Journal of Geophysical Research: Planets最新文献

We irradiate fine-grained regolith pure water ice in ultra high vacuum with $���5��\text{keV}��$ and $���10��\text{keV}��$ electrons to study the radiolysis of water ice. The ice regolith is designed to closely mimic the physical characteristics of the surfaces of the icy moons of the Solar System. We analyze the species sputtered by the electron irradiation using a time-of-flight mass spectrometer, recording the mass spectra before and during irradiations. We sublimate the ice with high-flux irradiations to release the trapped species. We find that oxygen and ozone are produced and stored in the ice. The ozone to molecular oxygen ratio is consistent with the observations of the surface of Ganymede. Since there are no other requirements than electron irradiation and water ice regolith, we expect ozone production to be widespread in the icy bodies in the Solar System.

As hydrous minerals have been observed in impact craters on Mars, impact-generated hydrothermal systems (IGHSs) have been considered as potential habitats for life on that planet. The Vargeão Dome, a 12 km wide impact structure in southern Brazil, was formed in basalts with at least two hydrothermal alteration stages. This structure is a rare terrestrial analog for IGHS evolution on Mars. However, the thermochemical evolution of these stages and their relationship to the impact remain unresolved. Two vein-forming alteration stages were identified by multidisciplinary sample analysis of Vargeão Dome. Fractured and deformed white vein fragments from the first stage occur within red veins from the second. This suggests reactivation of the white vein set during the crater excavation and modification stages. Rare Earth Element and Rb-Sr isotope data indicate different fluid sources for the white and red veins and an evolving fluid system. Thermodynamic modeling indicates a cooling sequence from 100–250 to <25°C for the white vein set, whereas goethite and hematite within the red veins indicate that most IGHS activity occurred at temperatures below 40°C. Considering this and accounting for gravity differences, life-supporting IGHSs on Mars may preferentially form impact structures over 28 km in diameter. Furthermore, impact-reactivation suggests elevated habitat potential in structures with pre-existing fault, fracture, and vein systems. These occur in volcanic areas or terrain older than 3.5 Ga, when Mars was wetter and geologically active. Therefore, the search for evidence of IGHS-supported life should be focused on these kinds of terrains.

Caloris basin on Mercury has a massive circular bulge topography imprinted with unique fault sets. A variety of deformational processes have been proposed to have influenced their formation, including both global contraction and basin evolution, as well as the deposition and weight of the volcanic infill. However, the relationship between these processes and the amount of influence that each has had on the present-day morphology is not yet constrained. Through a combination of elevation and gravity data, we analyzed faults and subsurface structures of the basin and its surroundings. Analyses revealed spatial relationships between interior grabens and the basin's topography, putative buried rings, and evidence that areas of the basin rim are controlled by faults of the surrounding external plains. The flexure surface to the circular bulge was computed, providing the maximum weight load (4.48 × 10¹⁸ N) required to produce the topography. In this scenario, late doming during deposition at the basin center formed radial grabens, followed by degassing and compaction. This led to increased density at the basin center, which altered the isostatic equilibrium and encouraged a flexural response, in turn producing a circular bulge and concentric grabens around a central depression. Regional warping from global contraction remains a possible influence; however, deformation of the basin's perimeter shows inconsistencies with the locations of long-wavelength folds from previous studies. Contemporaneously with the flexural response, southeasterly striking faults from west of Caloris and reactivated basin bounding faults from a relic basin at the northern border have altered the rim elevation and geometry.

The formation of gullies on Mars has often been attributed to the melting of (sub)surface water ice. However, melting-based hypotheses generally overlook key processes: (a) sublimation cooling by latent heat absorption, (b) the non-stability of ice where melting conditions can be reached, and (c) the particular microclimates of gullied slopes. Using state-of-the-art climate simulations, we reassess ice melting scenarios over the past 4 million years (obliquity $��\le �$35°), beyond the estimated period of gully formation. We find that the melting of opaque water snow or ice at the surface of Mars is unlikely anywhere due to sublimation cooling, while (quasi-) stable subsurface ice is typically too deep to reach melting temperatures. We propose an alternative mechanism in which seasonal $��{\text{CO}}_2�$ frost sublimation destabilizes the regolith and brings the underlying water ice close to the surface, allowing rapid heating. Even under these optimal conditions, melting requires unrealistic assumptions. Ice containing a small amount of dust could melt via a solid-state greenhouse effect, but both its possibility and frequency in Mars' recent past remain uncertain.

Surface temperature is a key component of the Martian climate system, modulating energy and momentum exchange between the atmosphere and surface. However, temporal shifts in the local time of Mars Climate Sounder (MCS) observations complicate comparisons of interannual variations. In this paper, we correct the observation-time bias in MCS-derived surface temperature records from Mars Years (MY) 29 to 36 using the Mars Climate Database. After correction, a statistically significant nighttime surface warming trend and tentative daytime warming are detected over the 8-year period. A multiple linear regression analysis reveals that variations in atmospheric dust opacity are the dominant driver of surface temperature variability, with dust, albedo, and thermal inertia (TI) together explaining 45.0% (29.1%–76.4%) of the daytime and 68.3% (55.4%–80.6%) of the nighttime global temperature variance. To assess the spatial response to each forcing, Martian surface temperature variations are simulated using the Mars Planetary Climate Model under variable surface albedo and TI scenarios. Simulations that incorporate surface property changes better reproduce observed spatial patterns, particularly during the daytime. Attribution using the optimal fingerprinting method shows that daytime warming can be explained primarily by surface albedo (0.52 K) and dust (0.24 K) changes, with TI exerting a slight cooling effect (−0.24 K). These findings emphasize the importance of correcting observational artifacts and highlight the roles of surface and atmospheric processes in recent Martian climate variability.

