Journal of Geophysical Research: Planets最新文献

Large topographic rises on Venus are regions thought to be formed in response to the presence of a mantle plume or mantle upwelling, equivalent to hot spots on Earth. In this work, we study the geology and evolution of one of these large topographic rises, Imdr Regio, based on geologic mapping and analysis of geophysical data of the area. Imdr Regio presents a complex structure with two very different areas: (a) an elevated southeast area that is dominated by volcanism associated with Idunn Mons, a large volcano that has been proposed as a site of recent or even active volcanism; (b) another elevated area in the northwest area that also has a large volcano (Arasy Mons), but that is dominated by volcanism and tectonic activity associated with the formation of the Olapa Chasma rift system. These two very differentiated topographically elevated areas also exhibit differences in their geology, volcanic and tectonic style, and geophysical characteristics, which leads us to suggest that more than the classic volcano-dominated rise classification attributed to Imdr Regio the area could rather be considered as an intermediate or hybrid volcano-rift dominated large topographic rise. The evaluation of the different genetic scenarios and its correspondence with the observed geology in the area suggests that the complex geology of Imdr Regio could be better explained if we consider models of hot spot evolution that involve the presence of several mantle plumes or secondary upwellings derived from a mantle plume emplaced at a deeper rheological boundary.

Venus' surface and interior dynamics remain largely unconstrained, due in great part to the major obstacles to exploration imposed by its 470°C, 90 bar surface conditions and its thick, opaque atmosphere. Flyby and orbiter-based thermal emission data provide opportunities to characterize the surface composition of Venus. However, robust interpretations of such data depend on understanding interactions between the planet's surface basaltic rocks and its caustic carbon dioxide (CO₂)-dominant atmosphere, containing trace amounts of sulfur dioxide (SO₂). Several studies, using remote sensing, thermodynamic modeling, and laboratory experiments, have placed constraints on basaltic alteration mineralogy and rates. However, constraints on the effects of SO₂-bearing reactions on basalts with diverse compositions remain incomplete. Here, we present new data from a series of gas-solid reaction experiments, in which samples of two basalt compositions were reacted in an SO₂-bearing CO₂ atmosphere, at relevant Venus temperatures, pressure, and oxygen fugacity. Reacted specimens were analyzed by scanning electron microscopy and scanning transmission electron microscopy using sample cross-sections produced with focused ion beam milling. Surface alteration products were characterized, and their abundances estimated; subsurface cation concentrations were mapped to show the depth of alteration. We demonstrate that the initial development of reaction products progresses rapidly over the course of 30-day runs. Alkaline basalt samples are coated by Na-sulfate (likely thenardite, Na₂SO₄) and amorphous calcium carbonate (CaCO₃) alteration products, and tholeiitic basalt samples are primarily covered by anhydrite (CaSO₄), Fe-oxide (Fe_xO_y: likely magnetite, Fe₃O₄), and other minor phases. These mineralogies differ from previous experiments in CO₂-only atmospheres.

Rampart craters are a class of lakes or depressions in Titan's north polar region that have morphological attributes suggestive of an explosive origin. Two previous studies have proposed that rampart craters form via nitrogen or methane vapor explosions analogous to terrestrial maar explosions. We propose a new terrestrial analog for rampart craters: gas emission craters (GECs) found in permafrost zones. We evaluate the explosive origin of Titan's rampart craters by modeling the dispersal of material from an explosive vent. The dimensions of nine rampart craters with radar-bright ramparts were used to model the explosion process. The model yields a range of explosion conditions (e.g., gas mass and reservoir depth) producing ejecta dispersal patterns matching the observed features. We find that gas masses of 10¹¹–10¹⁴ kg are required to produce a rampart crater. We examine two explosion scenarios: (a) rapid, maar-like vaporization and explosion of liquid nitrogen or methane, and (b) more gradual gas accumulation and explosion akin to a GEC driven by methane released from destabilizing clathrates. If Titan's crust is composed of pure water ice, the calculated gas pressures are consistent with a rapid, maar-like explosion mechanism. If the subsurface is predominantly composed of organic materials or clathrate, either scenario may be plausible. Further research on the composition and tensile strength of Titan's subsurface are required to discriminate between hypotheses. Nevertheless, we conclude that explosive dispersal of ejecta from a vent can account for the morphologies of Titan's rampart craters and may contribute to atmospheric methane replenishment.

The Tropical Cloud Oscillation (TCO) in the Martian atmosphere is a shift of clouds in the northern spring and summer tropical cloud belt between the eastern and western hemispheres on an intra-seasonal timescale of about 10–40 sols. The TCO is a significant intraseasonal variation and may strongly affect the Martian general circulation, water cycle, and dust cycle. We examine TCOs using multiple data sets with a focus on the clouds observed in Mars Daily Global Maps during Mars Year (MY) 29–35. One or more TCO cycles are observed in each MY and the phenomenon is most prominent during L_s = 135°–185°. Space-time spectral analysis shows a variety of waves which appear to follow the theoretical dispersion relationships of equatorial waves, such as Kelvin waves, Rossby waves, and Mixed Rossby Gravity waves. The TCO appears to be controlled by zonal wavenumber one traveling waves with Kelvin and Rossby wave characteristics and exhibits a fine-scale latitudinal structure that requires modeling with sufficient resolution. Issues with current data assimilation products for use in studies of Martian equatorial waves due to this fine-scale structure are discussed.

