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Over the next decade, the astronomical community will be commissioning multiple wide-field observatories well suited for studying stellar halos in both integrated light and resolved stars. In preparation for this, we use five high-resolution cosmological simulations of Milky Way–like galaxies from the FOGGIE suite to explore the properties and components of stellar halos. These simulations are run with high time (5 Myr) and stellar mass (1000 M _⊙ ) resolution to better model the properties and origins of low-density regions like stellar halos. We find that the FOGGIE stellar halos have masses, metallicity gradients, and surface brightness profiles that are consistent with observations. In agreement with other simulations, the FOGGIE stellar halos receive 30%–40% of their mass from in situ stars. However, this population is more centrally concentrated in the FOGGIE simulations and therefore does not contribute excess light to the halo outskirts. The remaining stars are accreted from ∼10–50 other galaxies, with the majority of the accreted mass originating in two to four galaxies. While the inner halo ( r < 50 kpc) of each FOGGIE galaxy has a large number of contributors, the halo outskirts of three of the five galaxies are primarily made up of stars from only a few contributors. We predict that upcoming wide-field observatories, like the Nancy Grace Roman Space Telescope, will probe stellar halos around Milky Way–like galaxies out to ∼100 kpc in integrated light and will be able to distinguish the debris of dwarf galaxies with extended star formation histories from the underlying halo with resolved color–magnitude diagrams.

Using an idealized climate model incorporating seasonal forcing, we investigate the impact of rotation rate on the abundance of clouds on an Earth-like aquaplanet, and the resulting impacts upon albedo and seasonality. We show that the cloud distribution varies significantly with season, depending strongly on the rotation rate, and is well explained by the large-scale circulation and atmospheric state. Planetary albedo displays nonmonotonic behavior with rotation rate, peaking at around 1/2Ω _E . Clouds reduce the surface temperature and total precipitation relative to simulations without clouds at all rotation rates, and reduce the dependence of total precipitation on rotation rate, causing nonmonotonic behavior and a local maximum around 1/8Ω _E ; these effects are related to the impacts of clouds on the net atmospheric and surface radiative energy budgets. Clouds also affect the seasonality. The influence of clouds on the extent of the winter Hadley cell and the intertropical convergence zone is relatively minor at slow rotation rates (

Dunes may form on Jupiter’s moon Io. Despite a tenuous atmosphere, interactions between widespread lava and sulfur dioxide frost may produce vapor flows dense enough to mobilize sand grains. Ridge-like features may be evidence of this phenomenon.

Comets are active geological worlds with primitive surfaces that have been shaped to varying degrees by sublimation‐driven sediment transport and mass wasting processes. Rosetta's rendezvous with comet 67P/Churyumov‐Gerasimenko (67P) in 2014 provided data with the necessary spatial and temporal resolutions to observe many evolutionary processes on micro‐gravity worlds. Rosetta's observations have thus far revealed that many changes to the surface occurred within 67P's smooth terrains, vast sedimentary deposits that blanket a significant fraction of the nucleus. Understanding the global context of these changes, and therefore the sediment transport pathways that govern the evolution of 67P's surface, requires a thorough description of their changing morphologies and an evaluation of existing global‐scale spatial and temporal trends. Accordingly, we present a time‐resolved synthesis of erosion and deposition activity on comet 67P as it passed through its 13 August 2015 perihelion from September 2014 to August 2016. Our mapping results indicate that, around perihelion, sediment is globally redistributed inter‐regionally from 67P's more active south to the north. Equally important, however, are local, topographically influenced sediment transport processes, with large volumes of sediment moving intra‐regionally over sub‐kilometer distances. We also show evidence for regions of near‐zero net erosion/deposition between approximately 30°N–60°N latitude, which may act as terminal sedimentary sinks, with the remobilization of these materials hindered by multiple factors. Our work therefore provides the most complete mapping of sediment transport processes and pathways across 67P, a critical step toward understanding the global landscape evolution of both 67P and other comets. Plain Language Summary: Comets are some of the oldest materials in the solar system. When these ice‐rich bodies approach the sun, the ice at the surface can sublimate, breaking down solid materials into sediment that can be mobilized across and off of the comet's surface. On comet 67P, sublimated vapor can move material in many ways. Determining how and where materials move across this comet can help us understand how its ice content has changed since it formed and how other comet surfaces may change over time. We found that the location and timing of sediment transportation are linked to the location of most direct sunlight on the comet's surface. Additionally, we only observed migrating scarps of smooth materials in either outer, equatorial regions of the comet or within "internal" regions that receive extra heat from the comet itself. We also found that sediments moved within and between regions after their initial deposition. However, we observed almost no sediment transport on the top of either lobe, perhaps indicating that the amount of deposited material is limited and may not be easily eroded after deposition. We predict that the processes transporting sediments on 67P will affect comets of similar orbits and masses. Key Points: Erosion and deposition follow the subsolar latitude and are strongly influenced by local topographyScarp activity is primarily limited to equatorial and internal regionsWe observe net zero erosion/deposition on the tops of the lobes, perhaps indicating the presence of terminal sinks for airfalling sediment [ABSTRACT FROM AUTHOR]

