Journal of Geophysical Research: Planets最新文献

Undifferentiated plains are the most common terrain type on Titan, yet their composition and geologic history remain poorly understood. To better characterize their physical properties, we combined Cassini RADAR measurements from nadir altimetry and side-looking SAR modes. We analyzed these data using radar backscatter models, finding that the multi-angle radar response from undifferentiated plains across Titan is remarkably consistent. This uniformity suggests globally similar properties and formation processes, permitting aggregate modeling. Our analysis reveals that canonical single-layer scattering models fail to reproduce the observed backscatter, particularly the bright near-nadir returns captured by altimetry, which proved critical for model discrimination and accurate parameter estimation. Instead, a two-layer model is required to fit the data across all incidence angles. Best-fit parameters indicate undifferentiated plains likely consist of a highly porous, low-density surface layer (effective permittivity 1.33) that is exceptionally smooth at radar wavelengths (RMS slope 2°), overlying a higher-density (effective permittivity >2.7) and rougher buried substrate. This surface layer is likely less than 1 m thick. The layered structure, along with the observed global uniformity and extreme flatness at multiple scales, is most consistent with long-term atmospheric deposition of organic particles (“tholin snow”), which are subsequently densified or buried, potentially during periods of different environmental conditions. The structure of the undifferentiated plains provides insights into organic processing and transport on Titan and potentially preserves a record of past environmental conditions. Measurements made by the upcoming Dragonfly mission may provide answers to questions raised by our analysis.

During its exploration of Neretva Vallis in Jezero crater, NASA's Mars 2020 Perseverance rover examined Mount Washburn, a set of light-toned sedimentary mounds that exhibited a diverse array of float rocks. Mastcam-Z visible/near-infrared (VISIR) multispectral imaging revealed five spectral types dominated by low-calcium pyroxene-bearing rocks, although olivine-bearing and ferric materials were also identified. SuperCam VISIR point spectroscopy of eight targets noted four spectral groups, including those with low-Ca pyroxene, hydrated alteration products, and phyllosilicates. SuperCam laser-induced breakdown spectroscopy data confirmed a wide compositional array among the targets. Overall, these data spanned much of the spectral and compositional range in rocks observed by Perseverance through Sol 1348, with no major outliers, although some rock textures were unique. The rock diversity at Mount Washburn is among the most extensive observed in situ at a single site on Mars, pointing to a multifaceted geological history. Bedrock beneath Mount Washburn exhibited high silica and iron content, closely resembling materials from the nearby Bright Angel sedimentary units and suggesting a possible stratigraphic link. Some geological models of Neretva Vallis are consistent with the idea that Bright Angel-like materials may have formed the foundation of Mount Washburn, with subsequent deposition of cobbles and boulders as late-stage fluvial events. These findings suggest that similar rocks—and possibly their source regions—may be encountered during future exploration of the Jezero crater rim and intracrater plains. Such materials would likely sample the olivine/carbonate-bearing regional units and/or ancient Noachian basement, reflecting the rich geological complexity of the Neretva Vallis watershed.

Intermittently sunlit areas near the lunar south pole are estimated to harbor thermal conditions permitting long-term stability of water ice and other volatiles. They are targets for future science and exploration missions due to the combination of sunlight availability for solar power generation, and the possibility for extraction of volatiles for scientific analysis and ISRU. We construct a geodatabase of spatially co-registered remote sensing and thermal model results, and perform a probabilistic analysis to determine the likelihood of successfully landing and operating on such locations for a quadrangular study area that bounds the 80°S parallel. In addition to water ice thermal stability, we consider factors relevant for the operation of solar-powered landed spacecraft: visibility to the Earth, visibility to the sun, and local slope. For two scenarios representing sets of most- and least-constrained landing site requirements, we find that circular landing ellipse diameters of ∼0.9 and 2.6 km, respectively, would allow to target available compliant terrains with 100% success. We quantify the reduction in success probability with increasing landing ellipse size. Further, we explore the distributions of geometric properties of compliant areas, and identify three sites of interest that support large areas of compliant terrain: near De Gerlache crater, near Shackleton crater, and Mons Mouton (informally named as Leibnitz-β massif). This study is provided to support planning for future lunar missions.

