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A main goal of human space exploration is to develop humanity into a multi-planet species where civilization extends beyond planet Earth. Establishing a self-sustaining human presence on Mars is key to achieving this goal.

The clockwise spiral of troughs marking the Martian north polar plateau forms one of the planet’s youngest megastructures. One popular hypothesis posits that the spiral pattern resulted as troughs underwent poleward migration. Here, we show that the troughs are extensively segmented into enclosed depressions (or cells). Many cell interiors display concentric layers that connect pole- and equator-facing slopes, demonstrating in-situ trough erosion. The segmentation patterns indicate a history of gradual trough growth transversely to katabatic wind directions, whereby increases in trough intersections generated their spiral arrangement. The erosional event recorded in the truncated strata and trough segmentation may have supplied up to ~25% of the volume of the mid-latitude icy mantles. Topographically subtle undulations transition into troughs and have distributions that mimic and extend the troughs’ spiraling pattern, indicating that they probably represent buried trough sections. The retention of the spiral pattern in surface and subsurface troughs is consistent with the megastructure’s stabilization before its partial burial. A previously suggested warm paleoclimatic spike indicates that the erosion could have occurred as recently as ~50 Ka. Hence, if the removed ice was redeposited to form the mid-latitude mantles, they could provide a valuable source of near-surface, clean ice for future human exploration.

Kuiper belt objects (KBOs) have diverse surface compositions, and the New Horizons mission to the Pluto-Charon system allows us to test hypotheses on the origin and evolution of these KBO surfaces. Previous work proposed that Charon’s organic-rich north pole formed from radiolytically processed volatiles sourced from Pluto’s escaping atmosphere. Here, we show an endogenic source of volatiles from Charon’s interior is plausible. We calculate that cryovolcanic resurfacing released 1.29 × 1015–3.47 × 1015 kg of methane to Charon’s surface from its interior. We modeled volatile transport and found the vast majority of this volcanically released methane migrates to Charon’s poles, with deposition rates sufficient to be processed into the observed organic compounds. Irradiated methane products appear on similarly sized KBOs that do not orbit a Pluto-sized object to draw an escaping atmosphere from, so interior-sourced volatiles could be a common and important process across the Kuiper belt.

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., Accepted in AAS Planetary Science Journal. arXiv admin note: text overlap with arXiv:2007.07284

Can fractal analysis enable us to classify a lava flow to a morphologic type (e.g., ‘a‘ā vs. pāhoehoe) solely by examining the geometry of the flow’s margin? If so, these classifications would prov...

International audience; Extensive evidence of landform-scale martian geomorphic changes has been acquired in the last decade, and the number and range of examples of surface activity have increased as more high-resolution imagery has been acquired. Within the present-day Mars climate, wind and frost/ice are the dominant drivers, resulting in large avalanches of material down icy, rocky, or sandy slopes; sediment transport leading to many scales of aeolian bedforms and erosion; pits of various forms and patterned ground; and substrate material carved out from under subliming ice slabs. Due to the ability to collect correlated observations of surface activity and new landforms with relevant environmental conditions with spacecraft on or around Mars, studies of martian geomorphologic activity are uniquely positioned to directly test surface-atmosphere interaction and landform formation/evolution models outside of Earth. In this paper, we outline currently observed and interpreted surface activity occurring within the modern Mars environment, and tie this activity to wind, seasonal surface CO2 frost/ice, sublimation of subsurface water ice, and/or gravity drivers. Open questions regarding these processes are outlined, and then measurements needed for answering these questions are identified. In the final sections, we discuss how many of these martian processes and landforms may provide useful analogs for conditions and processes active on other planetary surfaces, with an emphasis on those that stretch the bounds of terrestrial-based models or that lack terrestrial analogs. In these ways, modern Mars presents a natural and powerful comparative planetology base case for studies of Solar System surface processes, beyond or instead of Earth.

Mars Polar Science is an integrated, compelling system that serves as a nearby analogue to numerous other planets, supports human exploration, and habitability. Mars possesses the closest and most easily accessible layered ice deposits outside of Earth, and accessing those layers to read the climate record would be a triumph for planetary science.

