GROUNDWATER DETECTABILITY ON EARTH AND PLANETARY SUBSURFACES FROM VHF SOUNDING RADARS

Probing groundwater from orbital and airborne platforms can provide unique insights into our understanding of large-scale aquifer dynamics and their role in the geophysical evolution of both cold and warm deserts. On Earth little is known about the potential of VHF sounding radar for mapping aquifers in hyper-arid areas due to several environmental and operational constrains (e.g. frequency allocation, flight approvals, ionospheric distortion and others). Yet in planetary environments, several sounding radar experiments are being used to probe different bodies, exploring their potential for harboring subsurface volatiles (e.g. SELENE-Radar on the Moon, MARSIS & SHARAD on Mars, RIME & REASON on Jovian icy moons & CONSERT on Comet 67P). In particular, recent VHF radar sounding observations by the MARSIS instrument aboard Mars Express have suggested the occurrence of subglacial liquid water bodies beneath the Mars Northern Polar Layered Deposits. These first-of-kind observations of liquid water using a VHF sounder in a planetary environment has raised several questions on the detectability of groundwater from orbital platforms, as such observations have no direct analog on Earth. Herein, we quantify the penetration depth of radar waves down to saturated layers in desiccated desert sedimentary and volcanic grounds as well as for icy environments, which are representative of Earth's hyper-arid environments and Mars' Northern Polar Layered Deposits. In particular we quantify the total radar signal losses in terms of dielectric and scattering losses, as well as losses associated with surface clutter and ionospheric attenuation. We also present examples of airborne and ground probing of aquifers in desertic environments and assess the observed total losses and gained knowledge on the extension and shape of the water table of fossil aquifers in hyper-arid environments. We then derive the requirements in term of losses, dielectric contrasts and topography needed to identify the water table from VHF soundings. Finally, we reanalyze the losses in the latest MARSIS Northern Polar Layer observations by Orosei et al, 2018 that suggested the detection of subglacial melts and we provide additional insights on the detectability of subsurface water on Mars' and Earth's hyper-arid environments using VHF sounding radar.