SHARAD soundings and surface roughness at past, present, and proposed landing sites on Mars: Reflections at Phoenix may be attributable to deep ground ice

Abstract

We use the Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter to search for subsurface interfaces and characterize surface roughness at the landing sites of Viking Landers 1 and 2, Mars Pathfinder, the Mars Exploration Rovers Spirit and Opportunity, the Phoenix Mars lander, the Mars Science Laboratory Curiosity rover, and three other sites proposed for Curiosity. Only at the Phoenix site do we find clear evidence of subsurface radar returns, mapping out an interface that may be the base of ground ice at depths of~15-66 m across 2900 km 2 in the depression where the lander resides. At the Opportunity, Spirit, and candidate Curiosity sites, images and altimetry show layered materials tens to hundreds of meters thick extending tens to hundreds of kilometers laterally. These scales are well within SHARAD's resolution limits, so the lack of detections is attributable either to low density contrasts in layers of similar composition and internal structure or to signal attenuation within the shallowest layers. At each site, we use the radar return power to estimate surface roughness at scales of 10-100 m, a measure that is important for assessing physical properties, landing safety, and site trafficability. The strongest returns are found at the Opportunity site, indicating that Meridiani Planum is exceptionally smooth. Returns of moderate strength at the Spirit site reflect roughness more typical of Mars. Gale crater, Curiosity's ultimate destination, is the smoothest of the four proposed sites we examined, with Holden crater, Eberswalde crater, and Mawrth Vallis exhibiting progressively greater roughness.

Selection of the InSight Landing Site

“…These techniques complement smaller-scale information from the Arecibo data and pulse-width measurements as well as larger-scale (0.1-1.0 km) roughness estimates from MOLA (Kreslavsky and Head 2000;Anderson et al 2003). Past landing sites have been targeted extensively with SHARAD, yielding roughness estimates consistent with landed observations and other data (Putzig et al 2014).…”

“…Subsurface detections at landing sites with SHARAD data have thus far been confined to the 'Green Valley' of the PHX site, with returns from ∼25 m depth mapped over 2900 km 2 that may represent the base of ground ice (Putzig et al 2014). Important to these studies is the use of techniques to distinguish subsurface returns from off-nadir surface returns (clutter) and surface sidelobes.…”

“…This suggests the dielectric permittivity and/or density of the near surface of PHX and E9 is comparable and higher than that of Meridiani. The PHX site is known to have a shallow ice table extending between a few centimeters to ∼35 m depth (Putzig et al 2014). In contrast, Meridiani has a stack of sediments up to a few 100 m thick with no evidence in SHARAD « SPAC 11214 layout: Small Condensed v.2.1 file: spac321.tex (ELE) class: spr-small-v1.2 v.2016/06/09 Prn:2016/12/02; 14:37 p. 69/91» « d o c t o p i c : R e v i e w P a p e rn u m b e r i n g s t y l e : C o n t e n t O n l yr e f e r e n c e s t y l e : a p s » data for large dielectric contrasts (Putzig et al 2014).…”

“…The PHX site is known to have a shallow ice table extending between a few centimeters to ∼35 m depth (Putzig et al 2014). In contrast, Meridiani has a stack of sediments up to a few 100 m thick with no evidence in SHARAD « SPAC 11214 layout: Small Condensed v.2.1 file: spac321.tex (ELE) class: spr-small-v1.2 v.2016/06/09 Prn:2016/12/02; 14:37 p. 69/91» « d o c t o p i c : R e v i e w P a p e rn u m b e r i n g s t y l e : C o n t e n t O n l yr e f e r e n c e s t y l e : a p s » data for large dielectric contrasts (Putzig et al 2014). Considering the InSight landing site is within volcanic terrain (i.e., dense lava flows consisting of high permittivity) and extremely unlikely to host ground ice due to the low latitude, the similarity in power is likely explained by the presence of a low density material (e.g., regolith) overlying more intact rock within the upper range cell of the surface return (i.e., within 10-20 m depth), an interpretation also supported by RECs (Sect.…”

“…We search for potential subsurface reflections-returns delayed relative to the surface returns in the radargrams-using clutter simulations and custom processing as needed to identify and suppress off-nadir returns and sidelobes while enhancing subsurface returns (e.g., Holt et al 2008;Putzig et al , 2014. Where potential subsurface interfaces were found, we mapped their extent and provided an interpretation of the subsurface material properties.…”