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We analyzed spectral cubes of Callisto's leading and trailing hemispheres, collected with the NIRSpec Integrated Field Unit (G395H) on the James Webb Space Telescope. These spatially resolved data show strong 4.25-micron absorption bands resulting from solid-state 12CO2, with the strongest spectral features at low latitudes near the center of its trailing hemisphere, consistent with radiolytic production spurred by magnetospheric plasma interacting with native H2O mixed with carbonaceous compounds. We detected CO2 rovibrational emission lines between 4.2 and 4.3 microns over both hemispheres, confirming the global presence of CO2 gas in Callisto's tenuous atmosphere. These results represent the first detection of CO2 gas over Callisto's trailing side. The distribution of CO2 gas is offset from the subsolar region on either hemisphere, suggesting that sputtering, radiolysis, and geologic processes help sustain Callisto's atmosphere. We detected a 4.38-micron absorption band that likely results from solid-state 13CO2. A prominent 4.57-micron absorption band that might result from CN-bearing organics is present and significantly stronger on Callisto's leading hemisphere, unlike 12CO2, suggesting these two spectral features are spatially anti-associated. The distribution of the 4.57-micron band is more consistent with a native origin and/or accumulation of dust from Jupiter's irregular satellites. Other, more subtle absorption features could result from CH-bearing organics, CO, carbonyl sulfide (OCS), and Na-bearing minerals. These results highlight the need for preparatory laboratory work and improved surface-atmosphere interaction models to better understand carbon chemistry on the icy Galilean moons before the arrival of NASA's Europa Clipper and ESA's JUICE spacecraft., Comment: Accepted in AAS Planetary Science Journal, January 2024

We present a feasibility study for passive sounding of Uranian icy moons using Uranian Kilometric Radio (UKR) emissions in the 100 - 900 kHz band. We provide a summary description of the observation geometry, the UKR characteristics, and estimate the sensitivity for an instrument analogous to the Cassini Radio Plasma Wave Science (RPWS) but with a modified receiver digitizer and signal processing chain. We show that the concept has the potential to directly and unambiguously detect cold oceans within Uranian satellites and provide strong constraints on the interior structure in the presence of warm or no oceans. As part of a geophysical payload, the concept could therefore have a key role in the detection of oceans within the Uranian satellites. The main limitation of the concept is coherence losses attributed to the extended source size of the UKR and dependence on the illumination geometry. These factors represent constraints on the tour design of a future Uranus mission in terms of flyby altitudes and encounter timing.

Interstellar objects (ISOs) are fascinating and under-explored celestial objects, providing physical laboratories to understand the formation of our solar system and probe the composition and properties of material formed in exoplanetary systems. This paper will discuss the accessibility of and mission design to ISOs with varying characteristics, including a discussion of state covariance estimation over the course of a cruise, handoffs from traditional navigation approaches to novel autonomous navigation for fast flyby regimes, and overall recommendations about preparing for the future in situ exploration of these targets. The lessons learned also apply to the fast flyby of other small bodies including long-period comets and potentially hazardous asteroids, which also require a tactical response with similar characteristics, Comment: Accepted at IEEE Aerospace Conference

Solid body tides provide key information on the interior structure, evolution, and origin of the planetary bodies. Our Solar system harbours a very diverse population of planetary bodies, including those composed of rock, ice, gas, or a mixture of all. While a rich arsenal of geophysical methods has been developed over several years to infer knowledge about the interior of the Earth, the inventory of tools to investigate the interiors of other Solar-system bodies remains limited. With seismic data only available for the Earth, the Moon, and Mars, geodetic measurements, including the observation of the tidal response, have become especially valuable and therefore, has played an important role in understanding the interior and history of several Solar system bodies. To use tidal response measurements as a means to obtain constraints on the interior structure of planetary bodies, appropriate understanding of the viscoelastic reaction of the materials from which the planets are formed is needed. Here, we review the fundamental aspects of the tidal modeling and the information on the present-day interior properties and evolution of several planets and moons based on studying their tidal response. We begin with an outline of the theory of viscoelasticity and tidal response. Next, we proceed by discussing the information on the tidal response and the inferred structure of Mercury, Venus, Mars and its moons, the Moon, and the largest satellites of giant planets, obtained from the analysis of the data that has been provided by space missions. We also summarise the upcoming possibilities offered by the currently planned missions.

