Journal of Geophysical Research: Planets最新文献

Mantle flow throughout the lunar evolutionary history could be constrained by shear wave splitting if it is reliably observed in the moonquake waveforms. However, despite the long-standing availability of Apollo seismic data, it remains unclear whether the S-waves from moonquakes exhibit real splitting due to seismic anisotropy or whether apparent splitting arises instead from seismic wave propagation through isotropic heterogeneous media. To date, no systematic investigation has addressed this question. To fill this knowledge gap, we analyze direct S-wave splitting from deep moonquakes with high signal-to-noise ratios. Stacked waveforms from deep moonquakes show that the measurement results are inconsistent across different clusters at the same station and across different stations for the same cluster as well as across different frequency bands. Further analysis of unstacked individual events within the same cluster, differing in location by only a few kilometers, also exhibits large variability in the splitting parameters. Numerical simulations of seismic wave propagation through heterogeneous media demonstrate that the observed S-wave splitting of deep moonquakes can be explained by apparent splitting caused by scattering in addition to the potential intrinsic anisotropy. Therefore, even stacking waveforms of deep moonquakes do not improve the reliability of the splitting measurements. Our results suggest that there are no remnants of large-scale lateral mantle flow in the lunar mantle sampled by these deep moonquakes, or any existing anisotropy is obscured by the stronger effects of structural heterogeneity.

Mercury's exosphere is sustained by the continuous ejection of atoms from its surface, driven by solar wind, micro-meteoroid impacts, and surface heating. Due to its 3:2 spin-orbit resonance, some longitudes experience greater solar exposure, creating temperature variations from ∼90 to 700 K. This resonance also creates least exposed longitudes, called cold longitudes. These variations, combined with surface-solar interactions, lead to complex exospheric dynamics. Observations from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft revealed localized enhancements in sodium column density at cold longitudes near the aphelion, where these regions rotate into the day-side. Existing models do not explain this cold-pole enhancement but assume a smooth, impermeable surface, neglecting the highly porous nature of Mercury's regolith. This porosity allows for subsurface diffusion of volatiles through adsorption and desorption across regolith grains, influenced by temperature and species-specific surface binding energies. We couple a subsurface transport model with a 3D Monte Carlo-based Exosphere Global Model to investigate whether sodium accumulated in Mercury's regolith could explain the cold pole enhancement. Results suggest that at cold longitudes, low temperatures favor sodium retention near the surface. The gradual heating induces a release of sodium from the subsurface, producing the observed localized enhancements. This mechanism reconciles MESSENGER's findings with physical processes and highlights the significance of subsurface reservoirs in volatile dynamics. We highlight the need to consider regolith structure and subsurface processes in exosphere modeling. These results improve our understanding of Mercury's volatile cycle but also offer broader insights into the behavior of surface-bound species on airless planetary bodies.

The Mars 2020 Perseverance rover introduced Raman spectroscopy to in situ planetary exploration for the first time when it landed in Jezero crater on Mars in February 2021. The SuperCam instrument onboard Perseverance is a multi-analytical tool capable of acquiring time-resolved Raman data from Martian targets at standoff distances of a few meters. This is a particularly challenging task due to the operational constraints, the harsh conditions on the Martian surface, and especially the very fine-grained nature of the Martian soil. To address these challenges, the SuperCam Raman team has invested significant effort into optimizing both the acquisition and post-processing of Raman data collected on Mars, as detailed in this work. Additionally, this paper reviews and discusses the detections made by SuperCam Raman during the first 1,000 sols (almost 3 Earth years) of the Mars 2020 mission. During this period, SuperCam Raman data provided key insights into the mineralogy of Jezero throughout the Crater, Delta, and Margin Campaigns. Key detections include olivine, carbonates, perchlorates, and sulfates (such as anhydrite), identified in both abraded patches and natural surfaces. The high specificity of Raman spectroscopy enables the unequivocal identification of these minerals, allowing for rapid and direct interpretation of Jezero's mineralogy, especially when combined with other techniques from SuperCam or others on the rover. Furthermore, this paper compiles the spectra acquired from the SuperCam Calibration Target samples on Mars, including studies on the degradation of the Ertalyte (PET), an organic polymer sample and analyses of diamond, apatite, and other reference materials.

Earth's particular style of plate-tectonics—characterized by localized deformation along dynamic plate boundaries and long-lived stable plate interiors—appears to be unique among rocky objects in the solar system. However, it is entirely unknown how common plate tectonics and related lithospheric phenomena are among the vast population of exoplanets discovered astronomically or assumed to exist throughout the Universe. In this study, we explore the effect of planetary composition on mylonitization—a set of microphysical processes that is commonly associated with shear localization and plate boundary deformation on Earth. A model for planet compositions, based on stellar spectroscopy, is used to define a plausible range of theoretical mineral abundances in the mantles of rocky Earth-sized exoplanets. These mineral abundances, along with experimental rock rheology, are used to model microphysical evolution with two-phase mixing. The model is then used to determine the effect of composition on the time-scales for shear zone formation. We demonstrate that lithospheres composed of sub-equal proportions of two mineral phases will form shear zones over relatively short time-scales, a more favorable condition for forming Earth-like plate boundaries. In contrast, lithospheres that are nearly monomineralic may require unrealistically long time-scales to form plate boundary shear zones. Using this approach, we identify specific nearby stars with the optimal range of compositions to be targeted by future astronomical missions, including the Habitable Worlds Observatory.