This paper emphasizes the importance of using terrestrial analogues to improve our understanding of the role of ice on Mars through its associated landforms. We discuss terrestrial regions and techniques that can help understand Martian icy environments, and highlight the necessity to explore the Martian cryosphere as the next natural step.

Multiple nations and private entities are pushing to make landing humans on Mars a reality. The majority of proposed mission architectures envision ‘living off the land’ by leveraging Martian water-ice deposits for fuel production and other purposes. Fortunately for mission designers, water ice exists on Mars in plentiful volumes. The challenge is isolating accessible ice deposits within regions that optimize other preferred landing-site conditions. Here we present the first results of the Mars Subsurface Water Ice Mapping (SWIM) project, which has the aim of searching for buried ice resources across the mid-latitudes. Through the integration of orbital datasets in concert with new data-processing techniques, the SWIM project assesses the likelihood of ice by quantifying the consistency of multiple, independent data sources with the presence of ice. Concentrating our efforts across the majority of the northern hemisphere, our composite ice-consistency maps indicate that the broad plains of Arcadia and the extensive glacial networks across Deuteronilus Mensae match the greatest number of remote-sensing criteria for accessible ice-rich, subsurface material situated equatorwards of the contemporary ice-stability zone. The Mars Subsurface Water Ice Mapping (SWIM) project aims at determining the regions where near-surface ice is most likely to be present, according to the combination of all the available datasets. Focusing on the northern mid-latitudes, they identify in particular Deuteronilus Mensae and Arcadia Planitia as promising sites.

The presence of a liquid deposit of water below the South Polar Layered Deposits (SPLD) Region has been reported based on analysis of MARSIS radar data in the Planum Australe area (Orosei et al., 2018). These radar data show bright subsurface reflections that have been interpreted to be due to liquid water buried at a depth of 1.5 km (Orosei et al., 2018). The presence of such water would have important implications for the present-day thermal state of the region. Previous work based on the flexure of the lithosphere yielded a present surface heat flow of about 20–30 mW m-2 (Ruiz et al., 2010; Parro et al., 2017) at the south polar cap. Global thermal evolution models that included variations in crustal thickness and heat production estimated a global surface heat flow of 23.2–27.3 mW m-2 (Plesa et al. 2018). These estimations are difficult to reconcile with the high heat flows, in excess of 72 mW m-2 (Sori and Bramson, 2019), that are required to explain the existence of liquid water at 1.5 km deep. In this work, we aim to re-analyze the thermal state of the region and the thermal properties that are required to stabilize liquid water under the SPLD. In order to study the thermal conditions that are compatible with the liquid water deposit, we first recalculated the depth of the bright radar reflections using a temperature-dependent relative permittivity for the water ice (Fujita et al., 2000). The depth to the putative liquid water is important, as deeper depths require lower heat fluxes to reach the melting temperature, and vice versa. We obtained a new depth to the bright reflector of 1.7 km, assuming a surface temperature of 160 K and a melting temperature of 200 K, which is appropriate if calcium perchlorate is present in the area. Then, we calculated surface heat flows and subsurface temperatures by solving the stationary heat conduction equation. We assume that the composition of the SPLD region is a mixture of 85% of water ice and 15% of dust with a density of 1220 kg m-3 (Zuber at al., 2007). We use an ice thermal conductivity that is dependent on temperature following: where T is the temperature in Kelvins (Fukusako, 1990). For the dust component, we assume a thermal conductivity of 2 W m-1 K-1, and calculate the thermal conductivity of the dust-ice mixture as a geometric mean (Beardsmore and Cull, 2001). We also include a superficial layer of CO2 with a thickness of 1 m, a density of 1600 kg m-3, and a thermal conductivity of 0.02 W m-1 K-1. We find that a surface heat flow of 61 mW m-2 is needed to obtain melting at 1.7 km depth. This result is still higher than the values previously estimated from lithosphere flexure in the region, but somewhat lower than that reported by Sori and Branson (2019). Additionally, fractures or voids near the surface of the SPLD at this site could reduce the thermal conductivity of the region and lower the required heat flow even further. In order to account for this possibility, we calculated surface heat flows for additional models which include an intermediate layer of lower thermal conductivity placed between the CO2 and the more conductive dust-ice mixture. Models with an intermediate insulating layer 15 m thick and with thermal conductivities between 0.05 and 0.1 W m K-1 yield surface heat flows of 41 and 50 mW m-2, respectively, to stabilize liquid water at 1.7 km depth. Further work will realize a more complete assessment of this location and of the regional heat flow context. References Beardsmore, G.R. et al., (2001). Crustal Heat Flow: A Guide to Measurement and Modelling. Cambridge University Press, Cambridge, 324 pp. Fujita, S. et al., (2000). A summary of the complex dielectric permittivity of ice in the megahertz range and its applications for radar sounding of polar ice sheet. Physics of Ice Core Records: 185-212 Fukusako, S., (1990). Thermophysical properties of ice, snow and sea ice. Int.J. Thermophys.11, 353–372. Martin-Torres F.J. et al., (2015), Transient liquid water and water activity at Gale Crater on Mars, NatGeosci8:357–361. Orosei R. et al., (2018). Radar evidence of subglacial liquid water on Mars, Science 1126/science.aar7268. ParroM. et al., (2017). Present-day heat flow model of Mars, Sci. Rep. 7, 45629; doi: 10.1038/srep45629. Plesa, A. C. et al., (2018). The thermal state and interior structure of Mars. Geophysical Research Letters, 45(22), 12-198. Ruiz, J. et al.,(2010). The present-day thermal state of Mars. Icarus 207, 631-637. Sori, M. M., and Bramson, A. M., (2019). Water on Mars, with a grain of salt: Local heat anomalies are required for basal melting of ice at the south pole today. Geophysical Research Letters, 46, 1222–1231. Zuber, M. T. et al., (2007). Density of Mars' south polar layered deposits. Science, 317(5845), 1718-1719.

