Sazuna/UAT_keywords huggingface dataset:
All Comets are Somewhat Hyperactive and the Implications Thereof. We critically examine what hyperactivity on a comet entails, fully develop the A'Hearn Model for Hyperactivity based on the analyses of data collected for the Deep Impact encounter of comet 103P/Hartley 2, describe manifestations of hyperactivity suggested on many, if not all, comets, and give implications of hyperactivity for future cometary exploration. The A'Hearn model requires a highly volatile ice reservoir within a comet to undergo sublimation, escape the nucleus, and drive out less volatile ices along its path to the surface. Once in the coma, the less volatile ice eventually sublimates, creating a secondary source of that gas in the coma, which is generally displaced anti-sunward and not distributed symmetrically about the nucleus. The secondary source of gas increases the total production of the less volatile species in the coma, sometimes well above that expected if the total surface was undergoing sublimation. We argue that based on the simple assumptions of the A'Hearn model and the fact that several comets display one or more of the characteristics of hyperactivity detailed here, it is probable that nearly all comets experience some degree of hyperactivity. Of significance, the ice that is brought from deep within the nucleus into the coma via the process described by the A'Hearn model is the least thermally altered and is thus the most pristine ice in the comet. Therefore, it behooves future mission teams to consider cryogenically sampling coma ice, rather than or in addition to attempting a direct nucleus sample, for a better understanding of the unaltered ices and conditions present in the protoplanetary disk.

Keywords: Comet origins, Comae, Comets, Dirty snowball model, Comet volatiles
The Effect of Inefficient Accretion on Planetary Differentiation. Pairwise collisions between terrestrial embryos are the dominant means of accretion during the last stage of planet formation. Hence, their realistic treatment in N-body studies is critical to accurately model the formation of terrestrial planets and to develop interpretations of telescopic and spacecraft observations. In this work, we compare the effects of two collision prescriptions on the core-mantle differentiation of terrestrial planets: a model in which collisions are always completely accretionary ("perfect merging"), and a more realistic model based on neural networks that has been trained on hydrodynamical simulations of giant impacts. The latter model is able to predict the loss of mass due to imperfect accretion and the evolution of nonaccreted projectiles in hit-and-run collisions. We find that the results of the neural network model feature a wider range of final core mass fractions and metal-silicate equilibration pressures, temperatures, and oxygen fugacities than the assumption of perfect merging. When used to model collisions in N-body studies of terrestrial planet formation, the two models provide similar answers for planets more massive than ≈0.1 M⊕ (Earth masses). For less massive final bodies, however, the inefficient-accretion model predicts a higher degree of compositional diversity. This phenomenon is not reflected in planet formation models of the solar system that use perfect merging to determine collisional outcomes. Our findings confirm the role of giant impacts as important drivers of planetary diversity and encourage a realistic implementation of inefficient accretion in future accretion studies.

Keywords: Planetary system formation, Planetary interior, Solar system terrestrial planets
The In Situ Exploration of a Relict Ocean World: An Assessment of Potential Landing and Sampling Sites for a Future Mission to the Surface of Ceres. The Dawn orbiter's exploration of Ceres, the most water-rich body in the inner solar system after Earth, revealed the dwarf planet to be a relict ocean world of great interest to the astrobiology and ocean worlds communities. Evidence for an early global subsurface ocean is preserved in Ceres' surface minerals. While most of the subsurface ocean froze billions of years ago, its liquid (preserved in a deep brine reservoir) and solid remnants are responsible for spectacular surface features such as the faculae in the Occator crater, Ahuna Mons, and Haulani crater. Therefore, the next step in Ceres exploration is an in situ mission to the surface, with these three features being among the most scientifically compelling targets. Here we demonstrate the process of identifying potential landing/sampling sites. Using the highest resolution data obtained by Dawn (up to ∼5 m/pixel in Occator and up to ∼35 m/pixel elsewhere), we identify safe and scientifically compelling potential landing/sampling sites in these three regions. For future landing/sample return, higher spatial resolution data (a few decimeters per pixel to ∼1 m/pixel) than are currently available are needed to identify hazards on the scale of a ∼1-5 m lander. Such data could be obtained in an orbital reconnaissance phase prior to landing/sampling. Orbital reconnaissance, in combination with terrain relative navigation/hazard avoidance, and the ability to land within a ≦100 m diameter circle (&gt;800,000 of which fit within Occator), makes it highly likely that safe and scientifically compelling landing/sampling sites could be reached by a future in situ mission.

