Evaporation Effects on the Formation of Martian Gullies

Introduction: Gully features were first observed in the images taken by MOC in impact craters, south polar pits and large martian valleys [1], tending to midlatitudes [2]. HiRISE imagery supports the conclusion that the gullies were formed by fluvial erosion [3], but the source and composition of this fluid is unclear. The superposition of the gully aprons over aeolian features, e.g. dunes, suggests gullies to be relatively young. These features are usually interpreted in terms of groundwater surface runoff and infiltration, so their length is governed by loss of water due to infiltration into the regolith [1]. However, another factor possibly controlling gully length is the evaporation of the fluid carving the gullies, in which case we might observe a relationship between gully length and the structure of the adjacent dunes [4]. The number of dune crests per unit length (dune frequency) is a function of wind speed; all else being equal, and wind speed is a factor in determining evaporation rate of fluids on the surface of Mars. The linear increase of the evaporation rate of water for wind speeds ranging up to 12 m s at -15oC has been experimentally determined [5]. The dependence of evaporation on wind speed is not strong, and temperature plays a more important role in determining evaporation rates. However, at lower temperatures the data shows [5] that the dependence of evaporation rate on wind speed is more significant. This study explores the possible relationship between the gully length and wind speed as reflected in dune frequencies. We predict that if evaporation is a limiting factor in gully formation that a given location on Mars, and all else being equal, there should be a negative correlation between gully length and dune frequency. In addition, the slope of the correlation will be temperature dependent, the correlation will be steeper for gullies forming at low temperatures. The formation of gullies at temperatures much below zero Celsius requires that the gullies were formed from strong brine solutions. Brine solutions such as NaCl and CaCl2 have eutectic points near 30oC [6] while ferric sulfates have eutectic points as low as -98 to -68o C [7]. Methods: Images containing gullies and dunes from mid martian latitudes and varying longitudes were examined. Processed HiRISE images that have been radiometrically calibrated, stitched, and geometrically projected were used. After ensuring that the gullies and dunes were fresh and that the image contained at least five gullies suitable for these measureFigure 1. The distribution of images examined in this study overlain on a grayscale MOLA map (Courtesy GSFC/NASA). The images are distributed over a variety of landscapes, but are dominate in the southern hemisphere as most dunes on Mars are prevalent in southern hemisphere craters.