Cryovolcanism, defined here as the extrusion of icy material from depth, may be an important planetary phenomenon in shaping the surfaces of many worlds in the outer Solar System and revealing their thermal histories. However, the physics, chemistry and ubiquity of this geologic process remain poorly understood, especially in comparison to the better-studied silicate volcanism on the terrestrial planets. Ceres is the only plausibly cryovolcanic world to be orbited by a spacecraft up to now, making it the best opportunity to test the importance of cryovolcanism on bodies in the outer Solar System and compare its effects to silicate volcanism on terrestrial planets. Here, we analyse images from NASA’s Dawn mission and use the finite element method to show that Ceres has experienced cryovolcanism throughout its geologic history, with an average cryomagma extrusion rate of ~10^4 m^3 yr^(−1). This result shows that volcanic phenomena are important on Ceres, but orders of magnitude less so than on the terrestrial planets.

We provide a historical context of the International Conference on Mars Polar Science and Exploration and summarize the proceedings from the 6th iteration of this meeting. In particular, we identify five key Mars polar science questions based primarily on presentations and discussions at the conference and discuss the overlap between some of those questions. We briefly describe the seven scientific field trips that were offered at the conference, which greatly supplemented conference discussion of Mars polar processes and landforms. We end with suggestions for measurements, modeling, and laboratory and field work that were highlighted during conference discussion as necessary steps to address key knowledge gaps.

