Absolute model ages (AMAs) are derived from crater size-frequency distributions (CSFDs), but these estimates can be subject to biases introduced by local terrain properties and secondary craters. We investigated crater populations superposed on the ejecta deposits of five young Copernican-age craters, spanning a range of ages (similar to(1-44) & times; 10(6) yr) and sizes (2.3-24 km), and compared spatial crater densities with Lunar Reconnaissance Orbiter Diviner-derived rock abundance and regolith temperature maps. We found that crater spatial densities decrease with increasing rock abundance and regolith temperature at all locations studied, suggesting that impacts into boulders and melt deposits inhibit crater formation via an armoring or crater-scaling effect. This effect persists at rock fractions as low as similar to 5%. A direct consequence is that CSFDs in high rock abundance regions have shallower slopes and yield systematically younger AMAs. Conversely, areas with lower rock abundance exhibit CSFD slopes that are steeper than predicted by production functions, consistent with a pervasive presence of self-secondary craters. The opposing influences of target properties (shallower slope) and self-secondaries (steeper slope) result in a critical divergence in CSFDs at smaller diameters between areas with lower (<5%) and higher (>5%) rock abundance. The application of a regolith gardening model indicates that gardening depths of up to similar to 20 cm are insufficient to discernibly eliminate heterogeneous target properties. These effects confound straightforward interpretations of CSFDs for small, meter-to-decameter crater diameters. We recommend inspection of CSFD slopes against production functions and prioritization of larger crater diameters when deriving AMAs.

Subsurface water ice deposits on Mars are an important resource for potential future human exploration. They are also an indicator of the planet’s past climate. However, the distribution of subsurface water ice in Mars’s midlatitudes is uncertain because spacecraft imagery cannot directly observe subsurface ice in most cases. Various spacecraft remote sensing instruments are sensitive to subsurface water ice, including thermal imaging spectrometers, radar sounders, and neutron spectrometers. Geomorphic analyses of images can also implicate subsurface ice. Building upon the data products from the Mars Subsurface Water Ice Mapping project, we provide a probabilistic framework to jointly interpret existing data and estimate the likelihood of subsurface water ice in the Martian midlatitudes between 60 ^∘ S and 60 ^∘ N with uncertainty. Broadly, we find that near-surface ice is likely present poleward of ∼45 ^∘ in both the northern and southern hemispheres. However, closer to the equator, existing remote sensing data cannot uniquely constrain the presence of subsurface water ice. Our probabilistic results provide a framework for quantifying the abundance of ice on Mars, and our uncertainty estimates allow future analysis and exploration to target regions of high uncertainty.

Mars has an extensive yet poorly understood cryosphere. Nevertheless, both direct and indirect evidence indicates extensive buried ice across the midlatitudes, including locations where it is presently unstable. While much progress has been made in exploring the processes responsible for ice deposition and preservation during recent climatic fluctuations, a global assessment of the multiple ice reservoirs remains elusive. Motivated by science and the need to find suitable human landing sites, the Mars Subsurface Water Ice Mapping (SWIM) project has developed techniques to map out buried ice. Through integration of all appropriate orbital data sets, the SWIM project produces ∼3 km pixel ^−1 ice consistency maps over depth ranges of 0–1 m, 1–5 m, and >5 m. In concert with other studies, prior SWIM phases have recognized the uncertainty in our understanding of the geographic and vertical distribution of ice, especially between depths of 1 m and 10 m, creating a push for new ice-prospecting orbital missions, such as the International Mars Ice Mapper mission concept. Here we document the latest SWIM phase, which provides notional targeting maps of the lowest-latitude ice for future missions via a significant improvement in the geomorphic component of our work. The new mapping incorporates both an enhancement in our mapping of geomorphic features and surveys of thermal contraction crack polygons. Our results demonstrate the highly variable nature of the spatial distribution of the shallowest ground ice, with the most equatorward excursions occurring below 30° latitude N/S, locations thought to be out of equilibrium with the current climate.