Keywords: Orbiters, Ceres, Dwarf planets, Landers, Asteroid belt
Constraining the Regolith Composition of Asteroid (16) Psyche via Laboratory Visible Near-infrared Spectroscopy. (16) Psyche is the largest M-type asteroid in the main belt and the target of the NASA Discovery-class Psyche mission. Despite gaining considerable interest in the scientific community, Psyche's composition and formation remain unconstrained. Originally, Psyche was considered to be almost entirely composed of metal due to its high radar albedo and spectral similarities to iron meteorites. More recent telescopic observations suggest the additional presence of low-Fe pyroxene and exogenic carbonaceous chondrites on the asteroid's surface. To better understand the abundances of these additional materials, we investigated visible near-infrared (0.35-2.5 μm) spectral properties of three-component laboratory mixtures of metal, low-Fe pyroxene, and carbonaceous chondrite. We compared the band depths and spectral slopes of these mixtures to the telescopic spectrum of (16) Psyche to constrain material abundances. We find that the best matching mixture to Psyche consists of 82.5% metal, 7% low-Fe pyroxene, and 10.5% carbonaceous chondrite by weight, suggesting that the asteroid is less metallic than originally estimated (∼94%). The relatively high abundance of carbonaceous chondrite material estimated from our laboratory experiments implies the delivery of this exogenic material through low velocity collisions to Psyche's surface. Assuming that Psyche's surface is representative of its bulk material content, our results suggest a porosity of 35% to match recent density estimates.

Keywords: Asteroids, Carbonaceous chondrites, Iron meteorites, Infrared astronomy, Spectroscopy, Meteorites, Asteroid surfaces
Polarimetry as a Tool for Observing Orographic Gravity Waves on Venus. Planet-wide stationary gravity waves have been observed with the thermal camera on the Akatsuki spacecraft. These waves have been attributed to the underlying surface topography and have successfully been reproduced using the Institut Pierre Simon Laplace (IPSL) Venus Mesoscale Model (VMM). Here, we use numerical radiative transfer computations of the total and polarized fluxes of the sunlight that is reflected by Venus under the conditions of these gravity waves to show that the waves could also be observed in polarimetric observations. To model the waves, we use the density perturbations computed by the IPSL VMM. We show the computed wave signatures in the polarization for nadir-viewing geometries observed by a spacecraft in orbit around Venus and as they could be observed using an Earth-based telescope. We find that the strength of the signatures of the atmospheric density waves in the degree of polarization of the reflected sunlight depends not only on the density variations themselves, but also on the wavelength and the cloud top altitude. Observations of such wave signatures on the dayside of the planet would give insight into the occurrence of the waves and possibly into the conditions that govern their onset and development. The computed change in degree of polarization due to these atmospheric density waves is about 1000 ppm at a wavelength of 300 nm. This signal is large enough for an accurate polarimeter to detect.

Keywords: Planetary atmospheres, Venus, Radiative transfer, Polarimetry
Morphometry and Temperature of Simple Craters in Mercury's Northern Hemisphere: Implications for Stability of Water Ice. Multiple lines of evidence support the hypothesis that Mercury's polar regions host deposits of water ice in permanently shadowed regions, often within the interiors of craters. Pre-MErcury, Surface, Space, ENvironment, GEochemisty, and Ranging (MESSENGER) thermal modeling of the temperature of idealized simple craters found that the interiors of these craters were too hot to host near-surface ice on geologic timescales unless within 2° of the poles. However, results from the Arecibo Observatory and the MESSENGER mission identified many small, &lt;10 km diameter, simple craters that host radar-bright deposits located considerably farther than 2° from the pole. Here we investigate the location and morphometry of north polar craters with diameters of 5-10 km and find that the craters were, on average, 30% shallower than the idealized simple crater morphometry assumed in previous thermal studies. The craters that host radar-bright deposits have an asymmetric longitudinal distribution that cannot be fully explained by the thermal environment and may be related to the original water ice deposition. We also investigate the maximum and average temperatures of the shadowed regions in these craters and find that many of them possess temperatures that allow water ice to be stable under a thermally insulating layer on geologic timescales outside of 2° from the pole. Thus, while the presence of radar-bright deposits in 5-10 km craters is not necessarily a constraint on the stability of the deposits, it provides additional information on the distribution and deposition of the water ice.

Keywords: Impact phenomena, Planetary polar regions, Mercury (planet)
The Main Asteroid Belt: The Primary Source of Debris on Comet-like Orbits. Jupiter-family comets (JFCs) contribute a significant amount of debris to near-Earth space. However, telescopic observations of these objects seem to suggest that they have short physical lifetimes. If this is true, the material generated will also be short-lived, but fireball observation networks still detect material on cometary orbits. This study examines centimeter-to-meter-scale sporadic meteoroids detected by the Desert Fireball Network from 2014 to 2020 originating from JFC-like orbits. Analyzing each event's dynamic history and physical characteristics, we confidently determined whether they originated from the main asteroid belt or the trans-Neptunian region. Our results indicate that &lt;4% of sporadic meteoroids on JFC-like orbits are genetically cometary. This observation is statistically significant and shows that cometary material is too friable to survive in near-Earth space. Even when considering shower contributions, meteoroids on JFC-like orbits are primarily from the main belt. Thus, the presence of genuine cometary meteorites in terrestrial collections is highly unlikely.

Keywords: Near-Earth objects, Fireballs, Meteorite composition, Small Solar System bodies, Meteoroids, Orbits, Short period comets, Solar system, Orbital evolution, Meteors, Meteorites, Dynamical evolution
Photodetachment and Test-particle Simulation Constraints on Negative Ions in Solar System Plasmas. Negative ions have been detected in abundance in recent years by spacecraft across the solar system. These detections were, however, made by instruments not designed for this purpose and, as such, significant uncertainties remain regarding the prevalence of these unexpected plasma components. In this article, the phenomenon of photodetachment is examined, and experimentally and theoretically derived cross-sections are used to calculate photodetachment rates for a range of atomic and molecular negative ions subjected to the solar photon spectrum. These rates are applied to negative ions outflowing from Europa, Enceladus, Titan, Dione, and Rhea and their trajectories are traced to constrain source production rates and the extent to which negative ions are able to pervade the surrounding space environments. Predictions are also made for further negative ion populations in the outer solar system with Triton used as an illustrative example. This study demonstrates how, at increased heliocentric distances, negative ions can form stable ambient plasma populations and can be exploited by future missions to the outer solar system.

Keywords: Space plasmas, Exosphere, Neptunian satellites, Chemical abundances, Pickup ions, Atmospheric composition, Planetary magnetospheres, Galilean satellites, Saturnian satellites
Sampling Plume Deposits on Enceladus' Surface to Explore Ocean Materials and Search for Traces of Life or Biosignatures. Enceladus is unique as an astrobiology target in that it hosts an active plume sourced directly from its habitable subsurface ocean. Ice particles from the plume contain geochemical constituents that are diagnostic of the ocean conditions, and may hold traces of life and/or biosignatures, if they exist. Up to 93% of the plume particles fall back onto the surface of Enceladus. The low radiation environment and present-day activity are favorable to the preservation of any complex organics and putative biosignatures contained within these particles. Laboratory experiments and modeling suggest that plume deposits would likely be weakly consolidated and relatively easy to sample. Sampling systems like a dual rasp, under development to achieve technology readiness level (TRL) 5 in 2021, would enable a landed mission on Enceladus' surface to acquire large amounts of surface materials, a requirement for analysis of trace constituents. A landed mission on Enceladus could greatly enhance our understanding of the chemical makeup of plume particles and the subsurface ocean, and seek traces of life and/or biosignatures.

