Radiative Transfer Simulations for the Observed Decrease of Radio Brightness Temperature of Venus With Increasing Decimeter Wavelengths: Possible Existence of a Reflective or Quasi-Conductive Subsurface

Radiometric observations of Venus have revealed a monotonic decrease of brightness temperature (T-b) in the decimeter wavelength regime. Such a decrease has also been independently reported by the observations using interferometric radio-telescopes, like the Very Large Array (VLA) in the USA and the Giant Metrewave Radio Telescope (GMRT) in India. In this work, we have carried out microwave radiative transfer (RT) simulations of thermal emission from the Venusian surface at decimeter wavelengths to examine the role of subsurface properties of Venusian regolith in the continuous reduction of T-b at the microwave-radiowave spectral domain. These simulations are compared against spectral microwave measurements by GMRT over a wide decimeter wavelength regime ranging from similar to 23-128 cm. Good agreements are obtained for simulations that consider a two-layer Venusian surface (a low-loss medium overlaid over a reflecting/lossy medium) a situation that would arise if the subsurface layer has high dielectric properties due to the presence of semiconducting mineral assemblages such as pyrites, ferroelectric minerals, magnetite-hematite, magnetite-pyrite or magnetite-hematite-pyrite equilibrium assemblages. The thickness of the top layer, the possibilities of the formation of such layers and their geological and chemical evolutions are also discussed.