Radar Spectrometry for the Europa Explorer

We present the concept of using a lightweight, millimeter and submillimeter radar system, in orbit around Europa, to study its surface chemistry through measurements of its tenuous atmosphere. This technique adapts conventional radar concepts to the shorter wavelengths where molecular absorptions are common, easily resolved and well understood. A tunable mW-level solid-state transmitter on board the Europa Explorer will actively illuminate a spot on the surface and the back-scattered radiation will be detected by a heterodyne receiver system, enabling spectrometry of the exosphere. All molecules in the exosphere with an electric or magnetic dipole will absorb radiation in the millimeter and submillimeter wavelengths. Physical constraints including: practical mass and power of the instrument, two separate frequency bands, an altitude of 100-200 km and the scattering properties of the Europan surface were used to determine sensitivities to interesting species. Our calculations show that polar species present at ppm levels in the ice will be detectable during the sputter induced ballistic transits of the exosphere with roughly ten seconds or less of integration time from orbit. This will enable us to determine the chemical composition of surface species including salts and organic molecules, thus providing information on Europa's astrobiological potential. Additionally, measurements of water and oxygen will be routine and allow physical aspects of the exosphere, such as the sputtered velocity profiles and the local magnetic environment, to be probed remotely. The instrument will also be useful for measurements of surface temperatures, surface roughness and dielectric properties and surface to satellite ranging.