Distinct erosional progressions in the Medusae Fossae Formation, Mars, indicate contrasting environmental conditions Kathleen Mandt a,b, * , Shanaka de Silva c , James Zimbelman d , Danielle Wyrick e a University of North Dakota, Department of Space Studies, Grand Forks, ND 58202, United States b Space Science and Engineering Department, Southwest Research Institute, San Antonio, TX 78238, United States c Department of Geosciences, Oregon State University, Corvallis, OR 97331, United States d CEPS/NASM MRC 315, Smithsonian Institution, Washington, DC 20013, United States e Geosciences and Engineering Division, Southwest Research Institute, San Antonio, TX 78238, United States article info Article history: Received 17 January 2009 Revised 10 June 2009 Accepted 16 June 2009 Available online 7 July 2009 Keywords: Mars, Surface Mars, Climate abstract The form of erosional remnants of the Medusae Fossae Formation (MFF) on Mars provide evidence of their development progression and implicate two spatially distinct environments in the equatorial regions of Mars. Ubiquitous yardangs are clearly the product of strong unidirectional winds acting over time on variably indurated deposits. Yardang orientation is used as a proxy to map regional and local wind direction at meso-scale resolution. In other, more limited areas not subjected to strong unidirec- tional winds, randomly oriented kilometer-scale mesas and buttes are found to be remnants of progres- sive cliff recession through mass wasting as support is lost from within the MFF lithology at its margins. The differing processes that dominate the formation of the distinctive landforms have implications for meso-scale variations in climate that remain unresolved by current modeling efforts. Additionally, the variability of erosional forms within the deposit emphasizes the overall complexity of this extensive formation. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Erosional remnants on the surfaces of planetary bodies are valu- able as a record of past and current climatic and geologic processes on the planet. Among the most obvious features on the surface of Mars are erosional remnants like yardangs and less-than-kilome- ter-scale mesas. They provide insight into erosional processes and material properties of the formations on which they are found, such as the abundant yardangs in the Medusae Fossae Formation (MFF) that have been attributed to the mildly indurated nature of the MFF lithology and the presence of strong unidirectional winds (Schultz and Lutz, 1988; Scott and Tanaka, 1982; Mandt et al., 2008). To date, less-than-kilometer-scale mesas have been noted by very few authors, with little insight provided as to their source or significance (Bradley and Sakimoto, 2001; Zimbelman and Patel, 1998). As part of a comprehensive survey of 713 images of the MFF from the Mars Orbiter Camera (MOC) of the Mars Global Surveyor (MGS), we have inferred material properties that suggest that the lithology of the MFF is largely that of an ignimbrite (Mandt et al., 2008). Herein we focus on the development and evolution of yard- angs and less-than-kilometer-scale mesas and show that both are produced by a sequence of erosional stages that suggest differing processes for formation that are distinct from each other. The mechanism for forming yardangs is well understood (e.g. Breed et al., 1979) and their development within the MFF provides indi- cations of local climatic conditions and material properties within the MFF. The mechanism for forming the mesas is a strong indica- tor of material properties and shows a lack of the strong unidirec- tional winds that formed the yardangs. Both forms require a lithology that is indurated in its upper parts and more friable in its lower portions. The separation of areas dominated by these respective forms is on the scale of 50–350 km, suggesting contrast- ing local environmental conditions at this spatial scale. 2. The MFF Many geologic formations on Mars have origins that have yet to be agreed upon. One of the most prominent of these is the MFF, a deposit located along the equator stretching between 240° and 170°E Longitude. It is commonly described as enigmatic because its origin has been the subject of debate for decades. The MFF is lo- cated in the Amazonis Planitia region and lies between two major volcanic centers: Tharsis and Elysium (Fig. 1). In all places where they are in contact, the southern portion of the MFF overlies the 0019-1035/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2009.06.031 * Corresponding author. Address: Space Science and Engineering Department, Southwest Research Institute, San Antonio, TX 78238, United States. E-mail address: kathymandt@yahoo.com (K. Mandt). Icarus 204 (2009) 471–477 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus