Gully Formation on Mars and Earth: the Transition from Glacial Activity to Gully Depositional Phases

Introduction: The discovery of gullies on Mars [1,2] resulted in a wide variety of proposed candidate mechanisms for formation but ensuing analyses have shown very strong latitude and orientation dependencies on their distributions [3-6]. These constraints have been interpreted to require: 1) a volatile on or near the surface as a source of liquid water, and 2) insolation and slope orientation as important factors in the gully formation process [7]. Thus, recent studies have focused on two categories of interpretation for the source of the volatiles: 1) “breached groundwater aquifers” in which liquid water at shallow depth beneath the melting isotherm suddenly erupts, and 2) “surface/near surface melting” in which ice at the surface or within the uppermost layer of soil melts during optimal insolation conditions. Furthermore, a recent synthesis of gully observations [7] has shown that: 1) cold trapping of seasonal H2O frost occurs at the alcove/channel level on contemporary Mars, 2) gullies are episodically active systems, 3) gullies preferentially form in the presence of deposits plausibly interpreted as remnants of the Late Amazonian emplacement of ice-rich material, and 4) gully channels frequently emanate from the crest of alcoves instead of the base, arguing against a groundwater model [7]. On the basis of these developments, recent attention has focused on mechanisms that involve the episodic melting of atmospherically emplaced snow and ice under spin-axis/orbital conditions characteristic of the last several million years [8-15]. A key question is the relationship of the presence and accumulation of recent snow and ice in association with gullies to adjacent deposits of potential glacial or ice age origin [12, 1620]. What clues can we derive from examining the related ice-rich deposits about the nature and sources of water that appear to have played a key role in gully formation? In this analysis, we focus on an assessment of two specific examples on Mars (Fig. 1, 2,3) and a very similar feature in the North Fork of Upper Wright Valley in the Antarctic Dry Valleys (Fig. 4a,b). Newton Crater: A young 10 km diameter crater on the eastern margins of Newton Crater shows stratigraphic relationships that represent multiple episodes of glacial advance and retreat. Gullies are nested in spatulate depressions, interpreted to be the beheaded portions of the remnant glacial deposits. The gullies form from surface melting of snow and ice following the final stages of glacial recession [12]. Furthermore, multiple phases of gullies can be seen, with earlier gully fans being deformed and later ones superposed. Seasonal water snow/ice deposits are observed in the vicinity of the gully alcoves. The presence of gullies as the latest stage event in the declining glacial environment, particularly in the former glacial snow and ice accumulation zone, further implicated surface snow and ice deposits as a possible source of water to form the gullies [12].