Water ice in the dark dune spots of Richardson crater on Mars

In this study we assess the presence, nature and properties of ices - in particular water ice - that occur within these spots using HIRISE and CRISM observations, as well as the LMD Global Climate Model. Our studies focus on Richardson crater (72{\deg}S, 179{\deg}E) and cover southern spring and summer (LS 175{\deg} - 17 341{\deg}). Three units have been identified of these spots: dark core, gray ring and bright halo. Each unit show characteristic changes as the season progress. In winter, the whole area is covered by CO2 ice with H2O ice contamination. Dark spots form during late winter and early spring. During spring, the dark spots are located in a 10 cm thick depression compared to the surrounding bright ice-rich layer. They are spectrally characterized by weak CO2 ice signatures that probably result from spatial mixing of CO2 ice rich and ice free regions within pixels, and from mixing of surface signatures due to aerosols scattering. The bright halo shaped by winds shows stronger CO2 absorptions than the average ice covered terrain, which is consistent with a formation process involving CO2 re-condensation. According to spectral, morphological and modeling considerations, the gray ring is composed of a thin layer of a few tens of {\mu}m of water ice. Two sources/processes could participate to the enrichment of water ice in the gray ring unit: (i) water ice condensation at the surface in early fall (prior to the condensation of a CO2 rich winter layer) or during winter time (due to cold trapping of the CO2 layer); (ii) ejection of dust grains surrounded by water ice by the geyser activity responsible for the dark spot. In any case, water ice remains longer in the gray ring unit after the complete sublimation of the CO2. Finally, we also looked for liquid water in the near-IR CRISM spectra using linear unmixing modeling but found no conclusive evidence for it

Dark spots and cold jets in the polar regions of Mars: new clues from a thermal model of surface CO ice

International audienceObservations of the Martian CO ice cap in late winter and spring have revealed exotic phenomena. Unusual dark spots, fans and blotches form as the south-polar seasonal CO ice cap retreats. The formation mechanisms of these features are not clearly understood. Theoretical models suggest that photons could penetrate deep into the CO ice down to the regolith, leading to basal sublimation and gas and dust ejection. We have developed a detailed thermal model able to simulate the temporal evolution of the regolith-CO ice layer-atmosphere column. It takes into account heat conduction, radiative transfer within the ice and the atmosphere, and latent heat exchange when there is a phase transition. We found that a specific algorithm, fully coupling these 3 components, was needed to properly predict ice sublimation below the surface. Our model allows us to determine under what conditions basal sublimation is possible and thus when and where it can occur on Mars. Our results show that basal sublimation is possible if we consider large pathlengths and very little dust content within the ice. Moreover, the model can explain how dark spots can appear very early after the end of the polar night at high latitudes. We also evaluate the importance of the different parameters in our simulations. Contrary to what was suggested by theoretical models, the role of seasonal thermal waves is found to be limited. Solar radiation alone can initiate basal sublimation, which therefore only depends on the CO ice properties. Three main modes were identified: one where condensation/sublimation only occurs at the surface (in the case of small grains and/or high dust content), one where basal sublimation is possible (large pathlengths and very little dust content) and an intermediate mode where sublimation within the ice may occur. We suggest that these different modes could be keys to understanding many processes that occur at the surface of Mars, like the anticryptic area behavior or the recent reported activity in gullies

Optical properties of cometary particles collected by the COSIMA mass spectrometer on-board <i>Rosetta</i> during the rendezvous phase around comet 67P/Churyumov–Gerasimenko

