Radar Sounding for Planetary Subsurface Exploration: Translating the Mars Experience to Jupiter’s Icy Moons

Abstract:

Exploration of the subsurface of Mars using radar sounding began with MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) on Mars Express in 2005 and continued with SHARAD (Shallow Radar) on Mars Reconnaissance Orbiter in 2006. These instruments have been operating continuously since, providing a rich legacy of science return and observational experience in the highly variable environments and target sets at Mars. New missions to the icy moons of Jupiter, ESA’s JUICE (Jupiter Icy Moon Explorer) and NASA’s Europa Mission, will both carry radar sounders to probe the subsurface of several of the icy moons (Ganymede, Europa and Callisto by JUICE; Europa by the Europa Mission). The success of the Mars sounders demonstrated the scientific value of the technique and provided confidence that sounding of the icy moons is a promising endeavor. Icy targets at Mars have proven especially amenable to penetration by radar sounding. The polar layered deposits of Mars have been probed to their base (2-4 km deep) by MARSIS, operating at frequencies of 1.3-5.5 MHz. SHARAD, operating with a wider bandwidth at 15-25 MHz, provides higher vertical resolution that allows detection and imaging of fine details of interior layering in the ice deposits. The sounder planned for the Europa mission, REASON (Radar for Europa Assessment and Sounding, Ocean to Near-Surface), will utilize simultaneous dual frequency signals to obtain complementary deep sounding and high-vertical-resolution shallow observations. Co-located observations by MARSIS and SHARAD also demonstrate that high surface roughness (relative to the radar wavelength) affects the strength of the penetrating signals, and thus the capability to detect deep or low-contrast subsurface interfaces. The icy moon sounders’ wavelengths were selected, in part, to mitigate against this degradation of signals by the anticipated rough surfaces of Jupiter’s moons. This paper will discusss these and other examples of lessons learned from Mars sounding that will be applied to the exploration of the icy moons.