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Highlights • We test hypotheses for the formation of craters within martian glacial deposits. • Nine crater types and multiple degradation sequences observed. • There is a small size difference between ring-mold craters and bowl-shaped craters. • Flow lineations cross-cut older ring-mold craters but not other crater types. • Observations most consistent with formation by mantle emplacement and dissection. Abstract Lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF) on Mars, interpreted to be debris-covered glaciers, possess craters with a suite of distinct interior landforms (called "ring-mold craters") that have been attributed to the presence of glacial ice at depth or surface modification processes. We tested current hypotheses for the formation of ring-mold craters by conducting a comprehensive analysis of the size and morphology of 16,457 impact craters ≥125 m in diameter formed within glacial deposits in Deuteronilus Mensae. Two major groups, bowl-shaped craters and ring-mold craters, are found, with at least nine distinct crater types. While there is statistical difference in median diameters between these crater types, this difference is relatively small and is within the estimated uncertainty in diameter measurements and may be related to more enhanced erosion of the rims of ring-mold craters. Clear degradation sequences are observed, supporting a role for post-impact modification in producing at least some of the diversity in crater landforms. The spatial density of ring-mold craters is also directly correlated with the development of LDA, LVF, and CCF surface textures. Flow lineations cross-cut two ring-mold crater types but they maintain their circular planforms in some cases, suggesting that the craters initially formed completely within mantling layers deposited after glacial flow had ceased. We also find analogous craters in non-glacial units; glacial ice is therefore not required to form the observed morphologic diversity. Our observations are most consistent with formation of crater landforms by emplacement and modification of at least two depositional episodes of icy dust (i.e., "mantle"). This mantle was initially tens of meters in thickness to support crater formation, and has experienced much downwasting and erosion since emplacement. Derived crater retention ages of 460 Ma for LDA, LVF, and CCF features in the region therefore reflect deposition of mantle units and only give a very minimum age for the formation of LDA, LVF, and CCF. [ABSTRACT FROM AUTHOR]

Highlights • We constrain the physical characteristics of debris that covers glaciers on Mars. • Two main layers are observed: icy dust airfall superposes headwall-derived sediment. • In total, the supraglacial debris is a few meters to tens of meters in thickness. • Absence of near-surface radar reflectors may imply gradation in debris and ice. • Craters penetrate tens of meters into glacial ice yet maintain shape over ∼10 Ma. Abstract Geomorphic and geophysical evidence supports a debris-covered glacier origin for a suite of landforms at the mid-latitudes of Mars, including lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF). These large reservoirs of ice and their near-surface structure provide a rich record for understanding the planet's climate and history of global volatile exchange over the past billion years. LDA, LVF, and CCF are also potential sites for future robotic and human missions but the accessibility of glacial ice for direct sampling and in situ resource utilization depends largely on the geotechnical properties of the surface debris ("supraglacial debris"), including its thickness, grain sizes, and density structure. The physical properties of this supraglacial debris layer have been poorly constrained. We use images of morphology, digital elevation models, thermal inertia data, and radar sounding data to probe the near surface of LDA, LVF, and CCF in Deuteronilus Mensae in order to place constraints on the sources, grain sizes, thickness, and stratigraphy of supraglacial debris. We find evidence for at least a two-layer stratigraphy. Layered mantle consisting of atmospherically emplaced dust and ice superposes boulder-rich sediment sourced by rockfalls glacially transported downslope. High thermal inertia, boulder-rich termini and debris bands reminiscent of medial moraines are found throughout the study region, supporting a rockfall origin for at least a fraction of the debris exposed at the surface. This supraglacial debris layer would have thickened with time from sublimation of glacial ice and liberation of englacial sediment and dust. At present, the entire supraglacial debris package is a minimum of a few meters in thickness and is likely tens of meters in thickness in many locations, possibly thinning regionally at lower latitudes and locally thinning toward the headwalls. The lack of terracing or interior structures in craters formed within LDA, LVF, and CCF and the absence of near-surface reflectors in SHARAD radar data further suggest that no strong contrasts in permittivity or strength occur at the interface of the layers. [ABSTRACT FROM AUTHOR]

The mid-latitudes of Mars are host to a record of recent episodes of accumulations of ice-rich materials. The record includes debris aprons, interpreted to be debris-covered glaciers, that may represent the preserved remnants of a much more extensive ice sheet. We assessed the possibility of former glacial extents by examining debris aprons and the surrounding plains in Deuteronilus Mensae. Geomorphic units and stratigraphic relationships were mapped and documented from Mars Reconnaissance Orbiter (MRO) Context (CTX) and High Resolution Imaging Science Experiment (HiRISE) camera images, and crater retention ages were estimated from crater size–frequency distributions. Three major units are observed within the study area: debris aprons, lower plains, and upper plains. Debris aprons exhibit characteristics typical for these features documented elsewhere and in previous studies, including integrated flow lineations and patterns, convex-upward profiles, and knobby and brain terrain surface textures. A lower bound on the age for debris aprons is estimated to be 0.9 Ga. Debris aprons are superposed on a lower plains unit having a lower bound age of 3.3–3.5 Ga. A 50–100 m thick upper plains unit superposes both debris apron landforms and lower plains units and has a best-fit minimum age of 0.6 Ga. The upper plains unit exhibits characteristics of atmospherically-emplaced mantle material, including fine-grained nature, sublimation textures, cyclic layering, draping character, and widespread spatial distribution. Fracturing and subsequent sublimation/erosion of upper plains on debris aprons has contributed to many of the surface textures on debris aprons. The upper plains unit has also been eroded from the lower plains and plateaus, evidenced by isolated blocks of upper plains in the interiors of craters and on the walls and tops of plateaus. While no conclusive evidence diagnostic of former cold-based ice sheets are observed in the plains within the study region, such landforms and units may have been poorly developed or absent, as is often the case on Earth, and would have been covered and reworked by later mantling episodes. These observations suggest that emplacement of thick ice-rich mantle deposits extended at least to near the Early/Middle Amazonian boundary and overlapped with the waning stages of glaciation in Deuteronilus Mensae. [ABSTRACT FROM AUTHOR]