Journal of Geophysical Research: Planets最新文献

The Baltis Vallis channel on Venus preserves a record of long-wavelength deformation generated by a convecting mantle, providing a unique window into the planet's geodynamics. Notably, the observed topography along the channel is not downhill, suggesting complex interactions between surface processes and mantle dynamics. We statistically compare the observed dynamic topography of Baltis Vallis with dynamic topographies generated by a suite of stagnant-lid mantle convection models to constrain Venus' interior dynamics. Baltis Vallis's relatively young age (likely 250 Myr) and low root-mean-square relief of 217 m indicate vigorous convection in Venus's mantle, with a Rayleigh number $��>�$4 $��\times �$ $��{10}^8�$, implying a mantle viscosity 1–2 orders of magnitude lower than Earth's. This difference may result from either a water-rich, less-degassed interior or a higher-temperature mantle beneath an insulating lid. Additionally, our simulations suggest that melt advection may dominate heat transport on Venus, potentially leading to non-linear temperature profiles in the crust. Upcoming missions such as VERITAS and EnVision will deliver higher-resolution gravity and topographic data, providing further constraints on Venus's present-day internal dynamics and the origin of Baltis Vallis.

We present six newly identified examples of lobate ejecta in Mercury's south polar quadrangle (H15), providing the first evidence of syn-impact formation through our observation of perched impact melt at Nairne and Magritte craters. This finding challenges the idea that lobate forms could have developed post-impact via mass wasting or landslide processes, suggesting instead that many, if not all, lobate ejecta deposits formed contemporaneously with the impact event. We present detailed morphostratigraphic maps of two exemplary case studies: Nairne and Magritte, for which we used shadow measurements to better constrain the morphology of their lobes. Many examples globally, including Nairne, have been formed by deposition of the lobe material into a topographically lower antecedent crater. While we confirm that topography plays a significant role in the formation of lobate ejecta, it cannot be the sole factor, as similar impacts onto pre-existing craters do not always produce these features and not all lobate ejecta exhibit evidence for a topographic control. Our study also highlights the frequent association between lobate ejecta and characteristics of oblique impacts. Considering analogous features at Mars and the Moon in particular, lobate ejecta on Mercury is likely the result of either fluidized or dry granular flows, with minimal delay between impact and emplacement. Our findings suggest that lobate ejecta are more widespread on Mercury than previously recognized, and future studies and missions, such as a comprehensive global survey with higher resolution data from BepiColombo, will continue to constrain their formation mechanisms and prevalence.

Evidence shows that lava tubes on the Moon, which constitute intriguing targets for exploration and long-term habitation, could be much larger than terrestrial tubes. However, existing observation strategies cannot confidently constrain the exact dimensions of lunar tubes in anticipation of in situ exploration. We produced and analyzed numerical models of the elastic gravity-induced deformation field around lunar lava tubes to determine where failure may occur on their internal surfaces and how the lunar surface above them may deform. This analysis suggests ways to determine tube dimensions from lunar surface morphological observations. The models predict extensive failure on the tube floors and roofs, leaving relatively small sections of pristine floor near the tube walls even if the tube itself may not collapse. The most extensive type of internal failure, tensile floor failure, depends on the tube shape. Additionally, the lunar surface around lava tubes develops cracks and subtle topographic bulges running parallel to the tube axis at distances proportional to the tube width. If observed, these could be used to determine the tube's width remotely. We examined several lunar sinuous rilles using orbital data to search for the surface features we expect near tubes. Linear cracks consistent with the presence of lava tubes are present on the possible extensions of three rilles: Rimae Mairan, Sharp, and Marius. Other locations did not show similar features, possibly because of the resolution of available data, tube depths, or surface degradation. However, surface or low-altitude observations would be more sensitive to the expected structures.

Mercury has a global magnetic field that is unique in our solar system. It is dominated by a weak axial dipole and a substantial axial quadrupole contribution. The field has a dipole tilt which is smaller than one degree and an offset of the magnetic equator toward north by about 20% of the planetary radius. The only dynamo model that succeeds in continuously reproducing these features without having to rely on an unrealistic heat-flux pattern through the core-mantle boundary is a double-diffusive model by Takahashi et al. (2019), https://doi.org/10.1038/s41467-018-08213-7, where thermal effects cause a thick, stably stratified layer in the outer parts of the core. In this study we further show the sensitivity of this model to the choice of parameters. In the explored parameter set, the model yields solutions which are unrealistic for Mercury when convection becomes vigorous. We present a new single-diffusive model, that captures Mercury's field characteristics during long periods that are interrupted by dipole field reversals causing the magnetic equator to switch between the northern and southern hemisphere. The Mercury-like solutions are stable over a broader range of parameters. An important ingredient for the success is the fact that dynamo action sets in with the equatorially symmetric magnetic field family.

In photometric modeling, disk function plays a crucial role in describing scattering behaviors related to the local incidence and emission angles of an observed surface. Finding a proper disk function is important for normalizing spectra observed under different observing geometries. This study evaluates the performances of five different disk functions, including the Lommel-Seeliger function, and functions proposed by Minnaert, McEwen, and Akimov, using bidirectional reflectance distribution function (BRDF) data of lunar-type minerals and Apollo lunar soils. BRDF measurements of olivine, orthopyroxene, plagioclase, and ilmenite were conducted to examine the effects of mineralogy and particle size. Our results indicate that the Akimov empirical function is most effective in reducing discrepancies among data measured under varied incidence and emission angles for both pure minerals and lunar soil samples. Two empirical functions for the free parameter q in this model were generalized based on pure mineral data, but are unsuitable for lunar soil samples. The q derived for six Apollo lunar soil samples are much lower than those for pure silicate minerals but align well with remote sensing observations, with no distinct differences between lunar mare and highland soil samples, possibly due to extensive space weathering on the lunar surface. The q values at a phase angle of 70° show the strongest correlation with the widely used maturity index I_s/FeO, and two empirical functions between I_s/FeO and q(70°) for bands 550 and 750 nm were established, which may be used to quantify the degree of space weathering.

