The Formation and Evolution of Youthful Gullies on Mars: Gullies as a Late- Stage Product of Mars’ Most Recent Ice Age

Introduction: The discovery of youthful gullies on Mars [1] provided strong evidence for short-duration flow of liquid water within the last several million years, an era not generally considered conducive to the stability of liquid water at the surface [2]. Given average conditions below the melting point for all of Mars and the apparent correlation between bed-rock layering and gully alcoves, Malin and Edgett [1] invoked seepage from a subsurface aquifer to explain the observed morphology and distribution of gullies. The global distribution of gullies (only at mid-and high-latitudes, mostly on pole-facing slopes, etc.) implicates climatic processes in their formation, which Mellon and Phillips [3] used to argue that a subsurface liquid aquifer is dammed by a layer of ground ice at gully locations. Obliquity oscillations would yield freezing and expansion of the subsurface aquifer, breaching the ground ice layer along the slope face and carving channels downslope. Shortly after their discovery, Baker [4] documented gullies emanating from the crest of the central peak of Hale Crater in the southern mid-latitudes. The occurrence of gullies on isolated peaks unlikely to host sub-surface aquifers spawned a new suite of hypotheses for gully formation that centered around " top-down " melting of surface snowpacks [5-7], frost [8], and ground ice within the top meter [9-10]. These two end-member hypotheses (groundwater and " top-down ") have persisted without resolution through the end of the Mars Global Surveyor (MGS) mission and the beginning of the Mars Reconnaissance Orbiter (MRO) mission. Here, we synthesize the global observations made from MGS and use that as a foundation for the high-resolution (HiRISE) and high-spatial-context (CTX) data from MRO in an effort to test these hypotheses. Global Distribution: The tripartate morphology (head alcove/channel/distributory fan) defined by Ma-lin and Edgett [1] has been used to map the global distribution of gullies by several workers [1, 11-16]. Gully distribution is governed by several factors, including latitude [1], orientation [1,11-16], slope of the host surface [12, 15], and elevation [12, 15-16]. Latitude. Gullies do not occur equatorward of 27° [1]. In each hemisphere, gullies occur most frequently between 30°-42° [11], with a strong preference for the southern hemisphere. Some strong regional concentrations include the eastern rim of Hellas, Newton Basin, southern Utopia, and the northern rim of Argyre Basin.