Ice-giant-sized planets are the most common type of observed exoplanet, yet the two ice giants in our own solar system (Uranus and Neptune) are the least explored class of planet, having only been observed through ground-based observations and a single flyby each by Voyager 2 approximately 30 years ago. These single flybys were unable to characterize the spatial and temporal variability in ice giant magnetospheres, some of the most odd and intriguing magnetospheres in the solar system. They also offered only limited constraints on the internal structure of ice giants; understanding the internal structure of a planet is important for understanding its formation and evolution. The most recent planetary science Decadal Survey by the U.S. National Academy of Sciences, “Vision and Voyages for Planetary Science in the Decade 2013–2022,” identified the ice giant Uranus as the third highest priority for a Flagship mission in the decade 2013–2022. However, in the event that NASA or another space agency is unable to fly a Flagship-class mission to an ice giant in the next decade, this paper presents a mission concept for a focused, lower cost Uranus orbiter called OCEANUS (Origins and Composition of the Exoplanet Analog Uranus System). OCEANUS would increase our understanding of the interior structure of Uranus, its magnetosphere, and how its magnetic field is generated. These goals could be achieved with just a magnetometer and the spacecraft's radio system. This study shows that several of the objectives outlined by the Decadal Survey, including one of the two identified as highest priority, are within reach for a New-Frontiers-class mission.

NASA Planetary Geology and Geophysics Program NASA Planetary Science Division [NNX13AR14G, 80NSSC17K0307]; NASA Earth and Space Sciences Fellowship (NESSF) Program [NNX16AP09H]

Mars Polar Science is a subfield of Mars science that encompasses all studies of the cryosphere of Mars and its interaction with the Martian environment. Every 4 yr, the community of scientists dedicated to this subfield meets to discuss new findings and debate open issues in the International Conference on Mars Polar Science and Exploration (ICMPSE). This paper summarizes the proceedings of the seventh ICMPSE and the progress made since the sixth edition. We highlight the most important advances and present the most salient open questions in the field today, as discussed and agreed upon by the participants of the conference. We also feature agreed-upon suggestions for future methods, measurements, instruments, and missions that would be essential to answering the main open questions presented. This work is thus an overview of the current status of Mars Polar Science and is intended to serve as a road map for the direction of the field during the next 4 yr and beyond, helping to shape its contribution within the larger context of planetary science and exploration.

National Science Foundation (NSF) [DGE-1143953]; NASA Earth and Space Sciences Fellowship (NESSF) [NNX16AP09H]; NASA Mars Data Analysis Program (MDAP) award [NNX15AM62G]

Carbon dioxide has been detected on the trailing hemispheres of several Uranian satellites, but the exact nature and distribution of the molecules remain unknown. One such satellite, Umbriel, has a prominent high albedo annulus-shaped feature within the 131-km-diameter impact crater Wunda. We hypothesize that this feature is a solid deposit of CO 2 ice. We combine thermal and ballistic transport modeling to study the evolution of CO 2 molecules on the surface of Umbriel, a high-obliquity (∼98°) body. Considering processes such as sublimation and Jeans escape, we find that CO 2 ice migrates to low latitudes on geologically short (100s–1000 s of years) timescales. Crater morphology and location create a local cold trap inside Wunda, and the slopes of crater walls and a central peak explain the deposit's annular shape. The high albedo and thermal inertia of CO 2 ice relative to regolith allows deposits 15-m-thick or greater to be stable over the age of the solar system. We conclude that Wunda, located at low latitude (7.9° S) and near the center of the trailing hemisphere where CO 2 detections are strongest, likely contains a solid CO 2 ice deposit. We discuss prospects for similar CO 2 ice deposits on crater floors on the other major Uranian moons, and predict that they are present on Ariel, Titania, and possibly Oberon (but not Miranda or smaller satellites). Such deposits have likely not been observed due to the limited nature of Voyager 2 image coverage.

Water ice cliffs on MarsSome locations on Mars are known to have water ice just below the surface, but how much has remained unclear. Dundaset al.used data from two orbiting spacecraft to examine eight locations where erosion has occurred. This revealed cliffs composed mostly of water ice, which is slowly sublimating as it is exposed to the atmosphere. The ice sheets extend from just below the surface to a depth of 100 meters or more and appear to contain distinct layers, which could preserve a record of Mars' past climate. They might even be a useful source of water for future human exploration of the red planet.Science, this issue p.199

Mars Workshop on Amazonian and Present-day Climate; Lakewood, Colorado, 18–22 June 2018

