Remnant Buried Ice in the Arsia Mons

Introduction. The western and northwestern flanks of the Tharsis Montes volcanoes were covered by tropical mountain glaciers 125–220 million years ago [1], as evidenced by the morphology, stratigraphic relationships, and spatial distribution of landforms in the fanshaped deposits (FSDs) on each volcano [2-8] and by climate and glacial flow models that predict snow accumulation and ice flow in those regions during periods of high spin-axis obliquity [9-10]. The ~166,000 km Arsia Mons FSD [4,7,8] is the largest of the Tharsis Montes FSDs and has been in place for ~210 Ma [1]. The Smooth Facies, one of the geomorphologic units in the FSDs [11-12], has been interpreted as remnant debris-covered glaciers [4]. Shean et al. [5] suggest that lineated debris partially filling tectonic graben in the FSD also has ice at its core. The topography of these deposits [5], morphologic indicators of active flow [4] and the morphology of superimposed craters [13], suggest that ice 100-300 m thick may still be present at depth. Here, we review several other classes of landforms in the FSD that have not been previously described, and whose morphology indicates that they may still contain remnant tropical mountain glacier ice. Landforms Interpreted as Indicative of Remnant Ice. On Earth, glacier ice may be buried and preserved wherever overlying debris is sufficiently thick. The morphology of several landforms in the Arsia Mons FSD suggests that ancient ice is present in the FSD: Pit-and-knob terrain. Near the northern edge of the fan-shaped deposit is a field of mounds (“knobs”) and shallow topographic depressions (“pits”; Figure 1). Pits and knobs are similar in diameter (up to 1 km) and shape, and generally fall along lines concentric with the outline of the smooth facies. Many knobs are surrounded by shallow annular depressions (“moats”), and some pits have what appear to be degraded knobs at their centers. We propose that the terrain is ice-cored and that the landforms represent a progression in which gradual loss of ice via sublimation causes topographic inversion, with knobs becoming moated knobs, then pits with degraded knobs, and finally pits. The Arsia FSD pit-and-knob terrain may have formed by backwasting of smooth facies ice and subsequent burial of isolated ice blocks by volcanic or aeolian debris, analogous to the formation of terrestrial kettled outwash plains [e.g. 14-15]. Alternatively, the pit-and-knob terrain may have formed in a manner analogous to terrestrial “controlled moraine” [e.g. 1617]. When debris within or atop glaciers [e.g. 18] is concentrated into belts by glacial flow, bands of debris Figure 1. CTX mosaic (top) and sketch map (bottom) of typical pit (yellow in sketch map) and knob (red in sketch map) terrain.