The objective of this study is to examine the climatology of the residual mean circulation, and the roles of wave forcings by both resolved waves (RWs) and unresolved waves (UWs). The analysis is performed using data from the Ensemble Mars Atmosphere Reanalysis System (EMARS) over four Mars Years without global dust storms, based on the transformed Eulerian mean equation theory. While the RW forcing is estimated directly as Eliassen-Palm flux divergence, the forcing by UWs, including subgrid-scale gravity waves, is estimated indirectly using the zonal momentum equation. This indirect method, originally devised for studying the Earth's middle atmosphere, is applicable to latitudinal ranges where angular momentum isopleths are continuous from the surface to the top of the atmosphere, typically mid- and high-latitude regions. In low latitudes of the winter hemisphere, a strong residual mean flow toward the winter pole is observed in a pressure range between ∼20 and ∼0.5 Pa (∼30–60 km), where the latitudinal gradient of the absolute angular momentum is small. The strong poleward flow crosses the isopleths of angular momentum in the regions of its northern and southern ends, indicating the necessity of the wave forcing. Our results suggest that the structure of the residual mean circulation at mid- and high-latitude regions is largely determined by UW forcing, particularly above ∼2 Pa level, whereas the RW contribution is also significant below the ∼2 Pa level.

The Nadir and Occultation for MArs Discovery spectrometer on board Trace Gas Orbiter began science operations in April 2018, providing infrared and ultraviolet-visible spectra of the Martian atmosphere. This paper explores the application of the UVIS channel in solar occultation to study aerosols. We have developed a retrieval scheme that allows us to study the size and extinction of the aerosol as a function of altitude. Results from mid-MY 34 to the end of MY 36 are reported and discussed. Particle size is retrieved using a Mie code with log-normal distribution with an effective radius (r_eff), 0.1–0.8 μm and an effective variance (v_eff) equal 0.1. In this work, we show the presence of aerosol-detached layers, characterized by a local increase in particle size and extinction. These detached layers can be composed of dust, H₂O ice, or CO₂ ice. CO₂ ice clouds can be detected up to 80 km, while H₂O ice clouds are usually more present around 30–50 km. Symmetry of the particle effective radius between the northern and southern regions is observed. During northern/southern winter, the atmospheric aerosols are composed of smaller particles below 0.6 μm but in summer, the size increases to micron-sized particles that are larger than the sensitivity of UVIS. This increase in size in the northern/southern summer is not correlated with a similar increase in extinction. Our data support the previously observed relationship between water vapor and aerosol, especially the formation of high-altitude water ice cloud during the global dust storm event.

We present the detection and characterization of mesoscale waves on the lower clouds of Mars (20–40 km) using hyperspectral images from the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) onboard the European Mars Express space mission. We used image navigation and processing techniques based on contrast enhancement and geometrical projections to semi-manually detect and manually characterize morphological properties of the detected waves, such as horizontal wavelength or packet length. Our study covers 3 Martian years, spanning from January 2004 (Mars Year 26) to January 2010 (Mars Year 29). We detected 263 wave packets, of which we characterized 125, revealing an average horizontal wavelength of 21 km, with detected waves spanning horizontal wavelengths between 6 and 83 km. Wave activity exhibited spatial and temporal variability, with larger wave packets concentrated in the northern hemisphere and most detections occurring during daytime. Seasonal patterns revealed higher wave activity during northern spring and autumn and southern winter, linked to regional topography, atmospheric density perturbations, and diurnal heating cycles. These findings provide insights into Martian atmospheric gravity waves and demonstrate the OMEGA data set's value for future studies of Mars's atmospheric dynamics.

Volcanism is the primary endogenic geological process on the Moon, with mare basalts being the critical indicators of such activity. The abundance of titanium in these mare basalts reflects different lunar mantle sources, magma origins, and thermal evolution processes. The Chang'e-4 mission, which landed in the Von Kármán crater within the South Pole-Aitken Basin on the farside of the Moon, employed a lunar penetrating radar with a low-frequency channel to reveal the subsurface structures and abundance of FeO and TiO₂ of these structures down to approximately 300 m. The dielectric properties and iron-titanium content are crucial for interpreting basalt units. This study utilizes radar wave-impedance inversion to derive the permittivity from the low-frequency lunar penetrating data collected during the first 62 lunar days. The frequency shift method was used to calculate the loss tangent and estimate the abundance of FeO and TiO₂ in the subsurface materials at the landing area. Based on permittivity, variations in abundance of FeO and TiO₂, and impact crater statistics near the landing site, we infer at least four distinct periods of magma with varying FeO and TiO₂ beneath the Chang'e-4 landing site, spanning from the Nectarian to the Eratosthenian periods. High titanium and medium titanium basalts were found in the Nectarian and Imbrian periods. This study reveals the evolutionary process of multi-phase volcanic eruptions on the farside of the Moon, indicating not only temporal but also compositional variations in volcanic activities, thereby highlighting the complexity of volcanic processes on the lunar farside.