Geomorphological features potentially related to subsurface ice, such as scalloped depressions, expanded craters, pedestal craters, and banded terrain, are present in and around Hellas Planitia, Mars. We present a radar survey of the region using the Shallow Radar (SHARAD) instrument on board the Mars Reconnaissance Orbiter (MRO) to identify candidate subsurface reflectors that may be due to the presence of potentially ice-rich deposits. We found that the majority of radar returns are likely from off-nadir surface topography (“clutter”), arising from the rough topography of the region. There is no widespread radar return from any subsurface interfaces. However, we identify a group of six reflectors adjacent to each other on a plateau in Malea Patera in which we have higher confidence. Landforms associated with a likely ice-rich mantle are associated with the plateau, but the thickness of this mantle does not correspond to the expected depth of the reflectors. However, layers beneath the mantle and marginal pitting at the edge of the plateau are similar to those associated with pedestal craters, which may be ice rich and are a similar thickness to the expected depth of the reflectors. Malea Patera has been interpreted to be a volcanic caldera, so the reflectors may be associated with a volcanic deposit within the plateau, although the evidence for this is inconclusive. Because this radar detection is localized and its origin ambiguous, we cannot use it to make conclusions about the thickness of subsurface deposits in the Hellas region as a whole. The lack of widespread radar reflectors in this region, as compared to the northern mid-latitudes where extensive radar reflections have been mapped, may be due in part to higher surface roughness, which creates radar clutter that may obscure subsurface reflectors. However on the southern rim of the basin and south of the basin, the lack of reflectors may indicate that the possible ice-rich deposits observed geomorphologically in this region are too thin to be resolved by SHARAD, are dielectrically similar to the underlying unit, or have a gradual vertical transition in ice content that is not reflective for the radar. This would imply that recent climate processes may have favored widespread, thick ice deposition or preservation in the northern hemisphere as compared to the southern hemisphere.

Ahuna Mons is a 4-km-tall mountain on Ceres interpreted as a geologically young cryovolcanic dome. Other possible cryovolcanic features are more ambiguous, implying that cryovolcanism is only a recent phenomenon or that other cryovolcanic structures have been modified beyond easy identification. We test the hypothesis that Cerean cryovolcanic domes viscously relax, precluding ancient domes from recognition. We use numerical models to predict flow velocities of Ahuna Mons to be 10–500 m/Myr, depending upon assumptions about ice content, rheology, grain size, and thermal parameters. Slower flow rates in this range are sufficiently fast to induce extensive relaxation of cryovolcanic structures over 108–109 years, but gradual enough for Ahuna Mons to remain identifiable today. Positive topographic features, including a tholus underlying Ahuna Mons, may represent relaxed cryovolcanic structures. A composition for Ahuna Mons of >40% ice explains the observed distribution of cryovolcanic structures because viscous relaxation renders old cryovolcanoes unrecognizable.

The distribution of subsurface water ice on Mars is a key constraint on past climate, while the volumetric concentration of buried ice (pore-filling versus excess) provides information about the process that led to its deposition. We investigate the subsurface of Arcadia Planitia by measuring the depth of terraces in simple impact craters and mapping a widespread subsurface reflection in radar sounding data. Assuming that the contrast in material strengths responsible for the terracing is the same dielectric interface that causes the radar reflection, we can combine these data to estimate the dielectric constant of the overlying material. We compare these results to a three-component dielectric mixing model to constrain composition. Our results indicate a widespread, decameters-thick layer that is excess water ice ~10^4 km^3 in volume. The accumulation and long-term preservation of this ice is a challenge for current Martian climate models., Comment: 14 pages, 3 figures, 8 pages of supporting information included at the end

Understanding the environment in which a galaxy resides is crucial to our understanding of galaxy evolution. Most galaxies (~70%) live in groups and it is important to develop a quantitative understanding of how galaxies are distributed within groups and how groups are distributed in the larger scale structure. In addition to the traditional friends-of-friends algorithm, I have applied various concepts from networking algorithms to understand the network of galaxy groups in clusters, and to understand the substructure within groups. Doing so provides a new method of gauging the large scale environments of galaxies and groups, and will result in a more concrete way to define a group and to quantify the strength of the community structure that exists.