Distributions of trace hydrocarbons in the atmosphere of Titan possess a strong seasonality. In mid-winter, hydrocarbons are thought to be able to escape through the polar vortex and develop “tongues” of enriched air which spread equatorward from the poles. However, recent studies have shown that the mixing barrier associated with a polar vortex of similar structure to the ones found on Titan should still be strong enough to isolate these hydrocarbons. Here, we examine the horizontal mixing associated with the polar vortices throughout their lifetime by using simulations of passive chemical tracers and contour advection for two Titan-specific three-dimensional (3D) general circulation models. Our analysis reveals that during the fall, the mixing barrier associated with the vortex extends above ∼0.1 hPa and isolates the stratospheric polar regions, allowing for strong enrichment by mesospheric descent. During winter, however, the vertical extent of the polar vortex is reduced due to the winter weakening, opening a region directly above the polar vortex (∼1–0.1 hPa) where tracer-depleted air is transported poleward, locally reducing tracer concentrations in the upper stratosphere and creating an enriched tracer region below in the lower-mid stratosphere. The presence of an extremely broad mixing barrier within and below the polar vortex (>1 hPa) would highly limit the movement of material equatorward and may indicate that previous observations of hydrocarbon tongues on Titan may be due to the influx of material above the polar vortex rather than escape across the vortex; further work is needed to verify this theory.

Degradation of crater topography is important on planetary surfaces as it can record environmental conditions through rates and processes of erosion. Regolith creep is classically thought to dominate crater wall degradation, leading to smooth crater walls. Processes that create rough topography, such as chutes and alcoves, are often attributed to volatiles. Here we explore an alternate hypothesis for chute formation by erosion from dry rockfall. We mapped the western rim of Endeavour crater, Mars, including the Marathon and Perseverance valleys visited by NASA's Mars Exploration Rover (MER) Opportunity. Marathon Valley is a broad alcove with locally steep, rubbly outcrops, a moderately sloping (18°) bedrock floor dissected by a network of shallow grooves and boulders downslope. When initiated from these steep (>45°) outcrops, rockfall modeling shows focused impacts near the headwall of Marathon Valley and rocks that traverse the valley floor even at gradients below the angle of repose. Perseverance Valley is a smaller chute with relief that is too subtle to be captured in the digital elevation model, and therefore the model does not produce funneled rockfall there. Across the Endeavour crater rim, the rockfall-erosion hypothesis is consistent with locally steep rock outcrops as rockfall sources, boulder fields, and the rough chute-and-spur topography. We propose that through topographic steering, rockfall can be funneled into chutes and alcoves, concentrating impacts and erosion there, and further developing these landforms in the absence of flowing water.

Using observations from the Mars Climate Sounder (MCS) and images from the Mars Color Imager (MARCI) onboard the Mars Reconnaissance Orbiter (MRO), this study identifies a suppression of north polar ice cap expansion before the northern fall equinox (Ls ∼160°) in Martian Year 36 (MY36) compared to the same season in other years. This reduction coincides with an earlier-occurring tropical regional dust storm (Z storm). Simulations using the Mars Planetary Climate Model (PCM), along with a control experiment that excludes the Z storm, show an unusual decrease in atmospheric water vapor in the northern polar region, indicating that the ice cap anomaly is linked to dust storm–induced changes in atmospheric circulation and water transport. The Mars PCM simulations capture the key features of atmospheric temperature and water vapor changes during the Z storm period. To understand how the Z storm influenced polar water vapor, we conducted two PCM simulations for MY36: one representing the actual dust storm conditions and another representing a control experiment without the storm. Dynamical analysis showed that tropical dust activity excited anomalous planetary waves, which propagated poleward and affected the polar background circulation. As a result, circulation patterns in the lower polar atmosphere, including eastward zonal winds and meridional flows, were significantly disrupted, substantially reducing poleward water vapor transport to the northern polar region. These results provide new insights into the role of dust-related climate processes in modulating polar ice accumulation, contributing to a deeper understanding of the mechanisms driving long-term climate evolution on Mars.

Impact cratering redistributes geologic materials on up to global scales. On the Moon, cratering events contribute to the mixing of maria and the highlands. To better understand the role of impact ejecta in the redistribution of materials, we developed an analytical method to estimate the largest ejecta fragment sizes that could have been delivered to previous sample collection sites from selected primary impacts into lunar maria targets across the Moon. The ejecta fragments would not remain intact upon impact but would form secondary craters and be further eroded and mixed into the regolith. Here, we use maximum ejecta sizes as a proxy for the relative possible contributions of each assessed primary crater. All of the 45 primaries considered here (0.83–138 km in diameter) could potentially contribute sizable maria fragments to each landing site. The larger (>65 km diameter) and closer (<2,000 km distance) primaries resulted in the largest estimated fragment sizes (∼0.5–2.5 km), whereas the smaller (<65 km) and more distant (>2,000 km) primaries resulted in the smallest fragment sizes (up to ∼250 m). Depending on spatial proximity, both smaller and closer and larger and more distant primaries could also deliver relatively large (∼0.2–0.8 km) ejecta fragments. Our results provide context to understand how maria material may have become incorporated into distant surfaces and lunar sample locations.