Ring-mold Craters on Lineated Valley Fill, Lobate Debris Aprons, and Concen- Tric Crater Fill on Mars: Implications for Near-surface Structure, Composi-

Introduction: Ring-mold craters [1] comprise a suite of craters with unusual morphologies that have as yet been identified only on lineated valley fill (LVF), lobate debris aprons (LDA), and concentric crater fill (CCF) in the northern and southern mid-latitudes of Mars. Ringmold craters (RMCs) are generally rimless and consist of an outer annular trough surrounding a variety of interior morphologies. They have been interpreted as indicators of buried ice in the substrate at the time of formation, supporting a debris-covered glacier mode of origin for these types of deposits [1]. Previous analyses of RMCs were conducted on MOC and THEMIS data [1-4]. We expand investigations of these unusual craters to CTX data for comparison of the size-frequency distribution of the crater populations and derived ages of LVF, LDA and CCF in the Deuteronilus Mensae region (centered at around ~40N, 20E). In conducting these studies, we use the model of [1] that RMCs form by impacting into glacial ice buried beneath a relatively thin debris cover. Impacting into nearly pure ice causes the unusual morphologies and likely also causes crater diameters to be at least two times greater than for craters with otherwise identical impact parameters in basalt, as indicated by experimental results [1, 57] Observations: The principal studies of [1] and [2] were of RMCs on MOC images in Mamers Valles (Fig. 1). Mamers Valles is a ~950-km-long valley just south of Deuteronilus Mensae, ~5-10 km wide in its southern regions and ~20-30 km wide in its northern regions (“downstream” of the > 100 km crater it intersects). The narrower, southern parts of the valley are characteristically filled with LVF and the wider, northern parts with linear LDA that meet in the center of the valley and in some cases begin to flow down the local topographic gradient [8]. MOC images analyzed have resolutions of ~3-7 m/pixel, widths of ~3-4 km and lengths of tens of km, and they cover a cumulative area of ~2,500 km (Fig. 1). For comparison, we looked at a CTX image that covers the area where Mamers Valles transitions from 5-10 km wide to 20-30 km wide (Fig. 1). The CTX image has a resolution of nearly 6 m/pixel, a width of ~30 km and a length of ~140 km, giving wall-to-wall coverage of Mamers Valles at that location. The area of Mamers Valles covered in the image and used for crater counting is ~1,100 km (Fig. 1). We looked at CTX data of the T-shaped valley of [9], centered at ~37.5N, 24.2E, ~200 km east of Mamers Valles and a ~16-km-diameter crater at ~41.0N, 14.2E, in western Deuteronilus Mensae (Fig. 1). Resolution of these images was also ~6 m/pixel and there was complete CTX coverage for both areas. The areas used for crater counting are ~1,400 km and ~100 km, respectively (Fig. 1).