Onboard autonomy technologies such as planning and scheduling, identification of scientific targets, and content-based data summarization, will lead to exciting new space science missions. However, the challenge of operating missions with such onboard autonomous capabilities has not been studied to a level of detail sufficient for consideration in mission concepts. These autonomy capabilities will require changes to current operations processes, practices, and tools. We have developed a case study to assess the changes needed to enable operators and scientists to operate an autonomous spacecraft by facilitating a common model between the ground personnel and the onboard algorithms. We assess the new operations tools and workflows necessary to enable operators and scientists to convey their desired intent to the spacecraft, and to be able to reconstruct and explain the decisions made onboard and the state of the spacecraft. Mock-ups of these tools were used in a user study to understand the effectiveness of the processes and tools in enabling a shared framework of understanding, and in the ability of the operators and scientists to effectively achieve mission science objectives., Comment: 16 pages, 18 Figures, 1 Table, to be published in IEEE Aerospace 2022 (AeroConf 2022)

The 27 satellites of Uranus are enigmatic, with dark surfaces coated by material that could be rich in organics. Voyager 2 imaged the southern hemispheres of Uranus' five largest 'classical' moons Miranda, Ariel, Umbriel, Titania, and Oberon, as well as the largest ring moon Puck, but their northern hemispheres were largely unobservable at the time of the flyby and were not imaged. Additionally, no spatially resolved datasets exist for the other 21 known moons, and their surface properties are essentially unknown. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, our knowledge of the Uranian moons' surface compositions, and the processes that modify them, is limited to disk-integrated datasets collected by ground- and space-based telescopes. Nevertheless, images collected by the Imaging Science System on Voyager 2 and reflectance spectra collected by telescope facilities indicate that the five classical moons are candidate ocean worlds that might currently have, or had, liquid subsurface layers beneath their icy surfaces. To determine whether these moons are ocean worlds, and investigate Uranus' ring moons and irregular satellites, close-up observations and measurements made by instruments onboard a Uranus orbiter are needed., Comment: Accepted in AAS Planetary Science Journal. arXiv admin note: text overlap with arXiv:2007.07284

The ice-rich dwarf planet Ceres is the largest object in the main asteroid belt and is thought to have a brine or mud layer at a depth of tens of kilometers. Furthermore, recent surface deposits of brine-sourced material imply shallow feeder structures such as sills or dikes. Inductive sounding of Ceres can be performed using the solar wind as a source, as was done for the Moon during Apollo. However, the magnetotelluric method -- measuring both electric and magnetic fields at the surface -- is not sensitive to plasma effects that were experienced for Apollo, which used an orbit-to-surface magnetic transfer function. The highly conductive brine targets are readily separable from the resistive ice and rock interior, such that the depth to deep and shallow brines can be assessed simultaneously. The instrumentation will be tested on the Moon in 2023 and is ready for implementation on a Ceres landed mission.

This white paper advocates for the inclusion of small, captured Outer Solar system objects, found in the Ice Giant region in the next Decadal Survey. These objects include the Trojans and Irregular satellite populations of Uranus and Neptune. The captured small bodies provide vital clues as to the formation of our Solar system. They have unique dynamical situations, which any model of Solar system formation needs to explain. The major issue is that so few of these objects have been discovered, with very little information known about them. The purpose of this document is to prioritize further discovery and characterization of these objects. This will require the use of NASA and NSF facilities over the 2023 2032 decade, including additional support for analysis. This is in preparation for potential future insitu missions in the following decades., Comment: Community Science White Paper for the Planetary and Astrobiology Decadal Survey, 2023-2032

We review some key issues pertaining to NASA's Research and Analysis programs, and offer recommended actions to mitigate or resolve these issues. In particular, we recommended that NASA increases funding to support a healthy selection rate (~40%) for R&A programs, which underpin much scientific discovery with NASA mission data, and on which the majority of the U.S. planetary science community relies (either in part or wholly). We also recommend additional actions NASA can take to ensure a more equitable and sustainable planetary science research community in the U.S., including supporting the next generations of planetary researchers, working to minimize biases in peer review, and reducing the burden of scientists as they prepare R&A proposals., Comment: 8 pages, 3 figures, white paper prepared for the NASEM Planetary Science and Astrobiology Decadal Survey 2023-2032

