Ice-driven Degradation Styles in the Martian Mid-latitudes: Constraints from Lobate Debris Aprons, Lineated Valley Fill, and Small Flow Lobes

Introduction. The Martian mid-latitudes are regions of high scientific interest given recent descriptions of mantling deposits and glacial features believed to be relicts of recent ice ages on Mars [1-4]. Geomorphic indicators of ground ice have long been proposed to be concentrated in mid-latitude zones based on analyses of such features as lobate debris aprons, lineated valley fill, and concentric crater fill in Viking Orbiter (VO) images of Martian fretted terrain and the southern highlands [5-10]. Recent climate modeling also shows specific mid-latitude volatile accumulation zones that directly correspond to concentrations of these features [11-12]. The current investigation examines the geomorphic characteristics of lobate debris aprons, lineated valley fill, and small flow lobes found on crater rims and massifs in order to characterize emplacement styles for ice-rich flows on Mars. As part of this work, we examine topographic controls on planform shape and surface lineation patterns. Debris Aprons/Lineated Valley Fill. Studies based on VO images suggested that lobate debris aprons and lineated valley fill formed by flow of rock and ice mixtures, with latitudinal control attributed to seasonal frost deposition [6-10, 13]. Lineated valley fill covers canyon floors in the fretted terrain and within the circum-Hellas canyon systems. Debris aprons are found in similar locations, with prominent concentrations along the dichotomy boundary and in highland terrains of eastern Hellas [7-8, 14]. Debris aprons extend from massifs, mesas, canyon walls, and crater rims. Aprons frequently form complexes, in which multiple debris masses or lobes have coalesced to form a composite feature, with variable preservation of individual debris lobes. Detailed studies of the Deuteronilus Mensae [15-17], Tempe/Mareotis [18], and eastern Hellas [19-21] regions have incorporated new high-resolution imaging datasets (MOC, THEMIS, and MEX) and MOLA topography. Apron planform shapes, surface lineation patterns, and topographic profiles suggest viscous flow/deformation of apron masses, and surface textures indicate a complex history of surface mantling and subsequent degradation by aeolian processes and melting and/or sublimation of contained ice. The large range of potential terrestrial analogues (rock glaciers, debris-covered glaciers, and wet or icy debris flows) demonstrates significant