Small-scale morphologic properties of martian gullies: insights from analysis of HiRISE images

Introduction: The discovery of abundant, geologically recent gullies on Mars in Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images [1] has led to numerous studies examining potential mechanisms for recent fluid or fluid-rich surface processes [e.g., 1-2]. The process or processes involved in Martian gully formation provide important insights into Martian geologic and volatile history. New gully deposits have been found in comparison of MOC images acquired over the extent of the MGS mission [3], leading to consideration of contemporary fluid activity. Gullies appear to be the youngest features in most locations [4], serving as evidence of geologically recent surface flow and erosive processes [5]. Detailed analyses of Mars Reconnaissance Orbiter HiRISE images provide new insights into gully characteristics and formation processes. The purpose of this project is to document the small-scale morphologies of Martian gully systems in order to explore the potential diversity of gully formation processes and to determine if small-scale gully morphology can be used to distinguish between the various mechanisms proposed on the basis of larger-scale characteristics. The initial focus of this work has been on examination of boulder distributions and upslope initiation points. Background: As described by Gulick et al. [6], based on HiRISE image analysis, gullies exhibit a wide range of morphologies, sizes, and geologic settings. Multiple episodes of fluid flow causing erosion and the formation of gullies have taken place, explaining the broad range of features observed [7]. Martian gullies have a characteristic set of components. Gullies can be divided into an alcove at the top of the slope, one or more channels extending downslope, and debris fan at the base [1, 8]. Gullies tend to be concentrated in groups on the walls of craters, pits, and canyons. Although a cluster can differ greatly from an adjacent cluster [6], intra-cluster gullies tend to have similar morphologies. Methods: Geomorphologic analyses were facilitated by use of Geographic Information Systems (GIS) to correlate and analyze datasets. Over 40 HiRISE images (~1 meter/pixel) containing at least one grouping of gullies were imported into a GIS database with a THEMIS IR image mosaic (230 meters/pixel) and MOLA DEM (~128 pixels/degree) for geographic reference and topographic analyses. Gully features were qualitatively characterized and descriptions compiled, including information on boulder distributions, sinuosity ratio, dimensions of gully system components (length and widths of alcoves, channels, and debris aprons), types of initiation points, evidence for multiple episodes of activity, and other general qualitative observations. Results: Multiple stages of gully formation are clearly present in the population examined upon detailed analysis of over 40 HiRISE images. Gully systems exhibit different stages of development, with both simple, immature and complex, well-developed morphologies evident. Mature gullies tend to be welldeveloped with deeply incised and often sinuous channels, frequently terminating in heavily braided fans. In contrast, immature gullies are small, subtle features, visible only as poorly developed alcoves and/or channels. Different stages of maturity are intermixed within individual, isolated groupings of gullies (Figure 1).