40 000 collected cometary particles have been identified on the 21 targets exposed by the COSIMA experiment on-board Rosetta to the environment of comet 67P/Churyumov–Gerasimenko from 2014 August to 2016 September. The images of the targets where obtained by the COSIMA microscope (Cosiscope, 13.95 μm pixel−1) with near grazing incidence, which is optimal for the primary objective (detection of collected particles) but very challenging for photometry. However, more than 300 of the collected particles are larger than 100 μm which makes it possible to derive constraints on the optical properties from the distribution of light levels within the particles. Two types of particles collected by COSIMA (compact particles and cluster particles) have been identified in Langevin et al. The best estimate reflectance factors of compact particles range from 10 per cent to 23 per cent. For cluster particles (>90 per cent of large collected particles), the comparison of the signal profiles with illumination from two opposite directions shows that there is scattering within the particles, with a mean free path in the 20–25 μm range, which requires high porosity. The best estimate reflectance factors of cluster particles range from 3 per cent to 22 per cent. This range of reflectance factors overlaps with that obtained from observations of the cometary nucleus at macroscopic scales by OSIRIS and it is consistent with that measured for interplanetary dust particles collected in the stratosphere of the Earth

Modal mineralogy of planetary surfaces from visible and near-infrared spectral data

Real planetary surfaces are composed of several to many different minerals and ices. Deconvolving a reflectance spectrum to material abundance in an unambiguous way is difficult, because the spectra are complex nonlinear functions of grain size, abundance, and material opacity. Multiple scattering models can provide approximate solutions to the radiative transfer in a particulate medium. The paper examines the different approaches which deal with the theory of radiative transfer on atmosphereless bodies. We present the relative merits of two scattering theories based on the equivalent slab model: the extensively used Hapke theory [1] and the Shkuratov theory [2]. The performances of the two models for determining mineral abundance in multicomponent mixtures are also evaluated using laboratory data. Finally, one application on real planetary surfaces will be shown

Towards time-dependent current-density-functional theory in the non-linear regime.

Time-Dependent Density-Functional Theory (TDDFT) is a well-established theoretical approach to describe and understand irradiation processes in clusters and molecules. However, within the so-called adiabatic local density approximation (ALDA) to the exchange-correlation (xc) potential, TDDFT can show insufficiencies, particularly in violently dynamical processes. This is because within ALDA the xc potential is instantaneous and is a local functional of the density, which means that this approximation neglects memory effects and long-range effects. A way to go beyond ALDA is to use Time-Dependent Current-Density-Functional Theory (TDCDFT), in which the basic quantity is the current density rather than the density as in TDDFT. This has been shown to offer an adequate account of dissipation in the linear domain when the Vignale-Kohn (VK) functional is used. Here, we go beyond the linear regime and we explore this formulation in the time domain. In this case, the equations become very involved making the computation out of reach; we hence propose an approximation to the VK functional which allows us to calculate the dynamics in real time and at the same time to keep most of the physics described by the VK functional. We apply this formulation to the calculation of the time-dependent dipole moment of Ca, Mg and Na2. Our results show trends similar to what was previously observed in model systems or within linear response. In the non-linear domain, our results show that relaxation times do not decrease with increasing deposited excitation energy, which sets some limitations to the practical use of TDCDFT in such a domain of excitations.We thank French research funding agency ANR and Institut Universitaire de France for support during the realization of this work. J.M.E. acknowledges the support of EPSRC Grant No. EP/J015059/1. P.R. and J.M.E. would like to thank J. A. Berger for fruitful discussions.Copyright (2015) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in The Journal of Chemical Physics 2015, 142, 084118 and may be found at http://dx.doi.org/10.1063/1.4913291

Modelling Slope Microclimates in the Mars Planetary Climate Model

A large number of surface phenomena (e.g., frost and ice deposits, gullies, slope streaks, recurring slope lineae) are observed on Martian slopes. Their formation is associated with specific microclimates on these slopes that have been mostly studied with one-dimensional radiative balance models to date. We demonstrate here that any Martian slope can be thermally represented by a poleward or equatorward slope, i.e., the daily average, minimum, and maximum surface temperatures depend on the North-South component of the slope. Based on this observation, we propose here a subgrid-scale parameterization to represent slope microclimates in coarse-resolution global climate models. We implement this parameterization in the Mars Planetary Climate Model and validate it through comparisons with surface temperature measurements and frost detections on sloped terrains. With this new model, we show that these slope microclimates do not have a significant impact on the seasonal CO2 and H2O cycle. Our model also simulates for the first time the heating of the atmosphere by warm plains surrounding slopes. Active gullies are mostly found where our model predicts CO$_2$ frost, suggesting that the formation of gullies is mostly related to processes involving CO2 ice. However, the low thicknesses predicted there rule out mechanisms involving large amounts of ice. This model opens the way to new studies on surface-atmosphere interactions in present and past climates

