Color imaging of Mars by the High Resolution Imaging Science Experiment (HiRISE)

HiRISE has been producing a large number of scientifically useful color products of Mars and other planetary objects. The three broad spectral bands, coupled with the highly sensitive 14 bit detectors and time delay integration, enable detection of subtle color differences. The very high spatial resolution of HiRISE can augment the mineralogic interpretations based on multispectral (THEMIS) and hyperspectral datasets (TES, OMEGA and CRISM) and thereby enable detailed geologic and stratigraphic interpretations at meter scales. In addition to providing some examples of color images and their interpretation, we describe the processing techniques used to produce them and note some of the minor artifacts in the output. We also provide an example of how HiRISE color products can be effectively used to expand mineral and lithologic mapping provided by CRISM data products that are backed by other spectral datasets. The utility of high quality color data for understanding geologic processes on Mars has been one of the major successes of HiRISE.     

Investigating gully flow emplacement mechanisms using apex slopes

The origin of the martian gullies has been much debated since their discovery by the Mars Orbiter Camera (MOC, Malin, M.C., Edgett, K.S. [2000]. Science 288, 2330-2335). Several previous studies have looked at slope gradients in and around gullies, but none have used Digital Elevation Models (DEMs) from the High Resolution Imaging Science Experiment (HiRISE, McEwen, A.S., and 14 colleagues [2007]. J. Geophys. Res. 112 (E05), E0505S02), which has a pixel scale down to 25 cm/pixel. We use five 1 m/post HiRISE DEMs to measure gully apex slopes, the local channel gradient at the upslope extent of the gully debris apron, which marks a shift from erosion to deposition. The apex slope provides information about whether a flow was likely a typical dry granular flow (begins depositing on slopes ∼21°) or fluidized by some extra mechanism (depositing on shallower slopes). We find that 72% of the 75 gully fans studied were likely emplaced by fluidized flows. Relatively old gullies appear more likely to have hosted fluidized flows than relatively fresh gullies. This suggests a time and location dependent fluidizing agent, possibly liquid water produced in a different climate as previously proposed. Our results do not provide evidence for water-rich flows in gullies today.     

A search for ethane on Pluto and Triton

We present here a search for solid ethane, C₂H₆, on the surfaces of Pluto and Triton, based on near-infrared spectral observations in the H and K bands (1.4-2.45 μm) using the Very Large Telescope (VLT) and the United Kingdom Infrared Telescope (UKIRT). We model each surface using a radiative transfer model based on Hapke theory (Hapke, B. [1993]. Theory of Reflectance and Emittance Spectroscopy. Cambridge University Press, Cambridge, UK) with three basic models: without ethane, with pure ethane, and with ethane diluted in nitrogen. On Pluto we detect weak features near 2.27, 2.405, 2.457, and 2.461 μm that match the strongest features of pure ethane. An additional feature seen at 2.317 μm is shifted to longer wavelengths than ethane by at least 0.002 μm. The strength of the features seen in the models suggests that pure ethane is limited to no more than a few percent of the surface of Pluto. On Triton, features in the H band could potentially be explained by ethane diluted in N₂, however, the lack of corresponding features in the K band makes this unlikely (also noted by Quirico et al. (Quirico, E., Doute, S., Schmitt, B., de Bergh, C., Cruikshank, D.P., Owen, T.C., Geballe, T.R., Roush, T.L. [1999]. Icarus 139, 159-178)). While Cruikshank et al. (Cruikshank, D.P., Mason, R.E., Dalle Ore, C.M., Bernstein, M.P., Quirico, E., Mastrapa, R.M., Emery, J.P., Owen, T.C. [2006]. Bull. Am. Astron. Soc. 38, 518) find that the 2.406-μm feature on Triton could not be completely due to ¹³CO, our models show that it could not be accounted for entirely by ethane either. The multiple origin of this feature complicates constraints on the contribution of ethane for both bodies.     