A close partnership between people and partially autonomous machines has enabled decades of space exploration. But to further expand our horizons, our systems must become more capable. Increasing the nature and degree of autonomy - allowing our systems to make and act on their own decisions as directed by mission teams - enables new science capabilities and enhances science return. The 2011 Planetary Science Decadal Survey (PSDS) and on-going pre-Decadal mission studies have identified increased autonomy as a core technology required for future missions. However, even as scientific discovery has necessitated the development of autonomous systems and past flight demonstrations have been successful, institutional barriers have limited its maturation and infusion on existing planetary missions. Consequently, the authors and endorsers of this paper recommend that new programmatic pathways be developed to infuse autonomy, infrastructure for support autonomous systems be invested in, new practices be adopted, and the cost-saving value of autonomy for operations be studied., Comment: 10 pages (compared to 8 submitted to PSADS), 2 figures, submitted to National Academy of Sciences Planetary Science and Astrobiology Decadal Survey 2023-2032

Orbital geophysical investigations of Enceladus are critical to understanding its energy balance. We identified key science questions for the geophysical exploration of Enceladus, answering which would support future assessment of Enceladus' astrobiological potential. Using a Bayesian framework, we explored how science requirements map to measurement requirements. We performed mission simulations to study the sensitivity of a single spacecraft and dual spacecraft configurations to static gravity and tidal Love numbers of Enceladus. We find that mapping Enceladus' gravity field, improving the accuracy of the physical libration amplitude, and measuring Enceladus' tidal response would provide critical constraints on the internal structure, and establish a framework for assessing Enceladus' long-term habitability. This kind of investigation could be carried out as part of a life search mission at little additional resource requirements., Comment: 21 pages, 6 figures. A paper submitted to PSJ, which is an extension of the white paper previously submitted to the decadal survey

Solicited by the Small Bodies Assessment Group, we recommend a balanced program of telescopic observation (ground-based, airborne, and space-based), laboratory studies, theoretical research and missions to Main Belt Asteroids utilizing the full spectral range from ultraviolet to far-infrared to investigate these outstanding fundamental questions in the next decade., Comment: White Paper submitted to the Planetary Science Decadal Survey

The five classical Uranian moons are possible ocean worlds that exhibit bizarre geologic landforms, hinting at recent surface-interior communication. However, Uranus' classical moons, as well as its ring moons and irregular satellites, remain poorly understood. We assert that a Flagship-class orbiter is needed to explore the Uranian satellites.

The goal of Project GAUSS is to return samples from the dwarf planet Ceres. Ceres is the most accessible ocean world candidate and the largest reservoir of water in the inner solar system. It shows active cryovolcanism and hydrothermal activities in recent history that resulted in minerals not found in any other planets to date except for Earth's upper crust. The possible occurrence of recent subsurface ocean on Ceres and the complex geochemistry suggest possible past habitability and even the potential for ongoing habitability. Aiming to answer a broad spectrum of questions about the origin and evolution of Ceres and its potential habitability, GAUSS will return samples from this possible ocean world for the first time. The project will address the following top-level scientific questions: 1) What is the origin of Ceres and the origin and transfer of water and other volatiles in the inner solar system? 2) What are the physical properties and internal structure of Ceres? What do they tell us about the evolutionary and aqueous alteration history of icy dwarf planets? 3) What are the astrobiological implications of Ceres? Was it habitable in the past and is it still today? 4) What are the mineralogical connections between Ceres and our current collections of primitive meteorites? GAUSS will first perform a high-resolution global remote sensing investigation, characterizing the geophysical and geochemical properties of Ceres. Candidate sampling sites will then be identified, and observation campaigns will be run for an in-depth assessment of the candidate sites. Once the sampling site is selected, a lander will be deployed on the surface to collect samples and return them to Earth in cryogenic conditions that preserves the volatile and organic composition as well as the original physical status as much as possible., Comment: Section 3.4 revised; List of team members updated; Typos corrected

Centaurs--icy bodies orbiting beyond Jupiter and interior to Neptune--are believed to be dynamically related to Jupiter Family Comets (JFCs), which have aphelia near Jupiter's orbit and perihelia in the inner Solar System. Previous dynamical simulations have recreated the Centaur/JFC conversion, but the mechanism behind that process remains poorly described. We have performed a numerical simulation of Centaur analogues that recreates this process, generating a dataset detailing over 2.6 million close planet/planetesimal interactions. We explore scenarios stored within that database and, from those, describe the mechanism by which Centaur objects are converted into JFCs. Because many JFCs have perihelia in the terrestrial planet region, and since Centaurs are constantly resupplied from the Scattered Disk and other reservoirs, the JFCs are an ever-present impact threat., Comment: Companion paper to "It's Complicated: A Big Data Approach to Exploring Planetesimal Evolution in the Presence of Jovian Planets" in The Astronomical Journal: https://doi.org/10.3847/1538-3881/aae095

