STANFORD UNIVERSITY
EE 350 RADIOSCIENCE SEMINAR
Professor Umran Inan

Winter 2000-2001

Date: Wednesday, February 14, 2001
Time: 4:15-5:30 PM; Refreshments at 4:00 PM
Location:Bldg. 200, Rm. 34


Design of a radar sounder to detect and map subsurface liquid water on Europa

Howard A. Zebker
Stanford University

Abstract

Possibly the most exciting recent finding in the study of the Galilean satellites of Jupiter is that Europa, and perhaps Callisto, may possess liquid water in large quantities under an icy lithosphere, water which could harbor life with a chemistry not unlike our own. An orbiting radar sounding instrument, using energy that can penetrate an icy outer covering to view liquid water at depth, can help determine the presence or absence of a subsurface liquid ocean and to constrain its 3-D distribution. We examine here how the choice of a radar antenna and coherent signal processing techniques, optimized regarding likely mechanisms of radar scattering on Europa, aid in the design and operation a radar sounder for the proposed Europa space mission.

Gravity measurements by the Galileo spacecraft suggests that Europa is surrounded by an outer ice layer some 80-170 km thick. Energy dissipated in Europa's interior resulting from a slightly eccentric orbit is sufficient to melt some of the ice to liquid water. A planned Europa orbiter spacecraft will present the first opportunity to identify conclusively the presence of a liquid ocean below the icy outer lithosphere. If the orbiter is equipped with a radar sounding instrument designed to probe the ice layer covering the moon, radio frequency signals can penetrate many km into the icy lithosphere and be reflected from an ice/water interface at depth.

The design of the instrument and its data processing approach is dependent on expectations of the scattering mechanisms reflecting radar energy back to the instrument. Generally a radar sounding instrument is deployed on a sled in contact with the surface, or on low-flying aircraft to minimize corrupting reflections from surface scatterers. Here it is necessary to optimize antenna design to attenuate off-angle returns in order to isolate the weak echo from the desired region at depth. Signal processing techniques further reduce stray signals. We present a design that should provide visibility through the icy outer crust to layers of liquid water at depths of up to 20 km, perhaps more if the ice is exceedingly pure.