Rosetta observations of 67P/Churyumov‐Gerasimenko (67P) reveal that most changes occur in the fallback‐generated smooth terrains, vast deposits of granular material blanketing the comet's northern hemisphere. These changes express themselves both morphologically and spectrally across the nucleus, yet we lack a model that describes their formation and evolution. Here we present a self‐consistent model that thoroughly explains the activity and mass loss from Hapi's smooth terrains. Our model predicts the removal of dust via reradiated solar insolation localized within depression scarps that are substantially more ice rich than previously expected. We couple our model with numerous Rosetta observations to thoroughly capture the seasonal erosion of Hapi's smooth terrains, where local scarp retreat gradually removes the uppermost dusty mantle. As sublimation‐regolith interactions occur on rocky planets, comets, icy moons, and Kuiper belt objects, our coupled model and observations provide a foundation for future understanding of the myriad of sublimation‐carved worlds. Key Points: We report the discovery of transient, migrating depressions within the smooth terrains on comet 67PMigration is via uniform scarp retreat, which removes regolith completely from the nucleus and is driven by sublimating subsurface water iceWe develop a model that fully captures our own, and many previous, disparate observations, with implications for how comets seasonally erode [ABSTRACT FROM AUTHOR]

Abstract: Cassini observations of Titan's weather patterns over >13 years, almost half a Saturnian year, provide insight into seasonal circulation patterns and the methane cycle. The Imaging Science Subsystem and the Visual and Infrared Mapping Spectrometer documented cloud locations, characteristics, morphologies, and behavior. Clouds were generally more prevalent in the summer hemisphere, but there were surprises in locations and timing of activity: Southern clouds were common at midlatitudes, northern clouds initially appeared much sooner than model predictions, and north polar summer convective systems did not appear before the mission ended. Differences from expectations constrain atmospheric circulation models, revealing factors that best match observations, including the roles of surface and subsurface reservoirs. The preference for clouds at mid‐northern latitudes rather than near the pole is consistent with models that include widespread polar near‐surface methane reservoirs in addition to the lakes and seas, suggesting a broader subsurface methane table is accessible to the atmosphere. [ABSTRACT FROM AUTHOR]

The topography provided by altimetry, synthetic aperture radar-topography, and stereo radargrammetry has opened new doors for Titan research by allowing for quantitative analysis of morphologic form. Using altimetry measurements, we show that Titan's Maria are consistent with an equipotential surface but that several filled lakes are found to be hundreds of meters above this sea level, suggesting that they exist in isolated or perched basins. Within a given drainage basin, empty lake floors are typically higher than the liquid elevation of nearby lakes/seas, suggesting local subsurface connectivity. The majority of Titan's lakes reside in topographically closed, sharp-edged depressions whose planform curvature suggests lateral expansion through uniform scarp retreat. Many, but not all, empty lake basins exhibit flat floors and hectometer-scale raised rims that present a challenge to formation models. We conclude that dissolution erosion can best match the observed constraints but that challenges remain in the interpretation of formation processes and materials. [ABSTRACT FROM AUTHOR]

Alluvial rivers are conveyor belts of fluid and sediment that provide a record of upstream climate and erosion on Earth, Titan, and Mars. However, many of Earth's rivers remain unsurveyed, Titan's rivers are not well resolved by current spacecraft data, and Mars' rivers are no longer active, hindering reconstructions of planetary surface conditions. To overcome these problems, we use dimensionless hydraulic geometry relations-scaling laws that relate river channel dimensions to flow and sediment transport rates-to calculate in-channel conditions using only remote sensing measurements of channel width and slope. On Earth, this offers a way to predict flow and sediment flux in rivers that lack field measurements and shows that the distinct dynamics of bedload-dominated, suspended load-dominated, and bedrock rivers give rise to distinct channel characteristics. On Mars, this approach not only predicts grain sizes at Gale Crater and Jezero Crater that overlap with those measured by the Curiosity and Perseverance rovers, it enables reconstructions of past flow conditions that are consistent with proposed long-lived hydrologic activity at both craters. On Titan, our predicted sediment fluxes to the coast of Ontario Lacus could build the lake's river delta in as little as ~1,000 y, and our scaling relationships suggest that Titan's rivers may be wider, slope more gently, and transport sediment at lower flows than rivers on Earth or Mars. Our approach provides a template for predicting channel properties remotely for alluvial rivers across Earth, along with interpreting spacecraft observations of rivers on Titan and Mars.

Perseverance's Mastcam-Z instrument provides high-resolution stereo and multispectral images with a unique combination of spatial resolution, spatial coverage, and wavelength coverage along the rover's traverse in Jezero crater, Mars. Images reveal rocks consistent with an igneous (including volcanic and/or volcaniclastic) and/or impactite origin and limited aqueous alteration, including polygonally fractured rocks with weathered coatings; massive boulder-forming bedrock consisting of mafic silicates, ferric oxides, and/or iron-bearing alteration minerals; and coarsely layered outcrops dominated by olivine. Pyroxene dominates the iron-bearing mineralogy in the fine-grained regolith, while olivine dominates the coarse-grained regolith. Solar and atmospheric imaging observations show significant intra- and intersol variations in dust optical depth and water ice clouds, as well as unique examples of boundary layer vortex action from both natural (dust devil) and Ingenuity helicopter-induced dust lifting. High-resolution stereo imaging also provides geologic context for rover operations, other instrument observations, and sample selection, characterization, and confirmation.