Apatites record crucial information on the origin, composition, and chemical evolution of volatiles on terrestrial planets. As a martian intrusive rock, the gabbroic shergottite Northwest Africa (NWA) 13581 provides key information on the volatile evolution related to magmatic processes in the interior, shedding light on the intricate volatile circulation on Mars. The textural and chemical characteristics of the phosphates in NWA 13581 indicate a complex formation history involving fractional crystallization, degassing, and fluid interaction. Degassing of the NWA 13581 parent melt is capable of exsolving chlorine-rich fluids, resulting in the formation of notably fluorine-rich apatite with a high x-site occupancy of fluorine up to 90%. The degassed/exsolved volatile-rich fluids could subsequently continue to migrate and interact with surrounding magmatic suites, leading to highly heterogeneous compositions of active fluids. The crystallization of apatite is initiated by the interaction of fluids with merrillite at the late stage of the magmatic process, leading to the formation of phosphate intergrowths. Influenced by the composition and chemical evolution of volatiles in fluids and melts, apatite exhibits notable variability in chlorine compositions both within individual grains and among different grains. Moreover, the presence of magnetite associated with phosphate intergrowth highlights the transportation of metallic components in addition to volatiles from deep layers to shallower depths or to the surface of Mars. This process, which is observed in young shergottites, indicates the persistent presence of hydrothermal systems until recent geological periods, contributing to the generation and circulation of volatiles within the martian interior and on the surface.

Iron hydrides are a potentially dominant component of the metallic cores of planets, primarily because of hydrogen's ubiquity in the universe and affinity for iron. Using ab initio molecular dynamics, we examine iron hydrides with 0.1, 0.33, 0.5, and 0.6 mol fraction hydrogen up to 100 GPa between 3,000 and 5,000 K to describe how hydrogen content affects the melt structure, hydrogen speciation, equation of state (EOS), atomic diffusivity, and melt viscosity. We find that the addition of hydrogen decreases the average Fe–Fe coordination number and lengthens Fe–Fe bonds, while Fe–H coordination number increases. The pair distribution function of hydrogen at low pressure indicates the presence of molecular hydrogen. By tracking chemical speciation, we show that the amount of molecular hydrogen increases and the number of iron in H_x≥1Fe_y≥0 clusters decreases as the hydrogen concentration increases. We parameterize a pressure, volume, temperature, and composition EOS and show that the molar volume and Grüneisen parameter of the melts decrease while the compressibility and thermal expansivity increase as a function of hydrogen concentration. We find that hydrogen acts as a lubricant in the melts as the iron and hydrogen become more diffusive and the melts become more inviscid as the hydrogen concentration increases. We estimate 2.7 wt% hydrogen in the Martian core and 0.49–1.1 wt% hydrogen in Earth's outer core based on comparisons to seismic models, with the assumption that the cores are pure liquid iron-hydrogen alloy, and we compare the small exoplanet population with mass-radius curves of iron hydride planets.

Morphological characteristics were measured for barchan dunes on Earth (2,686 dunes in 30 barchan fields) and Mars (720 dunes in 10 barchan fields) using satellite images. The data were used to (a) develop a new barchan classification system; (b) compare characteristics of barchans on Earth and Mars; and (c) assess whether barchans, in bulk, display allometric or scale-invariant characteristics. Dimensional metrics were obtained for the width and length of barchan bodies, the width and length of barchans including the horns, and the length of each horn. Dimensionless metrics were derived for the ratios of the body width to the width between the tips of the horns (width ratio), the length of the entire barchan to the length of the body (length ratio), and the length of the longer horn to the shorter horn (symmetry ratio). The width, length, and symmetry ratios were used to classify barchans into eight types and compare the characteristics of their distributions on the two planets. From this analysis, it was established that, statistically, barchans on Earth are distinctive from those on Mars based on the morphometrics, with terrestrial barchans being, on average, of smaller size and more often symmetrical, while Martian barchans more often have convergent horns that are short relative to the central dune body and are more often asymmetrical. The analysis further reveals that barchan planform morphology can be considered scale-invariant, and we argue that body width is the most appropriate measure representing barchan size.

The lithosphere of Mars accommodates horizontal shortening through folding and faulting, producing landforms described as wrinkle ridges or lobate scarps. Despite this nomenclature, we lack a deep understanding of the drivers of morphological differences observed between landform types. This study aims to develop a quantitative model for shortening landform classification based on surface morphology, subsurface architecture, and strain accommodation, facilitating interpretations of where and how lithospheric stresses are recorded. We developed this model by mapping 100 shortening landforms in a Geographic Information System, recording 12 unique geomorphic parameters such as length and asymmetry, and estimating the strain of each landform. We conducted a Discriminant Function Analysis (DFA) using surface morphometrics. This DFA produced a predictive linear function for categorizing wrinkle ridges and lobate scarps and for quantifying which landforms were exemplars within those categories. The three most influential variables on the surface morphometry DFA were the maximum width, forelimb slope, and back limb length. We then modeled the subsurface structural geology of 50 landforms using MOVE Structural Geology Modeling Software and conducted a second DFA based on subsurface metrics. DFA was most influenced by the dip and depth of the lower ramp base. When both surface morphology and subsurface geometry are input into single DFA, wrinkle ridges and lobate scarps can be distinguished quantitatively 96% of the time. Our results also show that lobate scarps accommodate more strain and imply that studies should consider landform type when interpreting local, regional, and global geological stress histories.