Venus subsurface targets: assessment of detectability using radar sounder simulations
Authors: Thakur Sanchari , Sbalchiero Elisa , Paolaz Elisa , Bovolo Francesca , Bruzzone Lorenzo 
Published: October 2020

IntroductionFuture exploration of Venus with state-of-the-art instruments can significantly improve our knowledge of the planet. Following this objective, EnVision is selected as one of the three candidates for the ESA&#8217;s Cosmic Vision M5 missions, with the goal of understanding the evolution of surface and interior, present-day geological activity and the climate of Venus. Subsurface Radar Sounder (SRS) onboard EnVision, is a nadir-looking low-frequency radar capable of profiling the shallow subsurface of Venus. SRS aims to support the understanding of the geological history of Venus by searching for subsurface dielectric interfaces characterizing structural and compositional discontinuities. The ability of SRS in extracting valuable scientific information depends on the properties of the geological targets and the instrument parameters. The current phase A of SRS design addresses the complex task of characterizing the target properties despite the large uncertainty on them. Thus, for understanding their detectability, a structured simulation strategy is developed considering many different possible target hypotheses.This paper presents the simulation results and analysis of the SRS capability to detect three subsurface targets - buried craters, tesserae and lava flows. This is done considering the baseline design parameters of SRS (9 MHz central frequency, 6 MHz bandwidth) and a set of assumptions on the expected dielectric properties (real permittivity &#949;, loss tangent tan&#948;, and contrast &#916;&#949;) of basalts at Venus temperatures [1]. For generating the target geo-electrical models of the crater and tessera, 75 m resolution Magellan stereo DEMs [2] are used. Their radargrams are simulated using a multi-layered coherent radargram simulator (MRS) [3] (processed for range compression and unfocussed SAR and do not contain noise). For the lava flows, DEMs are not available. Thus, Mars-analog lava flow radargrams acquired by SHARAD have been reprocessed to simulate the fully processed SRS radargrams using analog-based simulation approach [4].Buried CraterCraters on Venus are modified volcanically or tectonically, even to the extent of being completely buried without any surface expression. EnVision will determine the nature and thickness of post-impact infilling as indicators of relative surface age and modification history. SRS will support this goal by searching for buried impact craters and stratigraphic horizons representing different phases of resurfacing in the modified craters. For this target, the DEM of a Venus crater is modelled as embayed by one or more smooth overlying interfaces, resulting in the three possible scenarios (Fig.1): (1) completely buried, (2) partially buried, and (3) modified in multiple episodes. The dielectric permittivity values of the target layers are selected assuming that the deeper layers have higher real permittivity, owing to compaction to a higher density. For the completely buried scenario, the crater is clearly detectable when tan&#948;