Journal of Geophysical Research: Planets最新文献

The morphology of fresh lunar craters contains information about the physical properties of both the impactors and the lunar surface, and is therefore crucial to our knowledge of the impact cratering process. Spectral analysis is a powerful tool to study crater morphology, as it can reveal the topographic variation on different scales. In this study, we calculate the power spectral densities of the radial distance and elevation of the rim crest, floor, and rim flank outlines of fresh lunar craters. The resulting power spectral density can be decomposed into an average component and a natural variability component. For the average component, we derive the classic morphometric parameter-crater diameter relations that are consistent with previous studies. For the natural variability component, we find that in general the spectral power increases with wavelength, which can be fitted by a piecewise function with four breakpoints. Among the four breakpoints, the power of the third breakpoint (i.e., the degree-2 power) is of particular interest, as it determines the ellipticity of the outline. The power of the third breakpoint is found to have a diameter dependence with a peak at 20 km, which indicates that transitional craters are more elliptical than simple and complex craters. The diameter dependence of the power spectral density enables us to generate the synthetic outlines of a crater of a particular size, which can be used to develop a preliminary 3-dimensional shape model for fresh lunar craters that is useful for improving Monte Carlo modeling of cratered surfaces on the Moon.

Space weathering records provide insights to better understand the formation and evolution of the lunar regolith. Ilmenite has contrasting responses to different space weathering processes. However, the atomic-scale structural modification of ilmenite induced using different space weathering processes remains poorly understood. Here, we investigate the effects of spacing weathering on lunar ilmenite grains returned from Chang'e-5 (CE-5) mission using a combination of transmission electron microscopy and thermodynamic modeling approaches. Experimental results show that melt shock induces the formation of twining structures and vein-like Si-Ca-rich nanostructures in the outermost and sub-outermost layers of ilmenite, respectively. In contrast, solar wind causes the formation of multilayered nanostructures surrounding the ilmenite grains. These structures are characterized by an outermost amorphous Si-rich vapor deposited layer, a middle layer rich in titanium (Ti) oxides and zero-valent iron (Fe⁰) nanoparticles, and an innermost layer hosting crystallographic orientation defect. The Ti oxides were identified as poorly crystallized anatase. Thermodynamic calculations indicate that the disruptive sputtering of solar wind and the reduction of hydrogen under lunar surface pressure conditions can promote ilmenite transformation into Fe⁰ and Ti oxides; nevertheless, the pressure increase associated with melt shock can lead to a rise in the decomposition temperature of ilmenite. In other words, solar wind irradiation plays a more significant role in promoting nanoparticle (such as anatase and Fe⁰) formation as compared to melt shock. Thus, unlike the chemical alteration of ilmenite induced by the solar wind irradiation, melt shock mainly causes physical changes in ilmenite grains.

Crater counting is a widely applied methodology for dating large areas of planetary surfaces, but is difficult to apply the method to constrain the durations of stratigraphic unconformities. Unconformities with exhumed craters are thought to indicate long hiatuses that can only be indirectly dated through stratigraphic relationships with other surfaces with uniform exposure ages. On Mars, sedimentary deposits with prominent unconformities with exhumed craters are found in layered deposits in the Arabia Terra region as well as Gale crater within Mount Sharp. In this work, we present a Linear Crater Counting methodology and apply it to constrain these unconformities observed in Arabia Terra and in Mount Sharp. The method applies a linear sampling domain correction to conventional two-dimensional crater size frequency distributions and Bayesian Poisson process statistics in order to constrain the likely durations of these unconformities. We found that unconformities in Arabia Terra were on the order of 0.1–1 Gyr in length and that the unconformity preserved at Mount Sharp is at least 0.2 Gyr in length given estimates of the ages of the host craters. Hiatuses of these lengths constrain the age of the overlying deposits to be Late Hesperian or Amazonian in age. Two utility plots are also provided, along with the derivation, for researchers to apply this method to dating arbitrary geologic contacts on Mars and to adapt it to other bodies.

The terrestrial planetary bodies display a wide variety of surface expressions and histories of volcanic and tectonic, and magnetic activity, even those planets with apparently similar dominant modes of heat transport (e.g., conductive on Mercury, the Moon, and Mars). Each body also experienced differentiation in its earliest evolution, which may have led to density-stabilized layering in its mantle and a heterogenous distribution of heat-producing elements (HPE). We explore the hypothesis that mantle structure exerts an important control on the occurrence and timing of geological processes such as volcanism and tectonism. We numerically investigate the behavior of an idealized model of a planetary body where HPE are assumed to be sequestered in a stabilized layer at the top or bottom of the mantle. We find that the mantle structure alters the patterns of heat flow at the boundaries of major heat reservoirs: The mantle and core. This modulates the way in which heat production influences geological processes. In the model, the mantle structure is a dominant control on the relative timing of fundamental processes such as volcanism, magnetic field generation, and expansion/contraction, the record of which may be observable on planetary body surfaces. We suggest that Mercury exhibits characteristics of shallow sequestration of HPE and that Mars exhibits characteristics of deep sequestration.

