Radar Backscatter and Emissivity Models of Proposed Pyroclastic Density Current Deposits on Venus

Magellan synthetic aperture radar observations of Venus revealed a small number of deposits in the highland regions that were suggested to have formed from pyroclastic density currents. Studying these deposits is useful for understanding the nature of pyroclastic activity and eruptive history on Venus. The proposed pyroclastic deposits occupy the uppermost unit in local stratigraphy and are found near exceptionally high reflectivity (R ∼0.6) units in the highlands. Their radar properties include high copolarized backscatter (∼−8 to −15 dB) and moderate emissivity values (∼0.70-0.88) in the 12.6 cm wavelength Magellan data acquired at incidence angles between ∼15° and 45°. We aim to characterize the structure of these deposits by modeling the observed backscatter and emissivity as a function of different physical and dielectric properties and shallow subsurface stratigraphy. Three different physical scenarios focusing on three different scattering mechanisms—surface scattering, subsurface scattering from buried dielectric horizons, and volume scattering from buried, distributed scatterers—are considered. By comparing the model results to Magellan observations, we narrow down likely pyroclastic deposit structures. We show that the deposits are likely analogous to dense, welded ignimbrites with high surface roughness. We also investigate other possible but less likely scenarios of a thin, low-density, low-loss mantling pyroclastic deposit on top of high reflectivity units and a thick, low-density, low-loss deposit with ∼5-10 volume % of scatterers of sub-wavelength size. Future multiwavelength, multipolarization radar observations from VERITAS and EnVision may enable unambiguous characterization of these deposits.