We report a comprehensive analysis of the global spectrophotometric properties of Ceres using Dawn Framing Camera images collected from April to June 2015 during the RC3 and Survey mission phases. The single-scattering albedo of Ceres at 555 nm is 0.14$\pm$0.04, the geometric albedo is 0.096$\pm$0.006, and the Bond albedo is 0.037$\pm$0.002. The asymmetry factors calculated from the best-fit two-term Henyey-Greenstein (HG) single-particle phase function (SPPF) show a wavelength dependence, suggesting that the phase reddening of Ceres is dominated by single-particle scattering rather than multiple scattering or small-scale surface roughness. The Hapke roughness parameter of Ceres is derived to be 20$^\circ\pm$6$^\circ$ with no wavelength dependence. The phase function of Ceres shows appreciably strong scattering around 90$^\circ$ phase angle that cannot be fitted with a single-term HG SPPF, suggesting possible stronger forward scattering than other asteroids previously analyzed with spacecraft data. We speculate that such a scattering characteristic of Ceres might be related to its unique surface composition. We grouped the reflectance data into a 1$^\circ$ latitude-longitude grid and fitted each grid independently to study the spatial variations of photometric properties. The albedo and color maps are consistent with previous studies. The SPPF over the surface of Ceres shows stronger backscattering associated with lower albedo and vice versa, consistent with the general trend among asteroids. The Hapke roughness parameter does not vary much across the surface of Ceres, except for the ancient Vendimia Planitia region that has a slightly higher roughness. Based on the wavelength dependence of the SPPF of Ceres, we hypothesize that its regolith grains either contain a considerable fraction of $\lessapprox\mu$m-sized particles, or are strongly affected by internal scatterers of this size., Comment: 43 pages, 3 tables, 17 figures, accepted by Icarus

The surface reflectance of planetary regoliths may increase dramatically towards zero phase angle, a phenomenon known as the opposition effect (OE). Two physical processes that are thought to be the dominant contributors to the brightness surge are shadow hiding (SH) and coherent backscatter (CB). The occurrence of shadow hiding in planetary regoliths is self-evident, but it has proved difficult to unambiguously demonstrate CB from remote sensing observations. One prediction of CB theory is the wavelength dependence of the OE angular width. The Dawn spacecraft observed the OE on the surface of dwarf planet Ceres. We characterize the OE over the resolved surface, including the bright Cerealia Facula, and to find evidence for SH and/or CB. We analyze images of the Dawn framing camera by means of photometric modeling of the phase curve. We find that the OE of most of the investigated surface has very similar characteristics, with an enhancement factor of 1.4 and a FWHM of 3{\deg} (broad OE). A notable exception are the fresh ejecta of the Azacca crater, which display a very narrow brightness enhancement that is restricted to phase angles $< 0.5${\deg} (narrow OE); suggestively, this is in the range in which CB is thought to dominate. We do not find a wavelength dependence for the width of the broad OE, and lack the data to investigate the dependence for the narrow OE. The prediction of a wavelength-dependent CB width is rather ambiguous. The zero-phase observations allow us to determine Ceres' visible geometric albedo as $p_V = 0.094 \pm 0.005$. A comparison with other asteroids suggests that Ceres' broad OE is typical for an asteroid of its spectral type, with characteristics that are primarily linked to surface albedo. Our analysis suggests that CB may occur on the dark surface of Ceres in a highly localized fashion., Comment: Credit: Schr\"oder et al, A&A in press, 2018, reproduced with permission, \copyright ESO