Limits on the trapping of atmospheric CH4 in martian polar ice analogs

Recent detection of methane (CH₄) on Mars has generated interest in possible biological or geological sources, but the factors responsible for the reported variability are not understood. Here we explore one potential sink that might affect the seasonal cycling of CH₄ on Mars - trapping in ices deposited on the surface. Our apparatus consisted of a high-vacuum chamber in which three different Mars ice analogs (water, carbon dioxide, and carbon dioxide clathrate hydrates) were deposited in the presence of CH₄ gas. The ices were monitored for spectroscopic evidence of CH₄ trapping using transmission Fourier-Transform Infrared (FT-IR) spectroscopy, and during subsequent sublimation of the ice films the vapor composition was measured using mass spectrometry (MS). Trapping of CH₄ in water ice was confirmed at deposition temperatures <100 K which is consistent with previous work, thus validating the experimental methods. However, no trapping of CH₄ was observed in the ice analogs studied at warmer temperatures (140 K for H₂O and CO₂ clathrate, 90 K for CO₂ snow) with approximately 10 mTorr CH₄ in the chamber. From experimental detection limits these results provide an upper limit of 0.02 for the atmosphere/ice trapping ratio of CH₄. If it is assumed that the trapping mechanism is linear with CH₄ partial pressure and can be extrapolated to Mars, this upper limit would indicate that less than 1% is expected to be trapped from the largest reported CH₄ plume, and therefore does not represent a significant sink for CH₄.     

Observations of metallic species in Mercury’s exosphere

From observations of the metallic species sodium (Na), potassium (K), and magnesium (Mg) in Mercury’s exosphere, we derive implications for source and loss processes. All metallic species observed exhibit a distribution and/or line width characteristic of high to extreme temperature - tens of thousands of degrees K. The temperatures of refractory species, including magnesium and calcium, indicate that the source process for the atoms observed in the tail and near-planet exosphere are consistent with ion sputtering and/or impact vaporization of a molecule with subsequent dissociation into the atomic form. The extended Mg tail is consistent with a surface abundance of 5-8% Mg by number, if 30% of impact-vaporized Mg remains as MgO and half of the impact vapor condenses. Globally, ion sputtering is not a major source of Mg, but locally the sputtered source can be larger than the impact vapor source. We conclude that the Na and K in Mercury’s exosphere can be derived from a regolith composition similar to that of Luna 16 soil (or Apollo 17 orange glass), in which the abundance by number is 0.0027 (0.0028) for Na and 0.0006 (0.0045) for K.     

Diagenetic haematite and sulfate assemblages in Valles Marineris

Previous orbital mapping of crystalline gray haematite, ferric oxides, and sulfates has shown an association of this mineralogy with light-toned, layered deposits on the floor of Valles Marineris, in chaos terrains in the canyon’s outflow channels, and in Meridiani Planum. The exact nature of the relationship between ferric oxides and sulfates within Valles Marineris is uncertain. The Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activite (OMEGA) spectrometer initially identified sulfate and ferric oxides in the layered deposits of Valles Marineris. The Thermal Emission Spectrometer (TES) has also mapped coarse (gray) haematite in or at the base of these deposits. We use Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectra and Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) imagery from the Mars Reconnaissance Orbiter (MRO) to explore the mineralogy and morphology of the large layered deposit in central Capri Chasma, part of the Valles Marineris canyon system that has large, clear exposures of sulfate and haematite. We find kieserite (MgSO₄·H₂O) and ferric oxide (often crystalline red haematite) in the lower bedrock exposures and a polyhydrated sulfate without ferric oxides in the upper bedrock. This stratigraphy is duplicated in many other basinal chasmata, suggesting a common genesis. We propose the haematite and monohydrated sulfate formed by diagenetic alteration of a sulfate-rich sedimentary deposit, where the upper polyhydrated sulfate-rich, haematite-poor layers either were not buried sufficiently to convert to a monohydrated sulfate or were part of a later depositional phase. Based on the similarities between the Valles Marineris assemblages and the sulfate and haematite-rich deposits of Meridiani Planum, we hypothesize a common evaporite and diagenetic formation process for the Meridiani Planum sediments and the sulfate-bearing basinal Interior Layered Deposits.     

Fast numerical method for growth and retreat of subsurface ice on Mars

Subsurface water ice on Mars evolves due to exchange of vapor with the atmosphere, in the form of loss of ice to the atmosphere or in the form of the growth of interstitial ice. Described here is an accelerated numerical method for the long-term evolution of subsurface ice. This accelerated method is five orders of magnitude faster than explicit vapor transport calculations, enabling fundamentally new types of climate models. Its speed matches that of purely thermal models. The speedup is achieved primarily by solving time-averaged equations for vapor transport and ice volume change. Processes incorporated are growth of interstitial pore ice, retreat of pore ice, retreat of an ice sheet, and retreat of pore ice due to geothermal heating from below. Two example applications illustrate this numerical method’s capabilities. Near the permafrost margin at 55° latitude, ice is periodically depleted and slowly recharged, leading to a pore ice layer estimated to be currently no more than a few meters thick. At the Phoenix Landing Site, it shows the formation of a three layered structure, whereby the layer of pore ice can be very thin.     

