Depths, Orientation and Slopes of Martian Hillside Gullies in the Northern Hemisphere

Introduction: Martian gullies were first identified by Malin and Edgett [1]. Previous work [2] has examined the characteristics of gullies in the southern hemisphere in detail. Less detailed work has been done for the northern hemisphere [3,4]. Gullies are found predominantly in the midlatitudes of both hemispheres between 30°-50° [1,2], which implies a thermal control on their formation. Several workers have suggested that the source material for the gullies is groundwater that flows along or within subsurface rock layers [1,2,3,5]. Other workers have suggested that insolation changes cause near surface ice or snow to melt, thus forming the gullies [6,7]. Here we attempt to better constrain the environment of gully formation by measuring orientation, slope, elevation and depths of gullied and non gullied slopes in the N. hemisphere. This work in an improvement on the methods of [3] and measures a larger set of parameters than [4]. We compare these measurements to published surveys of S. hemisphere gullies [4,5]. Methods: 160 gullies were identified in the northern hemisphere using MOC images, with a focus on the Acidalia and Utopia regions. Digital elevation models (DEMs) were created for the regions containing gullies using the Mars Orbiter Laser Altimeter (MOLA) data. These DEMs were created using the Generic Mapping Tools (GMT) software by a technique developed by Okubo et al. [8]. This interpolation method creates DEMs with an associated error comparable to MOLA measurements. Next, ISIS was used to create MOC image cubes, which were then converted into geotiffs. The DEM and MOC data were then overlaid in ArcGIS. Once the two data sets were properly aligned, the gullies were digitized. Using Easy Calculate, a set of free ArcGIS tools, the values of depth, slope, and orientation were extracted from the DEM using the regions defined by the digitized gully. This process was repeated for nongullied slopes. Gully elevation and depth were measured as the top of the gully; elevations of nongullied slopes were measured at the top of the slope. Nongullied slopes were identified primarily in MOC images that also included gullies, and often were measured on the walls of craters that also had gullies. Results: The gullied slopes appear to have an orientation preference, with low-latitude gullies facing poleward (orientations ~0o and 360o) and high-latitude gullies facing equatorward (orientations ~180o; Fig. 1). Fig.1. Orientation of gullied and nongullied slopes. North-facing slopes are oriented ~0o and 360o, while south-facing slopes are oriented ~180°.