Formation of Gullies on Mars by Water at High Obliquity: Quantitative Integration of Global Climate Models and Gully Distribution

Introduction: The latitude-dependent distribution [1-4] and non-uniform orientation [5-9] of gullies on Mars indicate that their formation is controlled by climate conditions at the surface within the last million years [10-11], while their global distribution [2,4] and morphology [12-13] show direct correlations with the ice-rich Latitude Dependent Mantle (LDM), a potential source for melting. Contemporary activity in gullies occurs at times that are consistent with the removal of CO 2 frost, leading to destabilization of loose material on steep slopes [14-17]. While this activity is capable of mobilizing fines within existing gully channels, the potential for this process to form new gully systems is unclear. In particular, this activity may not explain (1) gullies that exhibit sinuosity values in excess of what what is measured for dry channels on Earth [18], (2) gullies with alcove slopes below 25° [8], and (3) channels that incise through tens of meters of permafrost [19]. These properties suggest that many gullies may be initiated by flows that are comprised by some percentage of fluid, the most reasonable candidate for testing being liquid water [12,20-22]. Global Climate Model (GCM) simulations show that regions where liquid water could exist today are poorly correlated with gully locations [23], with the majority of gullies occuring at elevations with surface pressure below the triple point of H 2 O [8]. Melting of ice within 50 cm of the surface in the mid-latitudes is predicted at ~35° obliquity [20, 22], a scenario that has occurred within the last million years [24]. Ice is predicted to accumulate in the mid-latitudes at 35° obliq-uity [25], providing a replenishable source [4, 13]. Further, recently discovered massive CO 2 units presently sequestered in the South Polar Layered Deposits (SPLD) would be mostly sublimated at 35° obliquity, yielding average surface pressures > 10 mb [26]. This new information prompted us to quantitatively reassess the potential for liquid water at the surface at gully locations formed within the last million years. Methods: Simulations using the Laboratoire de Météorologie Dynamique (LMD) GCM [27] were run for three separate starting conditions thought to have occurred within the last million years [23]. (1) Present day (25° obliquity, 6 mb atmosphere); (2) ~380 kyr transitional scenario (30°, 8 mb); (3) ~625 kyr high-obliquity (35°, 10 mb). Age/obliquity values are from [24] and globally average pressure values are from [26], which calculat