Previous observations suggested that Ceres has active but possibly sporadic water outgassing, and possibly varying spectral characteristics in a time scale of months. We used all available data of Ceres collected in the past three decades from the ground and the Hubble Space Telescope, and the newly acquired images by Dawn Framing Camera to search for spectral and albedo variability on Ceres, in both a global scale and local regions, particularly the bright spots inside Occator crater, over time scales of a few months to decades. Our analysis has placed an upper limit on the possible temporal albedo variation on Ceres. Sporadic water vapor venting, or any possibly ongoing activity on Ceres, is not significant enough to change the albedo or the area of the bright features in Occator crater by >15%, or the global albedo by >3% over various time scales that we searched. Recently reported spectral slope variations can be explained by changing Sun-Ceres-Earth geometry. The active area on Ceres is less than 1 km$^2$, too small to cause global albedo and spectral variations detectable in our data. Impact ejecta due to impacting projectiles of tens of meters in size like those known to cause observable changes to the surface albedo on Asteroid Scheila cannot cause detectable albedo change on Ceres due to its relatively large size and strong gravity. The water vapor activity on Ceres is independent of Ceres' heliocentric distance, rulling out the possibility of comet-like sublimation process as a possible mechanism driving the activity., Comment: Accepted by ApJ Lett

The James Webb Space Telescope (JWST) has the potential to enhance our understanding of near-Earth objects (NEOs). We present results of investigations into the observability of NEOs given the nominal observing requirements of JWST on elongation (85-135 degrees) and non-sidereal rates ($<$30mas/s). We find that approximately 75% of NEOs can be observed in a given year. However, observers will need to wait for appropriate observing windows. We find that JWST can easily execute photometric observations of meter-sized NEOs which will enhance our understanding of the small NEO population., Comment: This paper is one of a series for a special issue on Solar System observations with JWST in PASP. Accepted 10/02/15. Preprint 22 pages, 4 figures

2017 and 2019 saw the discovery of the first twointerstellar objects (ISO) in the solar system, 1I ‘Oumuamuaand 2I Borisov, but left scientists with more questions thananswers as to the objects’ origins. To fully resolve these openquestions, scientists need a closer look at an interstellar objectvia a dedicated spacecraft and close approach. However, ISOspose unique engineering challenges for close encounters becauseof their extremely high relative velocity, unfavorable approachgeometry, and limited alert time; ISOs often are only discovereda few months before a spacecraft would need to launch toencounter it.Typical encounters with small bodies have occurred at relativevelocities on the order of 10 km/s with approach phase below90 deg (at least half-lit). Except for the Halley Armada in the80s, the most extreme flybys have been of comet Borrelly ata relative velocity of 16.6 km/s (but only a phase angle of 65deg) and Annefrank at a phase angle of 150 deg (but a relativevelocity of only 7.2 km/s). Furthermore, planetary missionsgenerally target bodies whose ephemerides are well constrainedbecause of many years of Earth based observations. On theother hand, ISOs typically have relative velocities of 30-90 km/s,phase angles biased towards over 90 deg, and larger positionuncertainties because of shorter ground-based observationalarcs, and thus represent much more difficult targets.Our team has been developing the flight system architecturesand technologies necessary to enable future reconnaissance ofan interstellar object by flyby, potentially including an impactor.This endeavor seeks to catalyze planetary exploration and itsintersection with exo-planet science. We establish encounterguidance, navigation, and control requirements within the tradespace of trajectories to a synthetic ISO population. Flightsystem constraints, Earth departure capabilities, and ability fora mission team to quickly respond to an inbound target limitthe accessible space for a future mission. This trade spaceanalysis reveals enabling mission architectures, and providesrecommendations for supporting investment to enable an ISOmission in the next decade.

We consider the Roche critical density (rho_Roche), the minimum density of an orbiting object that, at a given distance from its planet, is able to hold itself together by self-gravity. It is directly related to the more familiar "Roche limit," the distance from a planet at which a strengthless orbiting object of given density is pulled apart by tides. The presence of a substantial ring requires that transient clumps have an internal density less than rho_Roche. Conversely, in the presence of abundant material for accretion, an orbiting object with density greater than rho_Roche will grow. Comparing the rho_Roche values at which the Saturn and Uranus systems transition rapidly from disruption-dominated (rings) to accretion-dominated (moons), we infer that the material composing Uranus' rings is likely more rocky, as well as less porous, than that composing Saturn's rings. From the high values of rho_Roche at the innermost ring-moons of Jupiter and Neptune, we infer that those moons may be composed of denser material than expected, or more likely that they are interlopers that formed farther from their planets and have since migrated inward, now being held together by internal material strength. Finally, the "Portia group" of eight closely-packed Uranian moons has an overall surface density similar to that of Saturn's A ring. Thus, it can be seen as an accretion-dominated ring system, of similar character to the standard ring systems except that its material has a characteristic density greater than the local rho_Roche., Comment: 5 pages, 2 figures; Accepted to Astrophysical Journal Letters