Yearly and seasonal variations of low albedo surfaces on Mars in the OMEGA/MEx dataset: Constraints on aerosols properties and dust deposits

The time variations of spectral properties of dark martian surface features are investigated using the OMEGA near-IR dataset. The analyzed period covers two Mars years, spanning from early 2004 to early 2008 (includes the 2007 global dust event). Radiative transfer modeling indicates that the apparent albedo variations of low to mid-latitude dark regions are consistent with those produced by the varying optical depth of atmospheric dust as measured simultaneously from the ground by the Mars Exploration Rovers. We observe only a few significant albedo changes that can be attributed to surface phenomena. They are small-scaled and located at the boundaries between bright and dark regions. We then investigate the variations of the mean particle size of aerosols using the evolution of the observed dark region spectra between 1 and 2.5 {\mu}m. Overall, we find that the observed changes in the spectral slope are consistent with a mean particle size of aerosols varying with time between 1 and 2 {\mu}m. Observations with different solar zenith angles make it possible to characterize the aerosol layer at different altitudes, revealing a decrease of the particle size of aerosols as altitude increases

A Reappraisal of Near-Tropical Ice Stability on Mars

Two arguments have suggested the presence of subsurface water ice at latitudes lower than 30\textdegree~on Mars. First, the absence of CO2 frost on pole-facing slopes was explained by the presence of subsurface ice. Second, models suggested that subsurface ice could be stable underneath these slopes. We revisit these arguments with a new slope microclimate model. Our model shows that below 30{\deg} latitude, slopes are warmer than previously estimated as the air above is heated by warm surrounding plains. This additional heat prevents the formation of CO2 and subsurface water ice for most slopes. Higher than 30{\deg}S, our model suggests the presence of subsurface water ice. In sparse cases (steep dusty slopes), subsurface ice may exist down to 25{\deg}S. While hypothetical unstable ice deposits cannot be excluded by our model, our results suggest that water ice is rarer than previously thought in the +- 30{\deg} latitude range considered for human exploration

Recovery of surface reflectance spectra and evaluation of the optical depth of aerosols in the near-IR using a Monte-Carlo approach: Application to the OMEGA observations of high latitude regions of Mars

We present a model of radiative transfer through atmospheric particles based on Monte Carlo methods. This model can be used to analyze and remove the contribution of aerosols in remote sensing observations. We have developed a method to quantify the contribution of atmospheric dust in near-IR spectra of the Martian surface obtained by the OMEGA imaging spectrometer on board Mars Express. Using observations in the nadir pointing mode with significant differences in solar incidence angles, we can infer the optical depth of atmospheric dust, and we can retrieve the surface reflectance spectra free of aerosol contribution. Martian airborne dust properties are discussed and constrained from previous studies and OMEGA data. We have tested our method on a region at 90{\deg}E and 77{\deg}N extensively covered by OMEGA, where significant variations of the albedo of ice patches in the visible have been reported. The consistency between reflectance spectra of ice-covered and ice-free regions recovered at different incidence angles validates our approach. The optical depth of aerosols varies by a factor 3 in this region during the summer of Martian year 27. The observed brightening of ice patches does not result from frost deposition but from a decrease in the dust contamination of surface ice and (to a lower extent) from a decrease in the optical thickness of atmospheric dust. Our Monte Carlo-based model can be applied to recover the spectral reflectance characteristics of the surface from OMEGA spectral imaging data when the optical thickness of aerosols can be evaluated. It could prove useful for processing image cubes from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board the Mars Reconnaissance Orbiter (MRO)