Journal of Geophysical Research: Planets最新文献

Alkaline earth sulfides are possibly abundant in the mantle of Mercury, and knowledge of their melting and transport properties is needed to investigate the structure of the planet. We report electrical experiments at pressures in the range 2–5 GPa and at temperatures up to ∼2,400 K on proposed analogs of natural sulfides, that is, Ca_1-xMg_xS with minor impurities. Electrical conductivity increases nonuniformly with temperature with no systematic dependence on cation composition. At relatively low temperatures (near 1,100 K), the conductivities span a wide range, whereas at higher temperatures the values converge within ∼0.5–7 S/m at 1,800 K and 5 GPa. The conductivity trends are complex, and likely reflect contributions from divalent cations, alkali metal and carbon impurities, which would similarly contribute to the conductivity of Mercury's crust and mantle. Melting is identified by a sharp increase in conductivity between ∼1,850 and 2,100 K at 5 GPa. These transition temperatures are consistent with the presence of impurities. Using electrical studies on relevant silicate minerals and petrological observations, we developed electrical conductivity-depth profiles of Mercury's silicate portion. Depending on the interconnectivity of the sulfide phase, the conductivity at the base of the mantle containing 8 vol.% sulfide ranges from ∼0.2 to >8 S/m. Our results can be tested with future observations from the ESA-JAXA BepiColombo mission.

Hollows are small, shallow, irregularly shaped landforms, widespread across Mercury, interpreted to have formed via loss of volatiles. Here, we present the first global analysis of hollow degradation states using a new machine learning-derived global catalog. We define three classes, grading from younger/potentially active “Stage 1” (sharp morphology, high visible reflectance) to older/potentially expired “Stage 3” (softened morphology, reflectance similar to that of surroundings). Most analyzed hollows are Stage 1 (N = 1,545 individual hollows), which are more common than Stage 2 (N = 1,111) or Stage 3 (N = 10) hollows near the equator, consistent with the idea that insolation is a primary driver for hollow initiation/growth. Areas where Stage 2 hollows are more common than Stage 1 hollows may indicate regions of relative volatile depletion. Stage 3 hollows are rare, suggesting they are systematically missed during image review, or that hollows on Mercury are mostly young, have been recently reactivated, or are quickly erased once they become inactive. Temperature may limit hollow growth, given that only small hollows are identified in the coldest terrains. There is no meaningful difference in the distribution of hollow sizes between stages, suggesting that their morphological and reflectance properties are not substantially muted until they are fully grown. Stage 1 hollows are more commonly found on steeper slopes than nearby Stage 2 hollows, suggesting that slopes may be an important control on how long hollows remain active. Our hollow classifications are openly available and can help to inform global-scale studies of hollow evolution, and upcoming targeting efforts by the ESA/JAXA BepiColombo mission.

Submicroscopic metallic iron particles (SMFe) are unique components of lunar soil produced during long-term exposure on the Moon's surface. They can significantly alter the optical properties of lunar soil and this alteration is crucial for the interpretation of remote sensing data. The origin and formation of SMFe remain a subject of controversy, with multiple competing mechanisms coexisting. The newly returned Chang'e-5 (CE-5) samples provide a new opportunity to elucidate the formation of SMFe. Here, we conducted a systematical study on the morphology and chemical characteristics of metal globules in CE-5 impact melt splash. A total of 30,630 metal globules were identified with an average diameter of 222.87 nm. Most of them are nearly/perfectly spherical, but the others are irregular in shape. Three types of irregular metal globules have been found: Spindle type, deformation type, and coalescence type. Spindle and deformation types were formed under the influence of local thermal disequilibrium and/or differences in wettability, while the coalescence type reflects the growth of metal globules driven by the Oswald ripening. A series of metal globules at different coalescence stages were found, providing conclusive petrographic evidence for the long-term hypothesis of SMFe growth (e.g., Pieters & Noble, 2016, https://doi.org/10.1002/2016je005128). Geochemical analysis shows that meteoritic Fe-Ni metals (like iron meteorite) made a significant contribution to the formation of metal globules. This further indicates the contribution of exotic meteoroid materials to the CE-5 lunar soil.

Tectonic structures such as wrinkle ridges, lobate scarps, small-scale graben, and tectonic pits reveal the recent lunar activity and complex deformational processes. Despite numerous studies, the northeastern region of the Mare Serenitatis basin, including the Posidonius crater, has yet to be studied in detail. This research presents a comprehensive analysis of tectonic structures in this region, revealing ∼808 km of wrinkle ridges, ∼286 km of lobate scarps, ∼346 km of small-scale graben, and ∼269 tectonic pits. We identified 412 craters deformed by wrinkle ridges, 55 craters by lobate scarps, and 108 craters by small-scale graben, suggesting extensive recent deformation. Chronological analysis of wrinkle ridges and lobate scarps revealed young ages ranging from $��{�\sim �29�}_{�-�9�}^{�+�10�}�$ Ma to ∼120 ± 30 Ma close to the crater, whereas Posidonius crater resurfaced floor age tend to $��\sim �{2.8}_{�-�0.5�}^{�+�0.4�}�$ Ga. Our study suggests that the blind thrust fault deformed the western floor of the Posidonius crater, with small-scale graben with pits plausibly developed during the reactivation. Orthogonal transitions that occur between adjacent wrinkle ridges and lobate scarps in the study area could be a splay fault of the blind thrust fault and likely formed during the reactivation. This reactivation plausibly resulted from a combination of recession stresses, diurnal tidal stresses, and global contraction. Additionally, a combination of complex processes─intrusion, subsidence, and tectonics associated with the blind thrust fault plausibly influenced Rima Posidonius. Overall, this study suggests that the northeastern region of the Mare Serenitatis basin witnessed recent tectonic activity and could be a potential site for future exploration missions.

