Mathieu Vincendon

No signature of clear CO2 ice from the 'cryptic' regions in Mars' south seasonal polar cap

The seasonal polar ice caps of Mars are composed mainly of CO2 ice. A region of low (< 30%) albedo has been observed within the south seasonal cap during early to mid-spring. The low temperature of this ‘cryptic region’ has been attributed to a clear slab of nearly pure CO2 ice, with the low albedo resulting from absorption by the underlying surface. Here we report near-infrared imaging spectroscopy of the south seasonal cap. The deep and broad CO2 absorption bands that are expected in the near-infrared with a thick transparent slab of CO2 ice are not observed. Models of the observed spectra indicate that the low albedo results from extensive dust contamination close to the surface of a CO2 ice layer, which could be linked to atmospheric circulation patterns. The strength of the CO2 absorption increases after mid-spring, so part of the dust is either carried away or buried more deeply in the ice layer during the CO2 ice sublimation process.

Near-tropical subsurface ice on Mars

Near-surface perennial water ice on Mars has been previously inferred down to latitudes of about 45° and could result from either water vapor diffusion through the regolith under current conditions or previous ice ages precipitations. In this paper we show that at latitudes as low as 25° in the southern hemisphere buried water ice in the shallow (<1 m) subsurface is required to explain the observed surface distribution of seasonal CO2 frost on pole facing slopes. This result shows that possible remnants of the last ice age, as well as water that will be needed for the future exploration of Mars, are accessible significantly closer to the equator than previously thought, where mild conditions for both robotic and human exploration lie. Copyright 2010 by the American Geophysical Union.

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 μm. 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 μm. 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.

Dust aerosols above the south polar cap of Mars as seen by OMEGA

The time evolution of atmospheric dust at high southern latitudes on Mars has been determined using observations of the south seasonal cap acquired in the near infrared (1–2.65 μm) by OMEGA/Mars Express in 2005. Observations at different solar zenith angles and one EPF sequence demonstrate that the reflectance in the 2.64 μm saturated absorption band of the surface CO2 ice is mainly due to the light scattered by aerosols above most places of the seasonal cap. We have mapped the total optical depth of dust aerosols in the near-IR above the south seasonal cap of Mars from mid-spring to early summer with a time resolution ranging from one day to one week and a spatial resolution of a few kilometers. The optical depth above the south perennial cap is determined on a longer time range covering southern spring and summer. A constant set of optical properties of dust aerosols is consistent with OMEGA observations during the analyzed period. Strong variations of the optical depth are observed over small horizontal and temporal scales, corresponding in part to moving dust clouds. The late summer peak in dust opacity observed by Opportunity in 2005 propagated to the south pole contrarily to that observed in mid spring. This may be linked to evidence for dust scavenging by water ice-rich clouds circulating at high southern latitudes at this season.

67P/Churyumov-Gerasimenko surface properties as derived from CIVA panoramic images

The structure and composition of cometary constituents, down to their microscopic scale, are critical witnesses of the processes and ingredients that drove the formation and evolution of planetary bodies toward their present diversity. On board Rosetta’s lander Philae, the Comet Infrared and Visible Analyser (CIVA) experiment took a series of images to characterize the surface materials surrounding the lander on comet 67P/Churyumov-Gerasimenko. Images were collected twice: just after touchdown, and after Philae finally came to rest, where it acquired a full panorama. These images reveal a fractured surface with complex structure and a variety of grain scales and albedos, possibly constituting pristine cometary material.

Aphelion water-ice cloud mapping and property retrieval using the OMEGA imaging spectrometer onboard Mars Express

Mapping of the aphelion clouds over Tharsis and retrieval of their particle size and visible opacity are made possible by the OMEGA imaging spectrometer aboard Mars Express. Observations cover the period from MY26 Ls=330{degree sign} to MY29 Ls=180{degree sign} and are acquired at various local times, ranging from 8AM to 6PM. Cloud maps of the Tharsis region constructed using the 3.1µm ice absorption band reveal the seasonal and diurnal evolution of aphelion clouds. Four distinct types of clouds are identified: morning hazes, topographically controlled hazes, cumulus clouds and thick hazes. The location and time of occurrence of these clouds are analyzed and their respective formation process is discussed. An inverse method for retrieving cloud particle size and opacity is then developed and can only be applied to thick hazes. The relative error of these measurements is less than 30% for cloud particle size and 20% for opacity. Two groups of particles can be distinguished. The first group is found over flat plains and is composed of relatively small particles, ranging in size from 2 to 3.5µm. The second group is characterized by particle sizes of ~5µm which appear to be quite constant over Ls and local time. It is found west of Ascraeus and Pavonis Mons, and near Lunae Planum. These regions are preferentially exposed to anabatic winds, which may control the formation of these particles and explain their distinct properties. The water ice column is equal to 2.9pr.µm on average, and can reach 5.2pr.µm in the thickest clouds of Tharsis.

Identification of Mars gully activity types associated with ice composition

The detection of geologically recent channels at the end of the 20th century rapidly suggested that liquid water could have been present on Mars up to recent times. A mechanism involving melting of water ice during ice ages in the last several million years progressively emerged during years following the first observations of these gullies. However, the recent discovery of current activity within gullies now suggests a paradigm shift where a contemporary CO2 ice-based and liquid water-free mechanism may form all gullies. Here we perform a survey of near-infrared observations and construct time sequences of water and CO2 ice formation and sublimation at active gully sites. We observe that all major new erosive features such as channel development or lengthening systematically occur where and, if applicable, when, CO2 ice is observed or probable. CO2 ice layers are however estimated to be only 1 mm to 1 cm thick for low latitude sites, which may have implication for potential formation mechanisms. We also observe that part of current gully activity, notably the formation of some new deposits, is poorly compatible with the presence of CO2 ice. In particular, all new bright deposits reported in the literature have a low CO2 ice probability while water ice should be present at most sites. Our results confirm that CO2 ice is a key factor controlling present-day channel development on Mars and show that other mechanisms, potentially involving sublimation or melting of water ice, are also contributing to current gully activity.

Mars surface phase function constrained by orbital observations

The bidirectional photometric properties of the surface of Mars describe how remote measurements of surface reflectance can be linked to hemispherical albedo used for energy balance calculations. A simple Lamberts law is frequently assumed for global data processing, even though several local studies have revealed the complexity of Mars surface phase functions. In this paper, we derive a mean Bidirectional Reflectance Distribution Function (BRDF) representative of widespread typical Martian terrains. OMEGA and CRISM orbital observations are used to provide observational constraints at solar wavelengths over a wide range of viewing conditions all over the planet. Atmospheric contribution is quantified and removed using a radiative transfer model. We observe a common phase behavior consisting of a 5%–10% backscattering peak and, outside the backscattering region, a 10%–20% reflectance increase with emergence angles. Consequently, nadir measurements of surface reflectance typically underestimate hemispherical reflectance, or albedo, by 10%. We provide a parameterization of our mean Mars surface phase function based on Hapke formalism (ω=0.85, θ=17, c=0.6, b=0.12, B0=1 and h=0.05), and quantify the impact of the diffuse illumination conditions which change surface albedo as a function of local time and season. Our average phase function can be used as a refinement compared to the Lambertian surface model in global data processing and climate modeling.

Mars Express measurements of surface albedo changes over 2004–2010

Abstract The pervasive Mars dust is continually transported between the surface and the atmosphere. When on the surface, dust increases the albedo of darker underlying rocks and regolith, which modifies climate energy balance and must be quantified. Remote observation of surface albedo absolute value and albedo change is however complicated by dust itself when lifted in the atmosphere. Here we present a method to calculate and map the bolometric solar hemispherical albedo of the martian surface using the 2004–2010 OMEGA imaging spectrometer dataset. This method takes into account aerosols radiative transfer, surface photometry, and instrumental issues such as registration differences between visible and near-IR detectors. Resulting albedos are on average 17% higher than previous estimates for bright surfaces while similar for dark surfaces. We observed that surface albedo changes occur mostly during the storm season due to isolated events. The main variations are observed during the 2007 global dust storm and during the following year. A wide variety of change timings are detected such as dust deposited and then cleaned over a martian year, areas modified only during successive global dust storms, and perennial changes over decades. Both similarities and differences with previous global dust storms are observed. While an optically thin layer of bright dust is involved in most changes, this coating turns out to be sufficient to mask underlying mineralogical near-IR spectral signatures. Overall, changes result from apparently erratic events; however, a cyclic evolution emerges for some (but not all) areas over long timescales.

A radiative transfer model to simulate light scattering in a compact granular medium using a Monte‒Carlo approach: Validation and first applications

A new radiative transfer model to simulate light scattering in a compact granular medium using a Monte‒Carlo approach is presented. The physical and compositional properties of the sample can be specified at the grain scale, thus allowing to simulate different kinds of heterogeneties/mixtures within the sample. The radiative transfer is then calculated using a ray tracing approach between the grains, and probabilistic physical parameters such as a single scattering albedo and a phase function at the grain level. The reflectance and the albedo can be computed at different scales and for different geometries: from the grain scale to the sample one. The photometric behavior of the model is validated by comparing the bidirectional reflectance obtained for various media and geometries with the one of semi‒infinite multilayer models, and a few first applications are presented. This model will be used to refine our understanding of visible/NIR remote sensing data of planetary surfaces, as well as future measurements of hyperspectral microscopes which may be able to resolve spatial compositional heterogeneities within a given sample.