Keywords: Astrobiology, Laboratory astrophysics, Saturnian satellites, Solid matter physics
Physical Constraints on Motility with Applications to Possible Life on Mars and Enceladus. Motility is a ubiquitous feature of microbial life on Earth, and is widely regarded as a promising biosignature candidate. In the search for motile organisms, it is therefore valuable to have rough estimates for the number of such microbes that one may expect to find in a given area or volume. In this work, we explore this question by employing a simple theoretical model that takes into account the amount of free energy available in a given environment and the energetic cost of motility. We present heuristic upper bounds for the average biomass density and the number density of motile lifeforms for the Martian subsurface and the ocean of Enceladus by presuming that the motile microbes in question derive their energy from methanogenesis. We consequently demonstrate that the resultant densities of motile organisms might be potentially comparable to, or much lower than, the total microbial densities documented in various extreme environments on Earth.

Keywords: Ocean planets, Mars, Astrobiology, Xenobiology, Biosignatures, Galilean satellites, Saturnian satellites
Silicon and Oxygen Isotope Evolution of the Inner Solar System. Enstatite chondrites have been regarded as major building blocks of the Earth and other differentiated inner planetary bodies due to the similarity of Δ17O (deviation of the δ17O value from the terrestrial silicate fractionation line) and nucleosynthetic isotope anomalies. However, this hypothesis has been rebutted by the fact that the Earth and enstatite chondrites show distinct Si isotopic compositions. It has been debated whether the origin of this Si isotope difference is the result of nebular or planetary processes. Here we show that the δ30Si (deviation of 30Si/28Si relative to NBS 28 standard) and the Δ17O values of chondrules in unequilibrated enstatite chondrites are between -0.20‰ and -0.54‰ and -0.36‰ and +0.26‰, respectively. Furthermore, the chondrules with higher Δ17O values tend to have lower δ30Si. The data exhibit values consistent with most of the noncarbonaceous group differentiated planetary bodies. This consistency suggests that the Si and O isotopic compositions of enstatite chondrules record those of the major precursors that formed the differentiated planetary bodies in the inner solar system. Model calculations based on the results reveal that the Si and O isotope variations of the enstatite chondrite chondrules were generated by an interaction between the evaporation-driven SiO-rich gas and partially or fully melted forsterite-rich precursor chondrules. The Mg/Si of the evaporated dust-gas mixtures increased with increasing silicate/metal ratio in the evaporated dust, which may have increased the bulk Mg/Si and δ30Si value of the inner planetary bodies.

Keywords: Chondrites, Cosmochemistry, Chondrules, Solar system terrestrial planets, Protoplanetary disks
Framework for Coordinated Efforts in the Exploration of Volatiles in the South Polar Region of the Moon. The exploration of the lunar south polar region and the ground truthing of polar volatiles is one of the top priorities for several space agencies and private partners. Here we use Moon Mineralogy Mapper surficial water ice detections to investigate the location of water-ice-bearing permanently shaded regions (PSRs) near the south pole. We extract a variety of parameters such as their temperature regime, slope, hydrogen content, number of ice detections, depth stability for water ice and dry ice, and mobility aspects. We identify 169 water-ice-bearing PSRs and use their characteristics to identify sites that allow us to access the highest abundances of volatiles, sites that can be visited to characterize the lateral or vertical distribution of volatiles (water ice and dry ice), and sites that allow for the fastest recovery of a scientifically interesting sample. Collectively, 37 PSRs are identified as sites of interest, including 11 that would address more than one mission objective and may be, for that reason, higher-priority targets of exploration. These PSRs are found in Shoemaker, Faustini, Cabeus, Malapert, Nobile, Sverdrup, Wiechert J, and Haworth craters, as well as three unnamed craters (PSRs 57, 120, and 89). These sites are all located within 6° of the south pole. We present case studies for a relatively short traverse mission (20-50 km) to PSR 89, a medium-length traverse (∼100 km) to Sverdrup 1, and a longer traverse (∼300 km) to Cabeus that can serve as a guide in planning upcoming exploration missions.