Significant amounts of ice are located on the surface and in the subsurface of Mars. These polar and non-polar deposits are primarily water ice but, at the poles, carbon dioxide (CO2) ice exists on the surface where it exchanges seasonally with the atmosphere, while buried CO2 ice deposits have also been found. Analogous to Earth, Martian glacial ice deposits, as well as glacial remnants and landscapes from past glaciations, record how volatiles and components in the atmosphere, surface, and subsurface have interacted over time. Surface and subsurface expressions of past glaciations and deglaciations are critical to our understanding of the past climate on Mars, which is one of the highest priority goals in Mars science. Mars' ice and climate record is constrained by the glacial record that extends over the last ~1 billion years of the Amazonian Period. Imagery, elevation models, radar, and spectral data have revealed aspects of the setting and structure of glacial deposits, glacial remnants, and geomorphological signatures of receded glaciers. The stratigraphy of these landforms has the capacity to provide the most highly resolved record available of past climate conditions on Mars. We discuss three key questions, leading with: what history of the Late Amazonian Epoch climate is recorded in the Polar Layered Deposits? Then, what sequence of glaciation and deglaciation developed non-polar glacial remnants? Related to interpreting glacial landscapes, we discuss: how widespread were past warm-based conditions among extant Amazonian-aged buried glaciers? Addressing these questions is necessary as part of continued efforts to advance our understanding of ice and climate histories on Mars.

The distribution of impact craters on the ejecta of Giordano Bruno, a recent (<10 Ma) 22-km diameter crater within the lunar highlands, exhibits substantial variations. We surveyed craters D >= 10 m across a 1,323 km(2) area of Giordano Bruno's ejecta and compared the distribution of craters with variations in thermophysical properties derived from the Lunar Reconnaissance Orbiter Diviner instrument. We used Diviner-derived rock abundance and nighttime regolith temperatures along with thermal model-predicted surface temperatures for a diversity of terrains to identify and isolate areas of the ejecta based on thermophysical properties such as bulk density and thermal conductivity. We found that thermophysical properties of the ejecta vary considerably both laterally and vertically, and consistently differ from typical regolith, indicating the presence of higher thermal inertia materials. Crater-size frequencies are significantly lower in areas with terrain properties exhibiting higher: rock abundance, nighttime temperatures, and/or modeled thermal inertia. This discrepancy in crater distribution increases for craters smaller than similar to 25 m. These thermophysical variations indicate changes in the mechanical properties of the target materials. We suggest that these variations-specifically, terrain-dependent crater scaling variations and impactor-scale heterogeneities in material properties such as the presence or absence of large boulders-may influence crater diameters or inhibit crater production altogether in Giordano Bruno's ejecta; furthermore, these factors are size-dependent.

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.

Although ice in the Martian midlatitudes is typically covered by a layer of dust or regolith, it is exposed in some locations by fresh impact craters or in erosional scarps. In both cases, the exposed ice is massive or excess ice with a low lithic content. We find that erosional scarps occur between 50° and 61° north and south latitude and that they are concentrated in and near Milankovič crater in the northern hemisphere and southeast of the Hellas basin in the southern hemisphere. These may represent locations of particularly thick or clean bodies of ice. Pits created by retreat of the scarps represent sublimation‐thermokarst landforms that evolve in a manner distinct from other ice‐loss landforms on Mars. New impact craters reveal that clean subsurface ice is widespread at middle‐ and high‐latitudes in both hemispheres at depths less than 1 m. Both the depth to ice and the ice content appear to exhibit significant variability over tens to hundreds of meters. The lowest‐latitude exposed ice is near 39°N and is at the edge of a region where impact exposures between 40° and 50°N are common, consistent with other indications of a high ice content. This lowest‐latitude ice may be currently unstable and subliming. Impact craters on lineated valley fill excavate ice blocks that may represent the top of debris‐covered glacial ice. Together, these landforms indicate widespread, clean subsurface ice at middle‐latitudes on Mars. The distribution and properties of this ice could provide information about past climate conditions.

