The Maria asteroid family: Genetic relationships and a plausible source of mesosiderites near the 3:1 Kirkwood Gap

We present a mineralogical assessment of 12 Maria family asteroids, using near-infrared spectral data obtained over the years 2000-2009 combined with visible spectral data (when available) to cover the spectral interval of 0.4-2.5 μm. Our analysis indicates the Maria asteroid family, which is located adjacent to the chaotic region of the 3:1 Kirkwood Gap, appears to be a true genetic family composed of assemblages analogous to mesosiderite-type meteorites. Dynamical models by Farinella et al. (Farinella, P., Gunczi, R., Froeschlé, Ch., Froeschlé, C., [1993]. Icarus 101, 174-187) predict this region should supply meteoroids into Earth-crossing orbits. Thus, the Maria family is a plausible source of some or all of the mesosiderites in our meteorite collections. These individual asteroids were most likely once part of a larger parent object that was broken apart and dispersed. One of the Maria dynamical family members investigated, ((695) Bella), was found to be unrelated to the genetic Maria family members. The parameters of (695) Bella indicate an H-chondrite assemblage, and that Bella may be a sister or daughter of Asteroid (6) Hebe.     

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.     

Three‐dimensional shapes and Fe contents of Stardust impact tracks: A track formation model and estimation of comet Wild 2 coma dust particle densities

Abstract– We investigated three‐dimensional structures of comet Wild 2 coma particle impact tracks using synchrotron radiation (SR) X‐ray microtomography at SPring‐8 to elucidate the nature of comet Wild 2 coma dust particles captured in aerogel by understanding the capture process. All tracks have a similar entrance morphology, indicating a common track formation process near the entrance by impact shock propagation irrespective of impactor materials. Distributions of elements along the tracks were simultaneously measured using SR‐XRF. Iron is distributed throughout the tracks, but it tends to concentrate in the terminal grains and at the bottoms of bulbs. Based on these results, we propose an impact track formation process. We estimate the densities of cometary dust particles based on the hypothesis that the kinetic energy of impacting dust particles is proportional to the track volume. The density of 148 cometary dust particles we investigated ranges from 0.80 to 5.96 g cm^?3 with an average of 1.01 (±0.25) g cm^?3. Moreover, we suggest that less fragile crystalline particles account for approximately 5 vol% (20 wt%) of impacting particles. This value of crystalline particles corresponds to that of chondrules and CAIs, which were transported from the inner region of the solar system to the outer comet‐forming region. Our results also suggest the presence of volatile components, such as organic material and perhaps ice, in some bulbous tracks (type‐C).     

Highly reducing conditions during core formation on Mercury: Implications for internal structure and the origin of a magnetic field

The high average density and low surface FeO content of the planet Mercury are shown to be consistent with very low oxygen fugacity during core segregation, in the range 3-6 log units below the iron-wüstite buffer. These low oxygen fugacities, and associated high metal content, are characteristic of high-iron enstatite (EH) and Bencubbinite (CB) chondrites, raising the possibility that such materials may have been important building blocks for this planet. With this idea in mind we have explored the internal structure of a Mercury sized planet of EH or CB bulk composition. Phase equilibria in the silicate mantle have been modeled using the thermodynamic calculator p-MELTS, and these simulations suggest that orthopyroxene will be the dominant mantle phase for both EH and CB compositions, with crystalline SiO₂ being an important minor phase at all pressures. Simulations for both compositions predict a plagioclase-bearing “crust” at low pressure, significant clinopyroxene also being calculated for the CB bulk composition. Concerning the core, comparison with recent high pressure and high temperature experiments relevant to the formation of enstatite meteorites, suggest that the core of Mercury may contain several wt.% silicon, in addition to sulfur. In light of the pressure of the core-mantle boundary on Mercury (∼7 GPa) and the pressure at which the immiscibility gap in the system Fe-S-Si closes (∼15 GPa) we suggest that Mercury’s core may have a complex shell structure comprising: (i) an outer layer of Fe-S liquid, poor in Si; (ii) a middle layer of Fe-Si liquid, poor in S; and (iii) an inner core of solid metal. The distribution of heat-producing elements between mantle and core, and within a layered core have been quantified. Available data for Th and K suggest that these elements will not enter the core in significant amounts. On the other hand, for the case of U both recently published metal/silicate partitioning data, as well as observations of U distribution in enstatite chondrites, suggest that this element behaves as a chalcophile element at low oxygen fugacity. Using these new data we predict that U will be concentrated in the outer layer of the mercurian core. Heat from the decay of U could thus act to maintain this part of Mercury’s core molten, potentially contributing to the origin of Mercury’s magnetic field. This result contrasts with the Earth where the radioactive decay of U represents a negligible contribution to core heating.     

