titan’s atmosphere for several tens of thousands of years after a large cryovolcanic event.

The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) has imaged the sublimation of Mars’ seasonal CO2 polar cap with unprecedented detail for one complete martian southern spring. In some areas of the surface, beneath the conformal coating of seasonal ice, radiallyorganized channels are connected in spidery patterns. The process of formation of this terrain, erosion by gas from subliming seasonal ice, has no earthly analog. The new capabilities (high resolution, color, and stereo images) of HiRISE enable detailed study of this enigmatic terrain. Two sites are analyzed in detail, one within an area expected to have translucent seasonal CO2 ice, and the other site outside that region. Stereo anaglyphs show that some channels grow larger as they go uphill – implicating gas rather than liquid as the erosive agent. Dark fans of material from the substrate are observed draped over the seasonal ice, and this material collects in thin to thick layers in the channels, possibly choking off gas flow in subsequent years, resulting in inactive crisscrossing shallow channels. In some areas there are very dense networks of channels with similar width and depth, and fewer fans emerging later in the season are observed. Subtle variations in topography affect the channel morphology. A new terminology is proposed for the wide variety of erosional features observed.