FROM LRO DIVINER OBSERVATIONS. J.-P. Williams 1 , C. L. Gallinger 2 , P. O. Hayne 3 , D. A. Paige 1 , A. V. Pathare 4 , and E. S. Costello 5,6 , 1 Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, 2 Department of Earth Sciences, University of Toronto, ON, Canada, 3 Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, CO, 4 Planetary Science Institute, Tuscon, AZ, 5 Department of Geology and Geophysics, University of Hawai’i at Mānoa, Honolulu, HI, 6 Hawaii Institute of Geophysics and Planetology, Honolulu, HI.

Debris-covered glaciers from around the world offer distinct environmental, climatic, and historical conditions from which to study the effects of debris on glacier-ice evolution. A rich literature on debris-covered glaciers exists from decades of field work, laboratory studies, remote-sensing observations, and numerical modeling. In general, the base of knowledge established by studying periglacial, glacial, and paraglacial landforms on Earth has been applied to aid interpretation of ice-rich or ice-remnant landforms on Mars, but research has progressed on both planets. For Mars, the spatial distribution of lobate debris aprons and glacier-like forms, in particular, is critical to constraining past climate conditions when such features were active, reconstructing past ice extent, and estimating the total inventory of buried ice remaining in the mid-latitudes of Mars. This review spans a range of knowledge about debris-covered glaciers on Earth, in order to add context to investigations of dust and debris-covered ice on Mars and to put research on both planets in a perspective aimed at maximizing process-based understanding of glacier evolution. The state of knowledge and some gaps in knowledge on Mars are discussed in relation to possible avenues for future research in how landforms are classified, advances in comparative planetology, and new understanding from future missions. While this review is focused primarily on processes controlling active debris-covered glaciers, a key to understanding glacier change through time is to consider individual landforms in context with the full-system environment in which they are found. For Earth, this includes understanding local and regional controls on current glacier change, and how these processes relate to landform development in the past as well as what may develop in the future. For Mars, this includes evaluating how present-day landforms elucidate past ice activity and environmental conditions during epochs when orbital parameters, climate, and water ice distribution were substantially different.

Earth and Space Science Open Archive Presented WorkOpen AccessYou are viewing the latest version by default [v1]Underground Ice on Mars: Characterization Activities, Potential as an In Situ Resource, and Possible Destination for Human ExplorersAuthorsAliBramsoniDJenniferHeldmannNathanielPutzigGarethMorganMatthewGolombekNathanWilliamsiDColinDundasHannaSizemoreiDAlfredMcEwenEricPetersenMatthewPerryiDStefanoNerozziAsminPathareiDDavidBakerIsaacSmithSamuel WestonCourvilleiDJamesHeadiDDavidBeatyPaulWoosterSee all authors Ali BramsoniDCorresponding Author• Submitting AuthorPurdue UniversityiDhttps://orcid.org/0000-0003-4903-0916view email addressThe email was not providedcopy email addressJennifer HeldmannNASA Ames Research Centerview email addressThe email was not providedcopy email addressNathaniel PutzigPlanetary Science Instituteview email addressThe email was not providedcopy email addressGareth MorganPlanetary Science Instituteview email addressThe email was not providedcopy email addressMatthew GolombekJet Propulsion Laboratoryview email addressThe email was not providedcopy email addressNathan WilliamsiDJet Propulsion LaboratoryiDhttps://orcid.org/0000-0003-0602-484Xview email addressThe email was not providedcopy email addressColin DundasU. S. Geological Surveyview email addressThe email was not providedcopy email addressHanna SizemoreiDPlanetary Science InstituteiDhttps://orcid.org/0000-0002-6641-2388view email addressThe email was not providedcopy email addressAlfred McEwenUniversity of Arizonaview email addressThe email was not providedcopy email addressEric PetersenUniv of AK-Geophysical Instview email addressThe email was not providedcopy email addressMatthew PerryiDPlanetary Science InstituteiDhttps://orcid.org/0000-0003-3434-5333view email addressThe email was not providedcopy email addressStefano NerozziUniversity of Arizonaview email addressThe email was not providedcopy email addressAsmin PathareiDPlanetary Science InstituteiDhttps://orcid.org/0000-0001-8922-4470view email addressThe email was not providedcopy email addressDavid BakerNASA Goddard Space Flight Centerview email addressThe email was not providedcopy email addressIsaac SmithPlanetary Science Instituteview email addressThe email was not providedcopy email addressSamuel Weston CourvilleiDArizona State UniversityiDhttps://orcid.org/0000-0002-9539-1301view email addressThe email was not providedcopy email addressJames HeadiDBrown UniversityiDhttps://orcid.org/0000-0003-2013-560Xview email addressThe email was not providedcopy email addressDavid BeatyJet Propulsion Laboratoryview email addressThe email was not providedcopy email addressPaul WoosterSpaceXview email addressThe email was not providedcopy email address

