We investigated the local-time dependence of dust event formation around Thaumasia Plateau of Mars using 3,298 images acquired by the Emirates eXploration Imager (EXI) from Ls = 7° in MY 36 to Ls = 43° in MY 37. Through visual inspection of paired images separated by several hours, we determined formation times for small-scale dust events and distinguished them from water ice clouds using 320 nm water ice optical depth retrievals. Dust events occurred throughout the year but exhibited clear seasonal differences in their preferred formation times. During the southern winter, dust events were more likely to form in the afternoon, whereas during the southern spring and summer, several events initiated in the morning, particularly within the mountain ranges surrounding the plateau. Accounting for seasonal variations in observation opportunities, we found that morning dust lifting dominates in the southern spring and summer, while afternoon formation dominates in the southern winter. These results suggest that different mechanisms operate seasonally: morning events may be associated with the breakdown of nocturnal low‑level jets, while topographically driven slope winds also likely enhance dust lifting in the plateau. This study highlights the unique capability of the non-sun-synchronous EMM orbit to constrain Martian dust storm formation timing.
Dynamic weakening of rock friction at coseismic slip rates has been extensively studied in laboratory experiments using centimeter-scale samples. However, the potential scale dependence remains poorly understood. Recent studies suggest that the sample-size effect exists and may be explained by thermoelastic instability (TEI). In thermoelastic media, concentrated frictional heating and thermal expansion enhance normal stress heterogeneity. If this positive feedback dominates over diffusive heat transfer, the heterogeneity grows spontaneously, locally triggering dynamic weakening. The critical slip rate V_cr for TEI is proportional to the wavenumber and thus inversely proportional to sample size. Here, TEI coupled with temperature-weakening friction is investigated through numerical simulations employing a newly developed method for a simplified two-dimensional periodic system. The periodic length is interpreted as the sample size, and simulation conditions are chosen to be comparable to those in laboratory experiments. When the nondimensional slip rate V/V_cr is smaller than unity, the behavior is independent of sample size and characterized by gradual weakening due to nearly uniform temperature rise. When V/V_cr exceeds unity, TEI causes growing heterogeneity, but dynamic weakening does not necessarily occur by the end of the experiments. It becomes significant only when partial contact opening develops. In this regime, macroscopic frictional resistance is strongly correlated with the macroscopic contact fraction. At sufficiently large slip rates and cumulative slip, only a small fraction of the fault remains in contact, accompanied by intense localization of temperature rise and normal stress. Although V_cr depends inversely on sample size, the onset of dynamic weakening exhibits much weaker size dependence for sufficiently large systems, indicating the existence of a limiting length scale for the sample-size effect. This behavior is explained by the fact that the TEI growth rate is maximized at a wavenumber that is independent of the sample size: when the system is large enough to accommodate this mode, it dominates the evolution irrespective of sample size, whereas smaller systems are controlled by size-limited modes. These results suggest the existence of a representative length scale for constitutive description, providing a possible basis for upscaling laboratory friction laws to natural faults, although experimental validation is required.
Mercury undergoes the largest diurnal temperature variations in the Solar System, with surface temperatures oscillating from below 100 K at night to above 700 K at local noon. These extreme cycles, driven by the planet’s 3:2 spin–orbit resonance and strong solar irradiance, have profound implications for regolith evolution, surface modification processes, and the interpretation of remote-sensing data. In preparation for the BepiColombo SIMBIO-SYS observations, we developed a global thermophysical model of Mercury that solves the diurnal and seasonal evolution of surface and near-subsurface temperatures using a one-dimensional heat-conduction scheme coupled with MESSENGER topography. From these simulations, we derived planet-wide maps of maximum surface temperature T_max , diurnal temperature amplitude Δ T , longitudinal gradients of Δ T , and a thermal stress index S_therm = Δ T / t_day that quantifies the mean rate of temperature cycling over one Hermean solar day. The resulting products reproduce the characteristic two-lobed thermal structure imposed by the spin–orbit resonance, with peak T_max and Δ T at the hot longitudes and a smooth decrease toward the poles, and identify belts of enhanced horizontal thermal gradients. Building on these global fields, we further quantify how local surface morphology modulates the thermal environment by correlating T_max with slope, roughness, hillshade, and aspect derived from the global Mercury Digital Elevation Model. Pixel-by-pixel Pearson coefficients and a multiple linear regression show that these morphological parameters together explain only ∼ 4 T_max , indicating that latitude and insolation geometry overwhelmingly dominate the diurnal thermal regime, while local relief and roughness provide only a weak, second-order modulation. The thermophysical maps and tools presented here provide a quantitative framework for the scientific exploitation of SIMBIO-SYS data, supporting observation planning and the interpretation of future high-resolution imaging and spectroscopic datasets.
Satellite laser ranging (SLR) is a highly sophisticated observational technique that entails substantial installation and operational costs among space geodetic technologies. Despite these costs, SLR remains indispensable for determining fundamental geodetic parameters, including the Earth’s center of mass. Omni-SLR is a newly developed SLR concept aimed at increasing the global number of SLR observation sites by reducing system size and cost through a compact and affordable design. This paper describes the timing measurement technology implemented in Omni-SLR versions A25.02 and A25.09 released in February and September 2025 and presents its performance evaluation for the nighttime ranging. Demonstration experiments conducted at the Ishioka Geodetic Observatory (IGO) indicate that the timing measurement unit achieves data quality comparable to that of conventional, high-cost SLR systems. Furthermore, because Omni-SLR is applicable to optical communication technologies, such as frequency comparison, these evaluation results also provide useful guidance for the development of related emerging technologies.
Solar extreme ultraviolet (EUV) radiation heats and expands the thermosphere, modulating atmospheric density at altitudes in low Earth orbit and thereby increasing satellite drag and accelerating orbital decay. Previous studies have relied on empirical solar proxies, such as the F10.7 cm radio flux and Two-Line Element (TLE)-derived orbit solutions, which lack sufficient spectral resolution and temporal cadence to resolve wavelength-dependent thermospheric responses, and on orbital-decay metrics—most commonly the satellite altitude itself—that are not conserved quantities along the orbit and have rarely been validated against independent-density measurements. Here, we combine spectrally resolved EUV irradiance from the GOES-18 Extreme Ultraviolet and X-ray Irradiance Sensors at seven wavelengths (25.6–140.5 nm) with high-cadence GNSS position and velocity data from NinjaSat, an X-ray-astronomy 6U CubeSat orbiting at approximately 500–530 km altitude, over an 8-month period (April–November 2024). We derive the orbital-decay signal from the J_2 -corrected specific mechanical energy computed directly from the GNSS position and speed telemetry—a conserved quantity whose within-orbit scatter is about 50 times smaller than that of an altitude-based (point-mass energy) decay metric and which, unlike TLE/SGP4-based approaches, requires no propagated reference orbit. This energy-based decay rate correlates with independent thermospheric-density retrievals from the Swarm and GRACE-FO missions with Spearman coefficients of 0.85–0.92 (daily means), and its systematic-error budget (radiation pressure, unmodeled gravitational perturbations) remains below the statistical uncertainty of the daily decay rate. Cross-correlation analysis shows that the decay rate is strongly related to EUV and far-ultraviolet (FUV) irradiance (Spearman coefficients up to ∼ 0.8), with a positive lag of order 1–3 days. Using a block-bootstrap procedure that accounts for the strong autocorrelation of both series, we find that the 95
