Icarus 199 (2009) 295–302 Contents lists available at ScienceDirect Icarus www.elsevier.com/locate/icarus Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars Lynn M. Carter a,∗ , Bruce A. Campbell a , Thomas R. Watters a , Roger J. Phillips b , Nathaniel E. Putzig b , Ali Safaeinili c , Jeffrey J. Plaut c , Chris H. Okubo d , Anthony F. Egan b , Roberto Seu e , Daniela Biccari e , Roberto Orosei f a Smithsonian Institution, Center for Earth and Planetary Studies, MRC 315, PO Box 37012, Washington, DC 20013-7012, USA b Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA c Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109, USA d U.S. Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001, USA e Dipartimento INFOCOM, Università di Roma “La Sapienza,” I-00184 Rome, Italy f Istituto di Astrofisica Spaziale e Fisica Cosmica, Istituto Nazionale di Astrofisica, I-00133 Rome, Italy article info abstract Article history: Received 29 May 2008 Revised 18 September 2008 Accepted 2 October 2008 Available online 18 November 2008 Keywords: Mars Mars, surface Radar observations Volcanism The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155 ◦ E and east MFF between 209 and 213 ◦ E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD data. © 2008 Elsevier Inc. All rights reserved. 1. Introduction The Medusae Fossae Formation (MFF) stretches across the mar- tian equator and parts of the northern hemisphere lowlands from ∼140 to 240 ◦ E longitude. The surface of the MFF is dominated by wind erosion, with rough, parallel-grooved surfaces or yardangs present in most places (Hynek et al., 2003; Edgett and Malin, 2000). It is a geologically young deposit, overlying Amazonian-aged lava flows in the lowlands of Elysium Planitia and Noachian-aged cratered highlands along parts of the dichotomy boundary (Hynek et al., 2003; Bradley et al., 2002; Head and Kreslavsky, 2004). Ex- * Corresponding author. Fax: +1 202 786 2612. E-mail address: carterl@si.edu (L.M. Carter). humed and buried craters are common; towards the north, the formation is almost completely eroded to plains level (Hynek et al., 2003; Edgett and Malin, 2000). High-resolution images show slope streaks and small dune fields, which suggests that a fine- grained material probably makes up most of the deposit. Several hypotheses have been suggested to explain the origin of the MFF deposits, including pyroclastic density currents, volcanic fall de- posits and ignimbrites (Hynek et al., 2003; Muhleman et al., 1991; Zimbelman et al., 1997), aeolian deposits (Tanaka, 2000), or relic polar layered deposits (Head and Kreslavsky, 2004; Schultz and Lutz, 1988). The eastern parts of the MFF correspond to the radar-dark Stealth region identified in 3-cm and 13-cm ground-based radar images (Muhleman et al., 1991; Harmon et al., 1999). The dark- est part of the Stealth region, which has no received echo power 0019-1035/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2008.10.007