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°E and 77°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).
The Mawrth Vallis area displays some of the largest phyllosilicate-rich outcrops of Mars, on Noachian highlands. The Mawrth Vallis region is located just at the dichotomy boundary between the ...Noachian highlands and the younger, northern lowlands. A large, thick, layered clay-rich unit is present throughout the inter-crater plateaus. Clay-rich layers have also been identified in parts of the Mawrth Vallis and Oyama crater floors. The age of the alteration and its relationships with other processes such as fluvial activity is fundamental for estimating the timing of aqueous activity and habitability in this region, and on Mars. We have investigated the relative stratigraphy and ages of the regional plateau, of key surfaces of the inter-crater plateau, of Oyama crater's floor and of Chryse Planitia deposits in Mawrth Vallis' mouth to constrain the age of the clay unit and its alteration. According to the cratering model results, the main layered unit may have started forming prior to ∼4.0Ga ago, was largely deposited by ∼3.9Ga ago, and suffered erosion and redeposition up to ∼3.8Ga ago, as indicated by the latest age of the deposits on the floor of Oyama crater. Surface aqueous alteration stopped no later than 3.7–3.6Ga ago, corresponding to the age of the dark, non-altered material capping the region, and of the dark deposits in Mawrth Vallis' mouth. This work provides useful boundaries for constraining the time period of surface or shallow sub-surface water activity in this region. This preserved window into early phases of aqueous activity on Mars gives us a unique opportunity to study an aqueous environment of exobiological interest in the early solar system.
► The Mawrth Vallis region of Mars is studied through analysis of regional stratigraphy. ► Key surfaces are dated based on crater retention ages. ► Deposition of layers began ∼4.0Ga ago, until 3.8Ga with redeposition in Oyama crater. ► Aqueous alteration on the surface ended >3.7Ga ago. ► Mawrth Vallis provides an exceptional preserved window into the aqueous activity of the early Mars.
The ∼5 km of traverses and observations completed by the Opportunity rover from Endurance crater to the Fruitbasket outcrop show that the Meridiani plains consist of sulfate‐rich sedimentary rocks ...that are largely covered by poorly‐sorted basaltic aeolian sands and a lag of granule‐sized hematitic concretions. Orbital reflectance spectra obtained by Mars Express OMEGA over this region are dominated by pyroxene, plagioclase feldspar, crystalline hematite (i.e., concretions), and nano‐phase iron oxide dust signatures, consistent with Pancam and Mini‐TES observations. Mössbauer Spectrometer observations indicate more olivine than observed with the other instruments, consistent with preferential optical obscuration of olivine features in mixtures with pyroxene and dust. Orbital data covering bright plains located several kilometers to the south of the landing site expose a smaller areal abundance of hematite, more dust, and a larger areal extent of outcrop compared to plains proximal to the landing site. Low‐albedo, low‐thermal‐inertia, windswept plains located several hundred kilometers to the south of the landing site are predicted from OMEGA data to have more hematite and fine‐grained olivine grains exposed as compared to the landing site. Low calcium pyroxene dominates spectral signatures from the cratered highlands to the south of Opportunity. A regional‐scale model is presented for the formation of the plains explored by Opportunity, based on a rising ground water table late in the Noachian Era that trapped and altered local materials and aeolian basaltic sands. Cessation of this aqueous process led to dominance of aeolian processes and formation of the current configuration of the plains.
The Cassini visual and infrared mapping spectrometer (VIMS) investigation is a multidisciplinary study of the Saturnian system. Visual and near-infrared imaging spectroscopy and high-speed ...spectrophotometry are the observational techniques. The scope of the investigation includes the rings, the surfaces of the icy satellites and Titan, and the atmospheres of Saturn and Titan. In this paper, we will elucidate the major scientific and measurement goals of the investigation, the major characteristics of the Cassini VIMS instrument, the instrument calibration, and operation, and the results of the recent Cassini flybys of Venus and the Earth-Moon system.