Introduction: The Subsurface Water Ice Mapping (SWIM) project supports an effort by NASA’s Mars Exploration Program to determine in situ resource availability [1-2]. We are performing global reconnaissance mapping as well as focused multi-dataset mapping to characterize the distribution of water ice from 60oS to 60oN latitude. In 2019, we produced ice consistency maps for the northern hemisphere (0–60oN) from 0–225oE and 290–360oE longitude. In 2020, we are extending our mapping into the southern hemisphere (0– 60oS) and from 225–290oE longitude in the northern hemisphere at elevations <+1km. Our maps are being made available on the SWIM Project website (https://swim.psi.edu), and we intend to complete our global mapping by the summer of 2020. Follow us on Twitter @RedPlanetSWIM for project news and product release information. The SWIM Datasets: To search for and assess the presence of shallow ice across our study regions, we are integrating multiple datasets to provide a holistic view of the upper 10s of m of the Martian subsurface. The individual datasets and methods we employ include neutron-detected hydrogen maps (MONS), thermal behavior (TES, THEMIS, and MCS), multiscale geomorphology (HiRISE, CTX, HRSC, and MOLA), and SHARAD surface and subsurface radar echoes. Consistency Mapping: For the SWIM 2019 maps, we used the SWIM equation [2-3] to provide a quantitative assessment of how consistent (or inconsistent) the various remote sensing datasets are with the presence of shallow (<5 m) ice. The SWIM Equation yields values ranging between +1 and -1, where +1 means that all of the data are consistent with the presence of ice, 0 means that the data give no indications of the presence or absence of ice, and -1 means that the data are inconsistent with the presence of ice. Here, we focus on ice consistency values from geomorphic mapping. For more information on the project and its techniques and datasets, visit our website and associated presentations at this LPSC: Putzig et al. (summary of results), Perry et al. (SWIM Equation and methods), Sizemore et al. (thermal and neutron analysis), Morgan et al. (radar surface reflectivity), Petersen et al. (radar subsurface mapping), and Bain et al. (focused regions). SWIM 2019 Methods: The SWIM 2019 geomorphology ice consistency map [1,2,4] (Fig. 1) was based on previous and new mapping of the presence of periglacial and glacial landforms inferred to have required ice for their genesis. A sampling of 4x4° CTX image mosaics (beta01 versions from the Caltech Murray Lab [5]) within previously mapped geologic units [6] was used to tally the number of observed periglacial and glacial landforms and to extrapolate the observations to the geologic unit boundaries. The number of landforms was then normalized with equal weighting to yield ice consistency values between 0 and 1. Areas <27°N with no periglacial or glacial features were assigned a value of -1 and areas from 30° to 27°N were assigned an equatorward Gaussian decay in values from 0.1 to 0. Areas of scalloped terrain in Acidalia (070°W) were assigned a value of 0.75. Previous mapping was also used, including glacial features (lobate debris aprons, lineated valley fill, and concentric crater fill; [7])

