Evaluation of passive radio sounding of icy moons based on a radar equation

Passive radar sounding is one of the ways to observe surfaces and subsurfaces of planetary objects through reflection of natural radio emissions. Using observations of Radio and Plasma Investigations (RPWI) aboard Jupiter Icy Moons Explorer (JUICE), we have a plan to apply this technique to observe Jovian auroral radiation reflected on icy moons. For feasibility studies, we derived an expanded radar equation for planetary radio waves reflected on the moon’s surface, including the loss due to the moon’s spherical shape and scattering by surface roughness. Using this equation, we estimated the power and polarization characteristics of reflected radio emissions in two cases; terrestrial auroral kilometric radiation (AKR) reflected on the lunar surface and Jovian Hectometric radiation (HOM) reflected on the Ganymede surface. We showed the quantitative feasibility of the observation capability and potential of polarization measurements for detecting the permittivity of a surface. Additionally, we compared the estimation of the lunar case with a real case of AKR reflection observed using Lunar Radar Sounder (LRS) aboard Kaguya. The observed power ratio of reflected AKR to incident one was 2–5%, estimated from the interference fringe in the observed frequency-time diagram. This observed value agreed with the estimation from our radar equation.