SHARAD Constrains on Lava Flow Properties at Southeastern Utopia Planitia

The volcanic flows originated at the southwestern flanks of Elysium Mons extend over 1,000 km into Utopia Planitia and overlie the knobby and polygonally cracked Vastitas Borealis Formation (VBF). These flows display rough and smooth lobate morphologies (RL and SL) morphologies and occur in conjunction with sinuous channels (SC). Russell and Head [2003] described these morphologies and hypothesized that RL correspond to debris flows that arose as lahars from the interaction between magma and ground water or ice. The mapping of Tanaka et al. [2003] identified these features similarly, attributing them to volcanoclastic flows formed from magma-volatile interactions. Crater counts by Werner et al. [2011] support surface ages between 1 and 2 Gyr for these flows. Analysis of the radargrams acquired throughout this area o show unambiguous subsurface reflectors that, individually, are relatively short and laterally intermittent. As a group, however, these reflectors are distributed sparsely over the flow field and correlate very well with the SL units. Delays to reflectors beneath the surface are generally in the order of < ~1 μs. In one locale with a high concentration of subsurface reflectors, centered at 117.61°E and 31.31°N, a sequence of smooth lobate flows overlie a smooth volcanic unit. The lobate flow in immediate contact with the smooth unit possesses subsurface reflections that correlate well with the flow edges, and where this flow is overlain by another lobate flow these reflections vanish. We interpret these reflections as a reflector that corresponds to the interface between the lobate flow and smooth unit. The average delay to this reflector is 0.68 - 0.71 μs along its length. The thickness of this lobate flow, estimated from MOLA elevation data, ranges between 35 and 40 m. The thickness estimate from MOLA and the delay to reflector from SHARAD together constrain the relative permittivity of the flow to between 6.5 and 9.5. These values are consistent with the middle to upper range of permittivities for basalt, and, therefore, porosity does not dominate the volume of the lobate flow, and supports the morphologic interpretation of the flow and geologic history of this area offered by Russell and Head [2003] and Tanaka et al. [2003]. Russell P. S. and Head J. W. [2003] JGR, 108, 5064. Tanaka K. L. et al. [2005], USGS Sci. Invest. Maps, 2888. Werner S. C. et al. [2011], PSS, 59, 1143-1165.