Understanding Mars' deep interior is essential to reconstruct its geological history, thermal evolution, and present-day dynamics. To this end, the NASA InSight mission has provided unprecedented seismic observations. However, strong trade-offs between temperature and composition in seismic interpretations continue to limit our ability to resolve interior models. To address this challenge, we account for electromagnetic induction data from Mars Global Surveyor as an additional, independent constraint. We develop a joint probabilistic inversion framework that simultaneously fits seismic body wave arrival times, electrical conductivity, the $��k_2�$ Love number, and the moment of inertia. A key feature of our approach is the integration of Mars' long-term thermal evolution within the forward model, along with mineral physics and petrology data, to better constrain geodynamical parameters. We explore three different mantle compositions (Sanloup et al., 1999, https://doi.org/10.1016/s0031-9201(98)00175-7; Taylor, 2013, https://doi.org/10.1016/j.chemer.2013.09.006; Yoshizaki & McDonough, 2020, https://doi.org/10.1016/j.gca.2020.01.011) and consider both radially homogeneous and heterogeneous (with a basal molten layer (Samuel et al., 2023, https://doi.org/10.1038/s41586-023-06601-8)) mantle scenarios. For homogeneous mantle models, two families of solutions emerge regardless of the bulk composition: one with low Mg content and high potential temperature, which better reproduces electrical conductivity data due to a thicker lithosphere, and another with high Mg content and lower potential temperature. Models with a heterogeneous mantle reproduce electrical conductivity data less accurately, due to thinner lithospheres, and the mantle composition of Yoshizaki and McDonough (2020, https://doi.org/10.1016/j.gca.2020.01.011) appears to be less consistent with the full data set. To further refine models of Mars' interior, future efforts should focus on acquiring electromagnetic data with reduced uncertainties and seismically constraining more precisely the depth of mantle discontinuities associated with mineral phase transitions.

Space weathering constantly alters airless planetary bodies (e.g., the Moon), changing the composition and texture of their surface materials. However, how space weathering alters accessory minerals on the planetary surfaces is still unclear. This study uses transmission electron microscopy to investigate the space weathering features of diverse minerals from Chang'e-5 and Chang'e-6 lunar soils. Our results show that lunar baddeleyite (ZrO₂) exhibits less modification in response to space weathering, which is different from silicate minerals and ilmenite. Specifically, space weathering does not change the crystal structure and chemical composition of baddeleyite. We further reveal that bond energy and nuclear resistance are the two main factors affecting space weathering effects (particularly the formation of the amorphous layer). These results suggest that baddeleyite is a stable phase under space weathering conditions when compared with other lunar minerals. This work also provides mineralogical evidence for the existence of natural space weathering-resistant materials on the Moon.

Sedimentary texture, facies, and architecture can be used to reconstruct the palaeoenvironment under which sediments accumulate. On Mars, palaeoenvironmental reconstructions are used to understand ancient Martian atmospheric circulation, surface sedimentary processes, and the presence of water, to provide insight into ancient habitability and climate evolution. Exploration of the Stimson formation at the Naukluft plateau, Gale crater, by the MSL rover Curiosity yielded evidence of aeolian dunes recorded within the strata, and that the Stimson formation unconformably overlies the older, lacustrine Murray formation. Analysis of the strata revealed a hierarchy of bounding surfaces forming compound cross-bedding, indicating two scales of bedform: primary dunes (draa) and superimposed dunes. Cross-set dip-azimuths indicate a broad sediment transport direction toward the north, aligning with the north or north-east transport direction of previous work. Paired with textural analysis, the outcrops of the Naukluft plateau are interpreted to have formed within the central section of a large, dry aeolian dune field. The formation of concretions, veins and alteration halos, formed by diagenesis or fracturing associated with sub-surface fluids, coincide with the lithification of the Stimson formation. This hints toward an extended presence of water within Gale crater but at a much lower abundance than earlier during the crater fill sequence. The unconformity denotes a large climatic transition from the lacustrine Murray formation to dry aeolian Stimson formation, reflecting long-term drying of the ancient environment. Such observations enhance the understanding of the ancient Martian environment, allowing reconstructions of atmospheric conditions, climate and habitability on a regional scale.

The forthcoming MMX (Martian Moon eXploration) mission will carry the MIRS instrument (MMX Infrared Spectrometer) to study the composition of the Martian system. On airless bodies, like Phobos and Deimos, micrometeorite bombardment can strongly modify the spectral properties of their surface materials. To better identify the effects of micrometeorite bombardment on infrared spectroscopy data, we have simulated, using nanosecond pulse-laser irradiation experiments, micrometeorite impacts on minerals and rock relevant for the surface composition of Phobos and Deimos based on our current knowledge. Laser irradiation was performed on pure mineral phases (i.e., nontronite, antigorite, biotite, goethite) and basalt, and their reflectance spectral properties were acquired between 0.5 and 3.6 μm. Our results reveal that laser impacts can reduce the strength of specific absorption bands characteristic of these minerals. In addition, laser impacts can also significantly modify the position of specific absorption bands, especially for phyllosilicates. Iron features originally near 0.65–0.75 μm, and the $��\sim �$2.8 μm $��H_2�$O/OH bands are the most affected. They can shift up to a few tens of nanometers toward shorter or longer wavelengths, depending on the sample. The overlap between the iron band positions (e.g., between altered antigorite and pristine nontronite) suggests that space weathering can distort the accurate identification of minerals using infrared spectroscopy. Finally, we observed that the signature of antigorite subjected to micrometeorite bombardment is close to the signature observed on Phobos with CRISM data. Therefore, we believe that this mineral should be seriously considered as a potential constituent of the Phobos regolith.