Solar system

Abstract:

Abstract Laboratory spectral analysis of well‐characterized meteorite samples can be employed to more quantitatively analyze asteroid remote sensing data in conjunction with returned extraterrestrial samples. In this work, we examine the combined effects of physical (temperature, particle size) and chemical (petrologic type, metal fraction) variables on visible and near‐infrared (VNIR) spectra of ordinary chondrite meteorite powders. Six equilibrated ordinary chondrite meteorite falls were prepared at a variety of particle sizes to capture the spectral diversity associated with asteroid regoliths dominated by various grain sizes. Mineral compositions and abundance were determined from electron microprobe analysis of meteorite thick sections to precisely characterize changes in spectral features due to variations in mineralogy. VNIR spectra of the ordinary chondrites were measured under simulated asteroid surface conditions at a series of temperatures chosen to mimic near‐Earth asteroid surfaces. The resulting spectra show minimal variation in both major absorption bands across the simulated near‐Earth asteroid temperature regime. Changes in particle size result in variations in band centers and band area ratios for material of the same composition, two key parameters typically used to derive asteroid composition. Unlike previous spectral investigations of ordinary chondrites, we retained the metal fraction in our powders instead of analyzing only the silicate fraction. Metal has a subtle but non‐negligible effect on the VNIR spectra of ordinary chondrites. The more petrologically pristine samples from each ordinary chondrite group display relatively weaker absorption bands than their more thermally altered counterparts. The band centers shift to longer wavelengths as grain size and petrologic type increase. Plain Language Summary Remote interpretation of asteroid composition can be complicated by the physical (grain size, temperature) and chemical (mineral composition and abundance) properties of the asteroids themselves. To aid in understanding the effects of these variables and improve our remote interpretation of asteroid compositions from their spectra, we have systematically evaluated the effects of these variables on the visible‐near‐infrared spectra of well‐characterized asteroid samples (e.g., ordinary chondrite meteorites) measured in a simulated asteroid environment. Our results show that while the effect of near‐Earth asteroid temperatures on the spectra appears to be minimal, changes in particle size can mimic changes in spectral band parameters that are typically attributed to composition. It is therefore essential to account for particle size when interpreting composition from silicate‐dominated asteroid spectra. Key Points Near‐Earth asteroid temperatures (∼10–100°C) have a minimal effect on the visible and near‐infrared (VNIR) spectra of olivine and pyroxene dominated ordinary chondrite meteorites Particle size is a critical variable to account for when trying to derive composition from remotely sensed asteroid VNIR spectra Metal content has a subtle but non‐trivial effect on the VNIR spectra of ordinary chondrites

Abstract:

Plain Language Summary: In hot and wet “hothouse” climate conditions, rainfall transitions from a pattern that fluctuates from about a mean of 3 mm day − 1 ${\text{day}}^{-1}$ to more intense outbursts that are separated by multi‐day dry spells. Previous studies on hothouse climates did not consider the role of the diurnal cycle even though it strongly controls precipitation in Earth's current climate. This study uses radiative‐convective equilibrium simulations to investigate the impact of rising temperatures on the transition to hothouse conditions, incorporating the diurnal cycle with both swamp‐like and open ocean surface conditions. We find that episodic precipitation occurs at surface temperatures above 322 K even when accounting for the diurnal cycle. However, the diurnal cycle significantly influences the timing of convection and rainfall at high temperatures with precipitation primarily starting late at night or in the early morning.

