Preservation of layered paleodeposits in high-latitude pedestal craters on Mars

An outstanding question in Mars’ climate history is whether or not pedestal craters represent the armored remnants of ice-rich paleodeposits. We address this question using new high-resolution images; in a survey of several hundred high-latitude pedestal craters, we have identified 12 examples in which visible and/or topographically expressed layers are exposed on the marginal scarp of the pedestal. One example, located on the south polar layered deposits, preserves ice-rich layers that have otherwise been completely removed from the polar cap. These observations provide empirical evidence that the pedestal crater formation mechanism is capable of armoring and preserving ice-rich layered paleodeposits. Although layered exposures have not yet been observed in mid-latitude pedestal craters, high-latitude instances of discontinuous, partially covered layers suggest that layers can be readily concealed, likely through mantling and/or mass wasting processes along the marginal scarp. This interpretation is supported by the observation that high-latitude pedestals with exposed layers along their margins are, on average, taller than mid-latitude examples, and have larger, steeper marginal scarps, which may help to maintain layer exposures. These observations favor the interpretation that mid- to high-latitude pedestal craters represent the armored remnants of ice- and dust-rich paleodeposits, which occurred transiently due to changes in the climate regime. Preservation of fine-scale layering of ice and dust at these latitudes implies that the climate change did not involve regional melting conditions.     

Bounce Rock—A shergottite‐like basalt encountered at Meridiani Planum,Mars

Abstract– The Opportunity rover of the Mars Exploration Rover mission encountered an isolated rock fragment with textural, mineralogical, and chemical properties similar to basaltic shergottites. This finding was confirmed by all rover instruments, and a comprehensive study of these results is reported here. Spectra from the miniature thermal emission spectrometer and the Panoramic Camera reveal a pyroxene‐rich mineralogy, which is also evident in Mössbauer spectra and in normative mineralogy derived from bulk chemistry measured by the alpha particle X‐ray spectrometer. The correspondence of Bounce Rock’s chemical composition with the composition of certain basaltic shergottites, especially Elephant Moraine (EET) 79001 lithology B and Queen Alexandra Range (QUE) 94201, is very close, with only Cl, Fe, and Ti exhibiting deviations. Chemical analyses further demonstrate characteristics typical of Mars such as the Fe/Mn ratio and P concentrations. Possible shock features support the idea that Bounce Rock was ejected from an impact crater, most likely in the Meridiani Planum region. Bopolu crater, 19.3 km in diameter, located 75 km to the southwest could be the source crater. To date, no other rocks of this composition have been encountered by any of the rovers on Mars. The finding of Bounce Rock by the Opportunity rover provides further direct evidence for an origin of basaltic shergottite meteorites from Mars.     

Structural and isotopic microanalysis of presolar SiC from supernovae

Abstract– We report on the microstructure, crystallography, chemistry, and isotopic compositions of seven SiC X grains and two mainstream grains from the Murchison meteorite. TEM crystallographic analysis revealed that the X grains (approximately 3 μm) are composed of many small crystals (24–457 nm), while the similarly sized mainstream grains are composed of only a few crystals (0.5–1.7 μm). The difference in crystal size likely results from differences in their formation environments: the X grain crystals evidently formed under conditions of greater supersaturation and rapid growth compared to their mainstream counterparts. However, the same polytypes are observed in both mainstream and X grains. Six X grains and both mainstream grains are entirely the 3C‐SiC polytype and one X grain is an intergrowth of the 3C‐SiC and 2H‐SiC polytypes. EDXS measurements indicate relatively high Mg content in the X grains (≲5 atomic%), while Mg was undetectable in the mainstream grains. The high Mg content is probably from the decay of ²⁶Al into ²⁶Mg. Estimates of the ²⁶Al/²⁷Al ratios, which range from 0.44–0.67, were made from elemental Mg/Al ratios. This range is consistent with the ²⁶Al/²⁷Al ratios inferred from previous isotopic measurements of X grains. We also report the first direct observations of subgrains in X grains, including the first silicides [(Fe,Ni)_nSi_m]. Diffraction data do not match any previously observed presolar phases, but are a good fit to silicides, which are predicted stable SN condensates. Eight subgrains with highly variable Ni/Fe ratios (0.12–1.60) were observed in two X grains.     