Illumination conditions of the south pole of the Moon derived using Kaguya topography

We have used the Kaguya laser altimeter-derived topography to conduct a comprehensive study of the illumination conditions at the Moon’s south pole. We have determined, by comparing simulated and actual Clementine images, that the Kaguya topography can be used to generate realistic illumination conditions. We generated an average illumination map for the year 2020 for the lunar south pole region. From this we identified the areas that receive the most illumination. The place receiving the most illumination (86% of the year) is located close to the rim of Shackleton crater at 88.74°S 124.5°E. However two other areas, less than 10 km apart from each other, are collectively lit for 94% of the year. We found that sites exist near the south pole that are continuously lit for several months during summer. We were also able to map the locations and durations of eclipse periods for these areas. Finally we analyzed the seasonal variations in lighting conditions, from summer to winter, for key areas near the south pole. We conclude that areas exist near the south pole that have illumination conditions that make them ideal candidates as future outpost sites.     

Constraints on Mercury’s surface composition from MESSENGER and ground-based spectroscopy

The composition and chemistry of Mercury’s regolith has been calculated from MESSENGER MASCS 0.3-1.3 μm spectra from the first flyby, using an implementation of Hapke’s radiative transfer-based photometric model for light scattering in semi-transparent porous media, and a linear spectral mixing algorithm. We combine this investigation with linear spectral fitting results from mid-infrared spectra and compare derived oxide abundances with mercurian formation models and lunar samples. Hapke modeling results indicate a regolith that is optically dominated by finely comminuted particles with average area weighted grain size near 20 μm. Mercury shows lunar-style space weathering, with maturation-produced microphase iron present at ∼0.065 wt.% abundance, with only small variations between mature and immature sites, the amount of which is unable to explain Mercury’s low brightness relative to the Moon. The average modal mineralogies for the flyby 1 spectra derived from Hapke modeling are 35-70% Na-rich plagioclase or orthoclase, up to 30% Mg-rich clinopyroxene, <5% Mg-rich orthopyroxene, minute olivine, ∼20-45% low-Fe, low-Ti agglutinitic glass, and <10% of one or more lunar-like opaque minerals. Mercurian average oxide abundances derived from Hapke models and mid-infrared linear fitting include 40-50 wt.% SiO₂, 10-35 wt.% Al₂O₃, 1-8 wt.% FeO, and <25 wt.% TiO₂; the inferred rock type is basalt. Lunar-like opaques or glasses with high Fe and/or Ti abundances cannot on their own, or in combination, explain Mercury’s low brightness. The linear mixing results indicate the presence of clinopyroxenes that contain up to 21 wt.% MnO and the presence of a Mn-rich hedenbergite. Mn in M1 crystalline lattice sites of hedenbergite suppresses the strong 1 and 2 μm crystal field absorption bands and may thus act as a strong darkening agent on Mercury. Also, one or more of thermally darkened silicates, Fe-poor opaques and matured glasses, or Mercury-unique Ostwald-ripened microphase iron nickel may lower the albedo. A major part of the total microphase iron present in Mercury’s regolith is likely derived from FeO that is not intrinsic to the crust but has been subsequently delivered by exogenic sources.     

Aeolian processes and dune morphology in Gale Crater

Datasets at resolutions many times greater than previously available were used to study aeolian features within Gale Crater. High resolution thermal inertia data allowed for detailed particle size estimation, with the data sufficient to resolve dunefields. A wide range of grain sizes have now been identified in the Gale Crater dunefields, ranging from medium to very coarse sand. High Resolution Imaging Science Experiment (HiRISE) and THEMIS VIS data allowed for detailed analysis of the dune morphology and slip-faces, which shows that the dunes have responded to topographic influences on prevailing wind directions under a present day wind regime. This result was corroborated by a regional mesoscale model for the crater under dust storm conditions. The central mound and smaller scale crater floor topography has altered the prevailing wind regime and dune patterns. Aeolian activity has thus played, and continues to play, an important role in shaping many of the present surface features of Gale Crater. The arrival of a future lander mission such as the Mars Science Laboratory would be able to sample these surface features directly and add a wealth of data to the understanding of Gale Crater.