Abstract:

Recent James Webb Space Telescope observations of cool, rocky exoplanets reveal a probable lack of thick atmospheres, suggesting the prevalent escape of the “secondary” atmospheres formed after losing primordial hydrogen. Yet, simulations indicate that the hydrodynamic escape of secondary atmospheres, composed of nitrogen and carbon dioxide, requires intense fluxes of ionizing radiation (X-ray and extreme ultraviolet (XUV)) to overcome the effects of high molecular weight and efficient line cooling. This transonic outflow of hot, ionized metals (not hydrogen) presents a novel astrophysical regime ripe for exploration. We introduce an analytic framework to determine which planets retain or lose their atmospheres, positioning them on either side of the cosmic shoreline. We model the radial structure of escaping atmospheres as polytropic expansions—power-law relationships between density and temperature driven by local XUV heating. Our approach diagnoses line cooling with a three-level atom model and incorporates how ion–electron interactions reduce the mean molecular weight. Crucially, hydrodynamic escape onsets for a threshold XUV flux depend upon the atmosphere’s gravitational binding. The ensuing escape rates either scale linearly with XUV flux when weakly ionized (energy limited) or are controlled by a collisional–radiative thermostat when strongly ionized. Thus, airlessness is determined by whether the XUV flux surpasses the critical threshold during the star’s active periods, accounting for expendable primordial hydrogen and revival by volcanism. We explore atmospheric escape from the young Sun Mars and Earth, LHS 1140 b and c, and TRAPPIST-1 b. Our modeling characterizes the bottleneck of atmospheric loss on the occurrence of observable Earth-like habitats and offers analytic tools for future studies.

Abstract:

The recent discovery of strong tidal dissipation in Saturn’s interior has radically changed our view of the Saturnian system. While some questions are naturally answered by the new paradigm, others are emerging and require further measurement. This article presents the next key questions to be addressed by future space missions and analysis. Suggestions for space measurements to discriminate between different scenarios concerning the formation, evolution and internal state of the Saturnian system are given.

Abstract:

Abstract Intermittently sunlit areas near the lunar south pole are estimated to harbor thermal conditions permitting long‐term stability of water ice and other volatiles. They are targets for future science and exploration missions due to the combination of sunlight availability for solar power generation, and the possibility for extraction of volatiles for scientific analysis and ISRU. We construct a geodatabase of spatially co‐registered remote sensing and thermal model results, and perform a probabilistic analysis to determine the likelihood of successfully landing and operating on such locations for a quadrangular study area that bounds the 80°S parallel. In addition to water ice thermal stability, we consider factors relevant for the operation of solar‐powered landed spacecraft: visibility to the Earth, visibility to the sun, and local slope. For two scenarios representing sets of most‐ and least‐constrained landing site requirements, we find that circular landing ellipse diameters of ∼0.9 and 2.6 km, respectively, would allow to target available compliant terrains with 100% success. We quantify the reduction in success probability with increasing landing ellipse size. Further, we explore the distributions of geometric properties of compliant areas, and identify three sites of interest that support large areas of compliant terrain: near De Gerlache crater, near Shackleton crater, and Mons Mouton (informally named as Leibnitz‐β massif). This study is provided to support planning for future lunar missions. Plain Language Summary Researchers have identified areas near the lunar poles that receive occasional sunlight and could keep water ice and other resources stable over a long period of time. These spots are valuable for future lunar missions since they could provide solar power and possibly resources such as water for scientific study and on‐site use. To assess potential landing sites in the south polar region, we created a database combining remote sensing and thermal data set, then used it to calculate the likelihood of successful landing on accessible terrains with stable water ice conditions from the 80°S to the South Pole. The study looked at factors critical for solar‐powered landers: the terrain's visibility to Earth (for communication), sunlight access, and the slope of the ground. We analyzed two scenarios with different landing precisions. We found that landing areas with diameters of about 0.9 and 2.6 km could ensure a 100% success rate under the most‐ and least‐constrained scenarios, respectively. Larger landing areas decreased the success probability. We also mapped the physical characteristics of ideal areas and highlighted three promising locations near De Gerlache crater, Shackleton crater, and Mons Mouton. Key Points We identify intermittently sunlit areas that permit long‐term stability of sub‐surface water ice, and accessible by landed missions “Compliant terrains” in two scenarios range from 13,071 km² (least constrained) to 290 km² (most constrained) in the south polar region For areas ≥80°S, we recommend sub‐km landing precision for missions with success criteria involving exploration of lunar polar water ice