Keywords: The Moon, Lunar science, Polar caps, Surface ices, Lunar evolution
Narrowband Observations of Comet 46P/Wirtanen during Its Exceptional Apparition of 2018/19. II. Photometry, Jet Morphology, and Modeling Results. We report on our extensive photometry and imaging of comet 46P/Wirtanen during its 2018/19 apparition and use these data to constrain the modeling of Wirtanen's activity. Narrowband photometry was obtained in 9 epochs from 2018 October through 2019 March as well as 10 epochs during the 1991, 1997, and 2008 apparitions. The ensemble photometry reveals a typical composition and a secular decrease in activity since 1991. Production rates were roughly symmetric around perihelion for the carbon-bearing species (CN, C3, and C2), but steeper for OH and NH outbound. Our imaging program emphasized CN, whose coma morphology and lightcurve yielded rotation periods reported in a companion paper (Farnham et al. 2021). Here, we compare the gas and dust morphology on the 18 nights for which observations of additional species were obtained. The carbon-bearing species exhibited similar morphology that varied with rotation. OH and NH had broad, hemispheric brightness enhancements in the tailward direction that did not change significantly with rotation, which we attribute to their originating from a substantial icy grain component. We constructed a Monte Carlo model that replicates the shape, motion, and brightness distribution of the CN coma throughout the apparition with a single, self-consistent solution in principal axis rotation. Our model yields a pole having (R.A., decl.) = 319°, -5° (pole obliquity of 70°) and two large sources (radii of 50° and 40°) centered at near-equatorial latitudes and separated in longitude by ∼160°. Applications of the model to explain observed behaviors are discussed.

Keywords: Near-Earth objects, Comae, Comet nuclei, Short period comets, Comets, Comet volatiles
Neptune's Spatial Brightness Temperature Variations from the VLA and ALMA. We present spatially resolved (0.1 arcsec-1.0 arcsec) radio maps of Neptune taken from the Very Large Array and Atacama Large Millimeter/submillimeter Array between 2015 and 2017. Combined, these observations probe from just below the main methane cloud deck at ∼1 bar down to the NH4SH cloud at ∼50 bar. Prominent latitudinal variations in the brightness temperature are seen across the disk. Depending on wavelength, the south polar region is 5-40 K brighter than the mid-latitudes and northern equatorial region. We use radiative transfer modeling coupled to Markov Chain Monte Carlo methods to retrieve H2S, NH3, and CH4 abundance profiles across the disk, though only strong constraints can be made for H2S. Below all cloud formation, the data are well fit by ${53.8}_{-13.4}^{+18.9}\times$ and ${3.9}_{-3.1}^{+2.1}\times$ protosolar enrichment in the H2S and NH3 abundances, respectively, assuming a dry adiabat. Models in which the radio-cold mid-latitudes and northern equatorial region are supersaturated in H2S are statistically favored over models following strict thermochemical equilibrium. H2S is more abundant at the equatorial region than at the poles, indicative of strong, persistent global circulation. Our results imply that Neptune's sulfur-to-nitrogen ratio exceeds unity, as H2S is more abundant than NH3 in every retrieval. The absence of NH3 above 50 bar can be explained either by partial dissolution of NH3 in an ionic ocean at GPa pressures or by a planet formation scenario in which hydrated clathrates preferentially delivered sulfur rather than nitrogen onto planetesimals, or a combination of these hypotheses.

Keywords: Atmospheric composition, Neptune, Radio astrometry, Planetary atmospheres
TRAPPIST Habitable Atmosphere Intercomparison (THAI) Workshop Report. The era of atmospheric characterization of terrestrial exoplanets is just around the corner. Modeling prior to observations is crucial in order to predict the observational challenges and to prepare for the data interpretation. This paper presents the report of the TRAPPIST Habitable Atmosphere Intercomparison workshop (2020 September 14-16). A review of the climate models and parameterizations of the atmospheric processes on terrestrial exoplanets, model advancements, and limitations, as well as direction for future model development, was discussed. We hope that this report will be used as a roadmap for future numerical simulations of exoplanet atmospheres and maintaining strong connections to the astronomical community.

Keywords: Exoplanet atmospheres