Carbon dioxide on the satellites of Saturn: Results from the Cassini VIMS investigation and revisions to the VIMS wavelength scale

Several of the icy satellites of Saturn show the spectroscopic signature of the asymmetric stretching mode of C-O in carbon dioxide (CO₂) at or near the nominal solid-phase laboratory wavelength of 4.2675 μm (2343.3 cm⁻¹), discovered with the Visible-Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft. We report here on an analysis of the variation in wavelength and width of the CO₂ absorption band in the spectra of Phoebe, Iapetus, Hyperion, and Dione. Comparisons are made to laboratory spectra of pure CO₂, CO₂ clathrates, ternary mixtures of CO₂ with other volatiles, implanted and adsorbed CO₂ in non-volatile materials, and ab initio theoretical calculations of CO₂ * nH₂O. At the wavelength resolution of VIMS, the CO₂ on Phoebe is indistinguishable from pure CO₂ ice (each molecule’s nearby neighbors are also CO₂) or type II clathrate of CO₂ in H₂O. In contrast, the CO₂ band on Iapetus, Hyperion, and Dione is shifted to shorter wavelengths (typically ∼4.255 μm (∼2350.2 cm⁻¹)) and broadened. These wavelengths are characteristic of complexes of CO₂ with different near-neighbor molecules that are encountered in other volatile mixtures such as with H₂O and CH₃OH, and non-volatile host materials like silicates, some clays, and zeolites. We suggest that Phoebe’s CO₂ is native to the body as part of the initial inventory of condensates and now exposed on the surface, while CO₂ on the other three satellites results at least in part from particle or UV irradiation of native H₂O plus a source of C, implantation or accretion from external sources, or redistribution of native CO₂ from the interior.The analysis presented here depends on an accurate VIMS wavelength scale. In preparation for this work, the baseline wavelength calibration for the Cassini VIMS was found to be distorted around 4.3 μm, apparently as a consequence of telluric CO₂ gas absorption in the pre-launch calibration. The effect can be reproduced by convolving a sequence of model detector response profiles with a deep atmospheric CO₂ absorption profile, producing distorted detector profile shapes and shifted central positions. In a laboratory blackbody spectrum used for radiance calibration, close examination of the CO₂ absorption profile shows a similar deviation from that expected from a model. These modeled effects appear to be sufficient to explain the distortion in the existing wavelength calibration now in use. A modification to the wavelength calibration for 13 adjacent bands is provided. The affected channels span about 0.2 μm centered on 4.28 μm. The maximum wavelength change is about 10 nm toward longer wavelength. This adjustment has implications for interpretation of some of the spectral features observed in the affected wavelength interval, such as from CO₂, as discussed in this paper.     

EC 11481–2303—a peculiar subdwarf OB star revisited

EC?11481–2303 is a peculiar, hot, high-gravity pre-white dwarf. Previous optical spectroscopy revealed that it is a sdOB star with T _eff=41?790 K, log?g=5.84, and He/H = 0.014 by number. We present an on-going spectral analysis by means of non-LTE model-atmosphere techniques based on high-resolution, high-S/N optical (VLT-UVES) and ultraviolet (FUSE, IUE) observations.We are able to reproduce the optical and UV observations simultaneously with a chemically homogeneous NLTE model atmosphere with a significantly higher effective temperature and lower He abundance (T _eff=55?000 K, log?g=5.8, and He/H=0.0025 by number). While C, N, and O appear less than 0.15 times solar, the iron-group abundance is strongly enhanced by at least a factor of ten.     

