Ancient melting of mid-latitude snowpacks on Mars as a water source for gullies

Abstract We hypothesize that during past epochs of high obliquity seasonal snowfields at mid-latitudes melted to produce springtime sediment-rich surface flows resulting in gully formation. Significant seasonal mid-latitude snowfall does not occur on Mars today. General Circulation Model (GCM) results, however, suggest that under past climate conditions there may have been centimeters of seasonal mid-latitude snowfall [Mischna, M.A., Richardson, M.I., Wilson, R.J., McCleese, D.J., 2003. J. Geophys. Res. Planets 108, doi:10.1029/2003JE002051 . 5062]. Gully locations have been tabulated by several researchers (e.g. [Heldmann, J.L., Mellon, M.T., 2004. Icarus 168, 285–304; Heldmann, J.L., Carlsson, E., Johansson, H., Mellon, M.T., Toon, O.B., 2007. Icarus 188, 324–344; Malin, M.C., Edgett, K.S., 2000. Science 288, 2330–2335]) and found to correspond to mid-latitude bands. A natural question is whether the latitudinal bands where the gullies are located correspond to areas where the ancient snowfalls may have melted, producing runoff which may have incised gullies. In this study we model thin snowpacks with thicknesses similar to those predicted by [Mischna, M.A., Richardson, M.I., Wilson, R.J., McCleese, D.J., 2003. J. Geophys. Res. Planets 108, doi:10.1029/2003JE002051 . 5062]. We model these snowpacks under past climate regimes in order to determine whether snowmelt runoff could have occurred, and whether significant amounts of warm soil ( T > 273 K ) existed on both poleward and equatorward slopes in the regions where gullies exist. Both warm soil and water amounts are modeled because soil and water may have mixed to form a sediment-rich flow. We begin by applying the snowpack model of Williams et al. [Williams, K.E., Toon, O.B., Heldmann, J.E., Mellon, M., 2008. Icarus 196, 565–577] to past climate regimes characterized by obliquities of 35° (600 ka before present) and 45° (5.5 ma before present), and to all latitudes between 70° N and 70° S. We find that the regions containing significant snowmelt runoff correspond to the regions identified by Heldmann and Mellon [Heldmann, J.L., Mellon, M.T., 2004. Icarus 168, 285–304], Heldmann et al. [Heldmann, J.L., Carlsson, E., Johansson, H., Mellon, M.T., Toon, O.B., 2007. Icarus 188, 324–344] and Malin and Edgett [Malin, M.C., Edgett, K.S., 2000. Science 288, 2330–2335] as containing large numbers of gullies. We find that the snowmelt runoff (>1 mm, with equivalent rainfall rates of 0.25 mm/h) and warm soil (>1 cm depth) would have occurred on slopes within the gullied latitudinal bands. The snowfall amounts modeled are predicted to be seasonal [Mischna, M.A., Richardson, M.I., Wilson, R.J., McCleese, D.J., 2003. J. Geophys. Res. Planets 108, doi:10.1029/2003JE002051 . 5062], and our modeling finds that under the previous climate regimes there would have been meltwater present on the slopes in question for brief periods of time, on the order of days, each year. Our model provides a simple explanation for the latitudinal distribution of the gullies, and also suggests that the gullies date to times when water migrated away from the present poles to the mid-latitudes.