Active glacial environments exhibit characteristic landforms due to the interplay of ice, climate, soil, and rock. These landforms are used as indicators of past and present climate conditions, and the base of knowledge established by studying glacial morphologies on Earth has been applied to aid interpretation of ice-rich or ice-remnant landforms on Mars. We focus on how glaciers and glacial landforms act to erode their surrounding landscape when they are active, and how they are preserved on the landscape when climate changes and ice retreats. This includes specific study of glaciers, debris-covered glaciers, rock glaciers, and cirques because glaciers act to erode landscapes, and landscapes contribute debris that can preserve glacier ice. We contextualize lobate debris aprons and glacier-like forms on Mars with debris-covered glaciers on Earth in order to put the latest research on both planets in a perspective aimed at maximizing process-based understanding of glacier evolution and ice preservation. While we primarily focus on processes controlling active debris-covered glaciers, a key to understanding glacier change through time is to consider individual landforms in context with the full-system environment in which they are found. We discuss process-based progressions and relationships between glacial landforms as understood on Earth; for example, the development of clean-ice glaciers, debris-covered glaciers, rock glaciers, moraines, and talus may be determined as a function of ice movement and debris input. Building from our current knowledge of Mars, we show results from preliminary investigations of previously unmapped ice-remnant forms in Eastern Hellas and the Deuteronilis/Protonilus/Nilosyrtis Mensae regions that we have found using the recently available Context Camera (CTX) image mosaic (http://murray-lab.caltech.edu/CTX/). These landforms are newly identified small components of the martian glacial system, that are different from, but likely related to, glacier-like forms and recessional glacier-like forms. We also search for the cirque signature of ice erosion on Mars, and discuss how the timing of glacial, deglacial, and paraglacial activity may be further constrained by evaluating the existence and distribution of all possible components of a glacial landsystem. Interpretations of Mars from remote sensing alone can be evaluated against targeted interpretations on Earth using both remote sensing and field studies. In particular we will share on recent work studying debris sources and glacier evolution at Mt. Rainier, Washington state. By applying terrestrial understanding to Mars we aim to evaluate how present-day martian landforms are informative of past activity and conditions during times when orbital parameters, climate, and water-ice distribution were different.

Abstract Ice in both terrestrial and planetary settings often contains rock particles. Here we present an experimental investigation of the influence of intergranular particles on the rheological behavior of ice. Experiments were performed on samples fabricated from 10‐μm ice powders +1‐μm graphite or 0.8‐μm alumina particles and subjected to elevated confining pressures. A critical particle fraction, ∼6%, was observed, below which samples behave like pure ice and deform by both grain boundary sliding (GBS) and dislocation creep, and above which GBS creep is impeded. Above this critical fraction, ice grains occur in particle‐free clusters surrounded by bands of particles mixed with fine‐grained ice, resulting in the impedance of GBS in the bands as well as sliding between the ice clusters. Our results imply that South Polar Layered Deposits and midlatitude lobate debris aprons on Mars must contain >94% ice and that the shallow subsurface of Ceres could contain >90% ice.

We present improved Mars Odyssey Neutron Spectrometer (MONS) maps of near-surface Water Equivalent Hydrogen (WEH) on Mars that have intriguing implications for the global distribution of "excess" ice, which occurs when the mass fraction of water ice exceeds the threshold amount needed to saturate the pore volume in normal soils. We have refined the crossover technique of Feldman et al. (2011) by using spatial deconvolution and Gaussian weighting to create the first globally self-consistent map of WEH. At low latitudes, our new maps indicate that WEH exceeds 15% in several near-equatorial regions, such as Arabia Terra, which has important implications for the types of hydrated minerals present at low latitudes. At high latitudes, we demonstrate that the disparate MONS and Phoenix Robotic Arm (RA) observations of near surface WEH can be reconciled by a three-layer model incorporating dry soil over fully saturated pore ice over pure excess ice: such a three-layer model can also potentially explain the strong anticorrelation of subsurface ice content and ice table depth observed at high latitudes. At moderate latitudes, we show that the distribution of recently formed impact craters is also consistent with our latest MONS results, as both the shallowest ice-exposing crater and deepest non-ice-exposing crater at each impact site are in good agreement with our predictions of near-surface WEH. Overall, we find that our new mapping is consistent with the widespread presence at mid-to-high Martian latitudes of recently deposited shallow excess ice reservoirs that are not yet in equilibrium with the atmosphere. (C) 2017 Elsevier Inc. All rights reserved.