Super-resolution of THEMIS thermal infrared data: Compositional relationships of surface units below the 100 meter scale on Mars

The Thermal Emission Imaging System (THEMIS) has provided the highest spatial resolution (100 m/pixel) thermal infrared (TIR) data of the surface of Mars to date. These data have enabled the discovery of many small-scale compositional units and helped to better constrain surface processes operating at these scales. However, with higher-resolution visible (VIS) instruments revealing smaller-scale surficial differences, there exists a need to detect and map compositional variability using TIR data at scales below 100 m. Because it is unlikely there will be a higher-resolution TIR instrument sent to Mars in the near future, creative image processing techniques commonly classified under the umbrella of “super-resolution” can be employed to improve or enhance the spatial resolution of the THEMIS TIR data. These approaches typically integrate another higher-resolution dataset and can either be qualitative for visual appeal, quantitative for data accuracy, or some combination of both. The super-resolution approach presented here produces enhanced TIR images that are radiometrically accurate, but also visually appealing. For the technique to be successfully applied, multi- to hyper-spectral data from two different spectral regions are required (e.g., the THEMIS TIR and VIS datasets). The focus here is to introduce this new super-resolution methodology and demonstrate its ability using existing THEMIS IR and VIS data. The quartzofeldspathic deposit in northern Syrtis Major was selected because of the spectral variability detected using the original IR resolution data and to better constrain the relationship between the small-scale surface morphology and areal extent of the deposit as well as its formation process. Despite being associated with the central peaks of two craters, the results here show no positive correlation between the small rocky outcrops and the quartzofeldspathic unit. A gradational contact exists between the unit and basaltic sands within the intercrater eolian material. The super-resolution approach offers an alternative approach to traditional sub-pixel deconvolution identification and provides a higher-resolution IR dataset for thermophysical and spectral analysis on Mars.     

Analysis of Jupiter’s Oval BA: A streamlined approach

We present a novel method of constructing streamlines to derive wind speeds within jovian vortices and demonstrate its application to Oval BA for 2001 pre-reddened Cassini flyby data, 2007 post-reddened New Horizons flyby data, and 1998 Galileo data of precursor Oval DE. Our method, while automated, attempts to combine the advantages of both automated and manual cloud tracking methods. The southern maximum wind speed of Oval BA does not show significant changes between these data sets to within our measurement uncertainty. The northern maximum does appear to have increased in strength during this time interval, which likely correlates with the oval’s return to a symmetric shape. We demonstrate how the use of closed streamlines can provide measurements of vorticity averaged over the encircled area with no a priori assumptions concerning oval shape. We find increased averaged interior vorticity between pre- and post-reddened Oval BA, with the precursor Oval DE occupying a middle value of vorticity between these two.     

Analysis of Cassini/CIRS limb spectra of Titan acquired during the nominal mission II: Aerosol extinction profiles in the 600–1420 cm spectral range

We have analyzed the continuum emission of limb spectra acquired by the Cassini/CIRS infrared spectrometer in order to derive information on haze extinction in the 3–0.02 mbar range (∼150–350 km). We focused on the 600–1420 cm⁻¹ spectral range and studied nine different limb observations acquired during the Cassini nominal mission at 55°S, 20°S, 5°N, 30°N, 40°N, 45°N, 55°N, 70°N and 80°N. By means of an inversion algorithm solving the radiative transfer equation, we derived the vertical profiles of haze extinction coefficients from 17 spectral ranges of 20-cm⁻¹ wide at each of the nine latitudes. At a given latitude, all extinction vertical profiles retrieved from various spectral intervals between 600 and 1120 cm⁻¹ display similar vertical slopes implying similar spectral characteristics of the material at all altitudes. We calculated a mean vertical extinction profile for each latitude and derived the ratio of the haze scale height (H_haze) to the pressure scale height (H_gas) as a function of altitude. We inferred H_haze/H_gas values varying from 0.8 to 2.4. The aerosol scale height varies with altitude and also with latitude. Overall, the haze extinction does not show strong latitudinal variations but, at 1 mbar, an increase by a factor of 1.5 is observed at the north pole compared to high southern latitudes. The vertical optical depths at 0.5 and 1.7 mbar increase from 55°S to 5°N, remain constant between 5°N and 30°N and display little variation at higher latitudes, except the presence of a slight local maximum at 45°N. The spectral dependence of the haze vertical optical depth is uniform with latitude and displays three main spectral features centered at 630 cm⁻¹, 745 cm⁻¹ and 1390 cm⁻¹, the latter showing a wide tail extending down to ∼1000 cm⁻¹. From 600 to 750 cm⁻¹, the optical depth increases by a factor of 3 in contrast with the absorbance of laboratory tholins, which is generally constant. We derived the mass mixing ratio profiles of haze at the nine latitudes. Below the 0.4-mbar level all mass mixing ratio profiles increase with height. Above this pressure level, the profiles at 40°N, 45°N, 55°N, at the edge of the polar vortex, display a decrease-with-height whereas the other profiles increase. The global increase with height of the haze mass mixing ratio suggest a source at high altitudes and a sink at low altitudes. An enrichment of haze is observed at 0.1 mbar around the equator, which could be due to a more efficient photochemistry because of the strongest insolation there or an accumulation of haze due to a balance between sedimentation and upward vertical drag.     

Potential martian mineral resources: Mechanisms and terrestrial analogues

The future exploration of Mars is likely to utilize resources that can be extracted in situ. An overview of the geology of Mars has been presented and several mechanisms that could result in the formation of ore deposits have been identified. These include deposits caused by hydrothermal fluids resulting from volcanic activity, large igneous province formation and impact craters. Surface enrichment of mineral sand deposits is also discussed. Where appropriate, terrestrial analogues of these mechanisms have been discussed and supporting evidence from observations of Mars undertaken to date presented. Types of deposits that are unlikely to be found on Mars are also listed.     

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.