Hydrated minerals on Europa’s surface: An improved look from the Galileo NIMS investigation

The surface composition of Europa is of great importance for understanding both the internal evolution of Europa and its putative ocean. The Near Infrared Mapping Spectrometer (NIMS) investigation on Galileo observed Europa and the other Galilean satellites from 0.7 to 5.2 μm with spatial resolution down to a few kilometers during flybys by the spacecraft as it orbited Jupiter. These data have been analyzed and results published over the life of the Galileo mission and afterward. One result was the discovery of hydrated minerals at some locations on Europa and Ganymede. The data are noisy, especially for Europa, due to radiation affecting the NIMS electronics and detectors, and other artifacts are also present. The NIMS data are now being reprocessed using the accumulated knowledge gained over the entire missions to remove noise spikes and compensate for some other defects in the data. We are analyzing these reprocessed data in an attempt to defined better the nature of the hydrate spectral features and improve their interpretation. We report here on analyses of two NIMS reprocessed observations for the 0.7-3-μm region. A revised hydrate spectrum is calculated and mapped in detail across two lineaments. The spectrum shows the expected distorted water features but little or no spectral structure in these features. A narrow, weak spectral feature appears at 1.344 μm, which is weakly correlated with lower albedo. Several other weak features may be present but are difficult to confirm in these limited data sets. The hydrate signature shows the greatest strength within and toward the center of the lineaments, confirming and strengthening the association of the hydrate with these endogenic features. This trend may indicate that the material in the lineaments is youngest toward the center and has more water frost coverage toward the edge. A small, visually dark, circular feature has a spectrum that shows both hydrate and crystalline water ice features and perhaps contains a hydrate different in spectral characteristics and perhaps composition than found in the lineament.     

Identification of the Ca-sulfate bassanite in Mawrth Vallis, Mars

The region surrounding the Mawrth Vallis outflow channel on Mars hosts thick layered deposits containing diverse phyllosilicate minerals. Here we report detection of the Ca-sulfate bassanite on the outflow channel floor, requiring a more complex aqueous chemistry than previously inferred for this region. The sulfate-bearing materials underlie phyllosilicate-bearing strata, and provide an opportunity for testing proposed models of martian geochemical evolution with a future landed mission.     

Formation of an eroded lava channel within an Elysium Planitia impact crater: Distinguishing between a mechanical and thermal origin

A lava channel identified on the wall of an Elysium Planitia impact crater is investigated to identify the dominant erosion mechanism, mechanical vs. thermal, acting during channel formation. Observations of channel morphology are used to supplement analytical models of lava channel formation in order to calculate the duration of channel formation, the velocity of the lava flowing through the channel, and the erosion rate in each erosion regime considered. Results demonstrate that the channel observed in the Elysium Planitia impact crater formed primarily due to mechanical erosion. In a more general sense, results of this study suggest that lava channels can form primarily due to thermal erosion in the presence of more gradual slopes and more consolidated substrates whereas lava channels can form primarily due to mechanical erosion in the presence of more energetic flows on steeper slopes and more poorly consolidated substrates. Therefore, both erosion regimes must be considered when analyzing origins of eroded lava channels that cut through strata of different strengths.     

The geology of Australian Mars analogue sites

Australia has numerous landforms and features, some unique, that provide a useful reference for interpreting the results of spacecraft orbiting Mars and exploring the martian surface. Examples of desert landforms, impact structures, relief inversion, long-term landscape evolution and hydrothermal systems that are relevant to Mars are outlined and the relevant literature reviewed. The Mars analogue value of Australia's acid lakes, hypersaline embayments and mound spring complexes is highlighted along with the Pilbara region, where the oldest convincing evidence of life guides exploration for early life on Mars. The distinctive characteristics of the Arkaroola Mars Analogue Region are also assessed and opportunities for future work in Australia are outlined.