Hydrogen chloride (HCl) was recently discovered in the Mars atmosphere using the ESA's ExoMars Trace Gas Orbiter (TGO) onboard ESA's ExoMars mission. Its discovery is the first confirmation of an active presence of any chlorine-bearing species in the modern Mars atmosphere. TGO permitted investigations of HCl altitude profiles with high precision and showed that water vapor and ice clouds play an important role in the production and temporary loss of HCl. TGO cannot always sample the Martian atmosphere near the surface, and when those measurements are possible, they are highly affected by the increase in dust opacity, nor can TGO observe at equatorial latitudes with high cadence, due to orbital constraints, so its measurements are not suitable to obtain instantaneous global coverage. In this work, we present a methodic investigation of the Martian atmosphere, in support of the ExoMars TGO mission, targeting HCl and water using iSHELL at NASA/InfraRed Telescope Facility. Our observations mapped the Martian atmosphere, exploring three seasons in Martian Year 36. We observed the beginning of an increase in the HCl abundances around L_S = 249°–301°, followed by a drop in the abundances around L_S = 319°. We confirmed a strong correlation between the spatial distribution of water vapor and HCl—both globally and locally—suggesting that water vapor plays an important role in the production of HCl, in agreement with previous studies. Our observations also suggest the presence of two competing processes involving the dust, one supporting HCl production and another one contributing to its destruction.

The discovery of rounded, river-transported clasts near an alluvial fan on Mars by the Mars Science Laboratory (MSL) Curiosity rover provided the first ground-based evidence for sustained fluvial activity on Mars. However, questions remain about the paleoenvironment and provenance of these clasts. Clast shape and size are two quantitative metrics that can be used to reconstruct the history of sedimentary deposits. However, previous studies have not considered martian basaltic lithologies and thus may not be applicable to Mars. We use data from two alluvial fans in the Mojave Desert and a series of rotating drum experiments to quantify the difference between fluvial abrasion of basalt and granite to determine the effect of lithology on the relationship between clast roundness and transportation distance. We find that basalt rounds faster than granite both in the field and in the lab and that this effect is more pronounced in the field. Our rounding coefficient (p_c) values show the agreement between granitic clasts in the lab (0.028 ± 0.0008) and the field (0.029 ± 0.003 and 0.034 ± 0.007) as well as the faster rounding rate we see from the field (0.048 ± 0.001) than experimental basalt (0.039 ± 0.001). This may be due to the greater susceptibility of basalt to chemical weathering. When applying our relationship to rounded clasts observed in the Gale crater, we find that our transport values agree with previous interpretations that point to a sediment provenance near the crater rim.

The exploration of lunar lava tubes can address challenges of human habitation on the Moon by identifying natural shelters against radiation, temperature extremes, and micrometeoroid impacts. This study shows the feasibility of detecting lunar lava tubes with gravimetry using Ape Cave in Washington State, USA, as a terrestrial analog. Ape Cave's unique features, including its 3.5 km length, irregular geometry, and minimal surface expression, make it an ideal model for simulating lunar conditions. A high-resolution 3D model of the cave enabled the creation of forward gravity models, incorporating density and geometric variations of realistic terrestrial and lunar conditions. These models show significant negative gravity anomalies that closely fit field measurements along transects above Ape Cave, validating their use in predicting anomalies from lava tubes with comparable dimensions and complexity. Extending this to the Moon, two forward models simulated lunar lava tubes: one with Ape Cave's dimensions and another scaled to five times larger. Results show that while lunar anomalies are weaker due to lower surface gravity, instruments with resolutions of <25 μGal can nevertheless reliably detect tubes even at burial depths reaching >26 m. However, larger and more realistic lunar lava tubes produce detectable signals even with lower reading resolution instrument thresholds. This study underscores the benefit of integrating analog models with advanced gravimetric technologies for lunar exploration and highlights the importance of refining sensor capabilities to optimize the detection of lunar subsurface features. These findings contribute to the development of geophysical exploration strategies for future lunar missions to identify habitable environments.

We use data from the Jovian InfraRed Auroral Mapper (JIRAM) instrument on the Juno spacecraft, acquired during close fly-bys of Jupiter's moon Io during 2022–2024, to identify new characteristics of active lava flows and changes since observations from previous missions (Voyager, Galileo, and New Horizons). At least two new vents as well as a new flow field are identified along with some significant changes seen at other sites. Our new JIRAM observations of Culann, Tvashtar, Daedalus, Kanehekili, Masubi, Amirani, Prometheus, and Girru show that volcanism continues at these sites since previous missions, in some cases suggesting long duration (>45 years) eruptions. A large (∼130 × 40 km) active flow field was detected to the NE of Seth Patera as well as a probable new ∼120 km long active flow at a previously unrecognized site designated “Unknown 13.” Flows were also detected by JIRAM at Quzah, Xihe, and a large, recently emplaced flow was observed in the south polar region at Illyrikon Regio. Comparison with locations of plumes detected by Juno's camera shows that plumes coincide with distal margins of flow fields where brightness temperatures are high, indicating Promethean-type flow and plume activity. The spatial distribution of hotspots on most of the observed flows is consistent with earlier suggestions that the flows are inflated pāhoehoe-type flow fields, although activity at Kanehekili and Quzah Flucti may well display characteristics of higher-discharge rate, ‘a‘ā, eruptions.