The flapping motion of the current sheet is a common dynamic phenomenon in the planetary magnetosphere and plays an important role in the transportation of energy and disturbances. Based on the measurements from the Cassini spacecraft, we investigate the short-period flapping motions of the current sheet characterized by periods significantly much smaller than the planetary rotation cycle in Saturn's magnetosphere. Employing the Minimum Variance Analysis (MVA) method, a new technique is developed to distinguish the propagation of current sheet flapping in the radial and azimuthal directions. A total of 105 short-period current sheet flapping events have been detected in Saturn's magnetosphere. The global spatial distribution of these events is provided, and their respective propagation directions have been identified. Further discussions are conducted on the potential sources of the short-period current sheet flapping in Saturn's magnetosphere, including magnetic reconnection, Titan, fast plasma flow, planetary period oscillations, and instabilities near the magnetopause.

Recent numerical modeling and the detection of an Earth analog advanced our understanding of the formation mechanisms behind Europa's distinctive double ridges. Notably, a recent contribution by Cashion et al. (2024, https://doi.org/10.1029/2023je008007), demonstrated that ice wedging as a primary formation mechanism can accurately reproduce the topographic characteristics of double ridges. Building on previous theoretical foundations laid by Melosh and Turtle (2004, https://ui.adsabs.harvard.edu/abs/2004LPI….35.2029M/abstract), it hints at a complex hydrological system within the ice shell, characterized by local reservoirs and cycles of freezing that could also explain other observed geological formations, such as lenticulae and chaos terrain. This dynamic suggests the possibility of material exchange processes within the ice shell of Europa and enhances our understanding of the moon's habitability.

Crater chronology functions are used to estimate absolute surface ages using the number density of observed craters. The calibration of chronology functions is done using crater counts on regions with radiometrically dated samples. Both the Neukum Production Function (NPF) and Robbins Production Function (RPF) contain lunar chronology functions anchored by Apollo and Luna samples that have measured absolute ages and are known to be from surfaces with measured crater densities. However, these functions were constructed using different data and assumptions for their respective crater counts, and their functions differ in the rate of decline during the Imbrian period (3–3.9 Ga). In particular, the RPF suggests that the decline in impact rate was much more rapid than the decline suggested by the NPF. We use a numerical impact bombardment model called CTEM to track the production and transportation of impact melts. Using CTEM, we simulated the bombardment history of the Moon under each chronology function, and calculated the age distribution of impact melts mixed in the top meter of regolith at locations corresponding to the Apollo 14–17 landing sites. These results were then compared to the age distribution of Apollo impact melts. We find the rapid decline suggested by the RPF to be a better match to the age distribution of Apollo impact melts than the gradual decline of the NPF.

Chang’E-6 successfully completed the return of samples from the South Pole-Aitken (SPA) basin on the Moon, and future Chang’E-7 and Artemis missions will continue to focus on the lunar south polar region. FeO and TiO₂ are the primary chemical components of the lunar surface, and determining their contents in the south polar region is essential for investigating the geology and volcanism of this region. This work remapped the global FeO and TiO₂ content using Clementine data combined with updated lunar sample measurements. The new models for FeO and TiO₂ achieve determination coefficients (R²) of 0.921 and 0.817, and root mean square errors (RMSE) of 1.38 wt.% and 1.17 wt.%, respectively. The addition of sample measurements from Chang’E-3 and Chang’E-5 has resulted in higher estimates for FeO content in the Procellarum KREEP Terrane (PKT) and the SPA, as well as lower estimates for TiO₂ content in the maria. Furthermore, we present the Chang’E-6 landing zone in the PKT to illustrate the geological implications of the new oxide content maps. Estimates of surface FeO and TiO₂ content at the Chang’E-6 sampling site are close to recent sample measurements and are consistent with the characteristics of the young maria unit in the landing zone. Our results provide valuable information on FeO and TiO₂ content in the lunar south polar region, which will aid in the future analysis of Chang’E-6 samples and contribute to geological investigations in the region.

The Jau crater cluster on the north flank of Mt. Sharp in the Gale crater consists of 20–30 simultaneously formed impact structures. The craters are ∼2 to ∼25 m across (generally increasing in size from the ESE to WNW). Using Digital Terrain Models derived from orbital and rover orthomosaics of two of the larger craters, we evaluated the amount and processes of degradation based upon current versus expected original crater morphology. We conclude that the craters in the cluster are the result of a fragmenting primary impactor and that predominantly aeolian degradation at these two craters is responsible for ∼1–2 m wall back-wasting, 10s of cm of infilling by aeolian drift, and 20–40 cm rim lowering. Other craters in the cluster experienced similar degradation whose expression varied according to crater size: smaller-scale morphology at smaller craters is more modified by a given amount of erosion. Prevailing winds erode the downhill, NNE side of the craters more though raised rims persist at the larger craters. An estimated vertical erosion rate of 0.1 m/Myr and a horizontal erosion rate of several m/Myr predicted by others appear most consistent with the Jau crater morphology and setting relative to erosion estimates for elsewhere in Gale crater and other Mars landing sites. Factoring in uncertainties in our estimates, these rates are broadly consistent with the scale of erosion inferred for the evolution of larger, older features on Mt. Sharp and suggest that the cluster-forming impact occurred one to a few and less than ∼5 Ma.