Sharad Sounding and Surface Roughness of Once and Future Mars Landing Sites

Introduction: To search for subsurface interfaces and characterize surface roughness, the Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter is observing past, present, and proposed future Martian landing sites. Orbital and landed imagery and altimetry data show that most sites are located atop layered sequences that extend over tens to hundreds of kilometers and are hundreds of meters thick. With resolution of 0.3–6 km horizontally and 5–10 m vertically below the surface, SHARAD is capable of identifying subsurface interfaces at those scales, provided sufficient dielectric contrast between layers. In addition, analysis of the amplitude and time-delay characteristics of SHARAD echoes provides a measure of roughness on scales of meters to tens of meters. Previous Work: The SHARAD Team first made note of low-power apparent subsurface returns at the Mars Phoenix landing site in 2008 [1]. Subsequently, similar shallow returns were identified across the northern plains [2] and the southern highlands [3], suggesting widespread detections of the base of ground ice. However, further analysis has revealed that most of these far-ranging returns occur at delay times that correspond to those expected for sidelobes of the surface return. While the returns do have about twice the expected power of the sidelobes, the close correspondence in delay time casts great doubt on any sub-surface interpretation. Unpublished examinations of SHARAD data taken over Meridiani Planum revealed some of the highest-power surface returns on the planet, but no evidence of returns from subsurface interfaces, despite the several-hundred-meters stack of layers seen in great detail by orbital imagery and by the still-going Mars Exploration Rover (MER) Opportunity. Preliminary evaluations of SHARAD observations were also conducted for the prospective landing sites of the Mars Science Laboratory (MSL) Curiosity, now on its way to Gale Crater [4]. That work yielded tentative evidence of subsur-face returns only at Gale Crater and qualitative roughness information gleaned from surface power, with sites ranked from smoothest to roughest as Gale, Holden Crater, Eberswalde Crater, and Mawrth Vallis. All four were found to be substantially rougher than the Opportunity site by this measure. All of this earlier work employed a range of different SHARAD data products and the analyses were complicated by the differing sets of evolving approaches to focusing-parameter choices and corrections of ionospheric distortion.