Previous spectroscopic studies have shown the presence of hydrated minerals in various kinds of sedimentary accumulations covering and encircling the martian North Polar Cap. More specifically, ...gypsum, a hydrated calcium sulfate, has been detected on Olympia Planum, a restricted part of the Circum-Polar Dune Field. To further constrain the geographical distribution and the process of formation and accumulation of these hydrated minerals, we performed an integrated morphological, structural and compositional analysis of a key area where hydrated minerals were detected and where the main polar landforms are present. By the development of a spectral processing method based on spectral derivation and by the acquisition of laboratory spectra of gypsum–ice mixtures we find that gypsum-bearing sediment is not restricted to the Olympia Planum dunes but is also present in all kinds of superficial sediment covering the surface of the North Polar Cap and the Circum-Polar Dune Field. Spectral signatures consistent with perchlorates are also detected on these deposits. The interpretation of landforms reveals that this gypsum-bearing sediment was released from the ice cap by sublimation. We thus infer that gypsum crystals that are now present in the Circum-Polar Dune Field derive from the interior of the North Polar Cap. Gypsum crystals that were initially trapped in the ice cap have been released by sublimation of the ice and have accumulated in the form of ablation tills at the surface of the ice cap. These gypsum-bearing sublimation tills are reworked by winds and are transported towards the Circum-Polar Dune Field. Comparison with sulfates found in terrestrial glaciers suggests that gypsum crystals in the martian North Polar Cap have formed by weathering of dust particles, either in the atmosphere prior to their deposition during the formation of the ice cap, and/or in the ice cap after their deposition.
•Philae’s ROLIS camera acquired 6 images of the surface of 67P after the final landing.•Illumination of the comet surface was provided by LEDs of four colors.•A local horizon is visible in one image ...corner, beyond which we can see the coma.•Seen at low phase angle, the comet surface displays a bimodal brightness distribution.•The surface seems to consist of plates with a consolidated crust, separated by cracks.
After coming to rest on the night side of comet 67P/Churyumov-Gerasimenko, the ROLIS camera on-board Rosetta’s Philae lander acquired five images of the surface below the lander, four of which were with the aid of LED illumination of different colors. The images confirm that Philae was perched on a sloped surface. A local horizon is visible in one corner of the image, beyond which we can see the coma. Having spent a full day on the surface Philae was commanded to lift and rotate, after which a final, sixth, LED image was acquired. The change in perspective allowed us to construct a shape model of the surface. The distance to the foreground was about 80 cm, much larger than the nominal 30 cm. This caused stray light, rather than directly reflected LED light, to dominate the image signal, complicating the analysis. The images show a lumpy surface with a roughness of apparently fractal nature. Its appearance is completely different from that of the first landing site, which was characterized by centimeter to meter-sized debris (Mottola et al., 2015). We recognize neither particles nor pores at the image resolution of 0.8 mm per pixel and large color variations are absent. The surface has a bi-modal brightness distribution that can be interpreted in terms of the degree of consolidation, a hypothesis that we support with experimental evidence. We propose the surface below the lander to consist of smooth, cracked plates with unconsolidated edges, similar to terrain seen in CIVA images.
•The thermophysical properties of the martian surface are mapped using OMEGA/MEX data.•OMEGA thermal inertia is in good agreement with previous studies.•Diurnal thermal behaviors attributable to ...anisothermality are observed.
The thermophysical structure of the martian surface is the result of various processes that have shaped the martian surface through time. Previous dedicated heliosynchronous measurements of the thermal infrared (IR) flux of the martian surface have revealed the diversity of martian surface thermal properties, as well as its complexity linked to the heterogeneous nature of terrains. We present the first retrieval of thermophysical properties of the martian surface using near-infrared (NIR) Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) onboard Mars Express (MEX) thermal measurements from 5 to 5.1μm. MEX orbit around Mars is elliptical and therefore OMEGA has performed surface temperature measurements at various local times and seasons over more than 4 full martian years. We have developed a method to exploit these unprecedented measurements using a one-dimensional energy balance code derived from a Global Climate Model that allows retrieval of the thermal properties of the martian surface using OMEGA data. Regional maps of the thermal inertia at a resolution up to 32 pixels per degree and a global map at 4 pixels per degree are presented. OMEGA-derived thermal inertia values agree with previous mappings by the Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor (MGS) and Thermal Emission Imaging Spectrometer (THEMIS) onboard Mars Odyssey and highlight the key role of dust for the thermal behavior of the martian surface. OMEGA directly reveals for the first time some diurnal variations of apparent TI attributable to surface heterogeneities at macroscopic scale and enables to quantify these heterogeneities. In Nili Patera and Tharsis, local surface heterogeneities are modeled with layering and horizontal admixture of divergent slopes respectively.
Context. On 12 November 2014, the European mission Rosetta delivered the Philae lander on the nucleus of comet 67P/Churyumov-Gerasimenko (67P). After the first touchdown, the lander bounced three ...times before finally landing at a site named Abydos. Aims. We provide a morphologically detailed analysis of the Abydos landing site to support Philae’s measurements and to give context for the interpretation of the images coming from the Comet Infrared and Visible Analyser (CIVA) camera system onboard the lander. Methods. We used images acquired by the OSIRIS Narrow Angle Camera (NAC) on 6 December 2014 to perform the analysis of the Abydos landing site, which provided the geomorphological map, the gravitational slope map, the size-frequency distribution of the boulders. We also computed the albedo and spectral reddening maps. Results. The morphological analysis of the region could suggest that Philae is located on a primordial terrain. The Abydos site is surrounded by two layered and fractured outcrops and presents a 0.02 km2 talus deposit rich in boulders. The boulder size frequency distribution gives a cumulative power-law index of −4.0 + 0.3/−0.4, which is correlated with gravitational events triggered by sublimation and/or thermal fracturing causing regressive erosion. The average value of the albedo is 5.8% at λ1 = 480.7 nm and 7.4% at λ2 = 649.2 nm, which is similar to the global albedos derived by OSIRIS and CIVA, respectively.
The Mars Express Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) collected an unprecedented visible and near-infrared hyperspectral dataset covering the low albedo regions ...of Mars. We investigate the ability to infer modal abundance of surfaces of these regions from a radiative transfer model developed by Shkuratov et al. Shkuratov, Y., Starukhina, L., Hoffmann, H., Arnold, G., 1999. Icarus 137, 235–246 and adapted to basaltic surfaces by Poulet and Erard Poulet F., Erard, S., 2004. J. Geophys. Res. 109 (E2),
doi:10.1029/2003JE002179. From OMEGA measurements of mafic surfaces, we develop several sensitivity tests to assess the extent to which the model can be applied to predict pyroxene composition (high-calcium phase and low-calcium phase), abundance of almost neutral components (plagioclase) in the near-infrared wavelength as well as grain sizes, by using a library of selected end-members. Results of the sensitivity tests indicate that the scattering model can estimate both abundances and grain sizes of major basaltic materials of low albedo regions within uncertainties (±5 to 15 vol%). The model is then applied to data from dissected cratered terrains located in Terra Meridiani. The derived grain size including uncertainties is in the 50–500 μm range. This is consistent with the thermal inertia and albedo of this region, which indicates a fine sand-sized surface with little dust. The abundances of plagioclase (43–57%) and pyroxenes (
35
–
45
±
10
%
, including
11
±
5
%
of low-calcium phase) are in good agreement with previous basalt-like compositions of low albedo regions from thermal infrared spectral measurements. The method presented in this paper will provide a valuable tool for evaluating the modal mineralogy of other mafic regions of Mars observed in the near-infrared wavelength range.
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