The Mars 2020 Perseverance rover discovered fine-grained clastic sedimentary rocks in the “Hogwallow Flats” member of the “Shenandoah” formation at Jezero crater, Mars. The Hogwallow Flats member shows evidence of multiple phases of diagenesis including Fe/Mg-sulfate-rich (20–30 wt. %) outcrop transitioning downward into red-purple-gray mottled outcrop, Fe/Mg clay minerals and oxides, putative concretions, occasional Ca sulfate-filled fractures, and variable redox state over small (cm) spatial scales. This work uses Mastcam-Z and SuperCam instrument data to characterize and interpret the sedimentary facies, mineralogy and diagenetic features of the Hogwallow Flats member. The lateral continuity of bedrock similar in tone and morphology to Hogwallow Flats that occurs over several km within the western Jezero sedimentary fan suggests widespread deposition in a lacustrine or alluvial floodplain setting. Following deposition, sediments interacted with multiple fluids of variable redox state and salinity under habitable conditions. Three drilled sample cores were collected from this interval of the Shenandoah formation as part of the Mars Sample Return campaign. These samples have very high potential to preserve organic compounds and biosignatures. Drill cores may partially include dark-toned mottled outcrop that lies directly below light-toned, sulfate-cemented outcrop. This facies may represent some of the least oxidized material observed at this interval of the Shenandoah formation. This work reconstructs the diagenetic history of the Hogwallow Flats member and discusses implications for biosignature preservation in rock samples for possible return to Earth.

The projected evolutionary history of the Moon and observed occurrence of moonquakes suggest that brittle faulting is present in the shallow lunar crust. The main component of the lunar crust, plagioclase, shows velocity-strengthening behavior in the range of crustal temperatures. Chang'e 5 samples of lunar regolith show a mineral composition almost identical to basaltic bedrock. We measured the friction-stability characteristics of dry synthetic gouges representative of basaltic faults assumed to be present in the lunar crust. Frictional strengths are ∼0.7 and exhibit an overall velocity-strengthening response but transition to velocity-weakening at intermediate temperatures (∼200–300°C) and stresses (∼25–100 MPa). Bounding temperature profiles representative of the lunar crust suggest that moonquakes are feasible in the lunar crust. The rheological heterogeneity of mineral fragments in basalt is a potential cause of unstable sliding on faults with the related steady-state stress drop close to the minimum of the estimated dynamic stress drop. This suggests that some events with small stress drops are associated with the instability of mature basalt faults. However, observations of shallow moonquakes with high stress drop but merely moderate magnitude suggest that high degrees of healing on immature faults, small seismic nucleation lengths, or the failure of intact crust are present. We emphasize that moonquakes may arise from stress transfer and accumulation due to processes such as cooling contraction.

Accurate thermoelastic properties and thermal conductivity are crucial for understanding the thermal evolution of the Martian core. A fitting method based on ab initio calculated pressure-volume-temperature data was proposed for the formulation of the equation of state with high accuracy, by which the pressure and temperature dependent thermoelastic properties can be directly calculated by definitions. Ab initio results showed that Fe_0.75S_0.25 liquid under Martian core conditions was thoroughly in a magnetic state without existing spin crossover. The Fe_0.75S_0.25 liquid in magnetic calculations had a low thermal conductivity (21–23 W/m/K) when compared with non-magnetic calculations at the same state. Based on Insight's estimated Martian core properties (Stähler et al., 2021, https://doi.org/10.1126/science.abi7730) and ab initio calculated properties of the Fe_0.75S_0.25 liquid, the scenario for the thermal evolution of the Martian core is the iron-snow model crystallization regime. The parameter uncertainty effect on the cessation time of the dynamo and zone of iron snow was systematically analyzed.

The Emirates Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission (EMM) Hope probe images Mars at wavelengths extending from approximately 100 to 170 nm. EMUS observations began in February 2021 and cover over a full Mars year. We report the first limb scan observations at Mars of ultraviolet emissions Ar I 106.6 nm, N I 120 nm, and carbon monoxide (CO) Fourth Positive Group (A − X) band system excited by electron impact on CO. We use EMUS limb scan observations to retrieve number density profiles of argon, molecular nitrogen, atomic oxygen, and CO in the upper atmosphere of Mars from 130 to 160 km. CO is a sensitive tracer of the thermal profile and winds in Mars' middle atmosphere and the chemistry that balances CO₂ in the atmosphere of Mars. EMUS insertion orbit special observations demonstrate that far ultraviolet limb measurements of the Martian thermosphere can be spectroscopically analyzed with a robust retrieval algorithm to further quantify variations of CO composition in the Martian upper atmosphere.