ICARUS 134, 292–302 (1998) ARTICLE NO. IS985961 Radar Detectability of a Subsurface Ocean on Europa Christopher F. Chyba 1 Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721-0092 E-mail: chyba@lpl.arizona.edu Steven J. Ostro Jet Propulsion Laboratory, California Institute of Technology, MS 300-233, Pasadena, California 91109-8099 and Bradley C. Edwards Los Alamos National Laboratory, MS D436, Los Alamos, New Mexico 87544 Received October 9, 1997; revised May 5, 1998 ocean may provide a model for early Earth at the time of the terrestrial origin of life (Bada et al. 1994). The A spacecraft in orbit around Jupiter’s moon Europa can use ice penetrating radar to probe for a possible liquid water ocean search for and initial characterization of a Europan ocean beneath Europa’s surface ice and to characterize other impor- via an orbiting spacecraft is now one of the highest tant subsurface structure. Consideration of available con- priority objectives of Solar System exploration. straints on the properties of Europa’s ice, possible subsurface It has been suggested (Squyres 1989) that an orbiting temperature gradients, and possible impurities in the ice places radar sounder might be able to detect the presence of an upper limit of about 10 km on the depth to which an ocean a subsurface Europan ocean. The radar detectability of might be detectable with an orbiting radar.  1998 Academic Press the putative ocean depends on the transparency of the overlying ice, which in turn will depend on the ice’s purity and temperature gradient, as well as the radar I. INTRODUCTION wavelength. Here we demonstrate that consideration of those factors plus available constraints on the properties The initial reconnaissance of the jovian system by the of Europan ice places a limit of about 10 km on the Voyager spacecraft (Smith et al. 1979a,b, Malin and Pieri depth to which an ocean might be detectable with an 1986) and models of tidal heating of natural satellites orbiting radar. suggested the possibility of a liquid water ocean beneath the ice of Jupiter’s moon Europa (Cassen et al. 1979, II. PREVIOUS RADAR OBSERVATIONS 1980, 1982, Squyres et al. 1983, Ross and Schubert 1987, Ojakangas and Stevenson 1989). Recent high resolution imaging of Europa from the Galileo spacecraft (Belton Extensive radar sounding of Greenland and polar ice sheets, sea ice, and glaciers provides a baseline of terrestrial et al. 1996, McKinnon 1997a, Carr et al. 1998) is consistent with this interpretation, and suggests that in some loca- experience (Evans and Smith 1969, Gudmandsen 1971, Drewry 1981, Robin et al. 1983, Daniels et al. 1988, Fisher tions, liquid water may lie within a few kilometers or less of Europa’s surface. Since liquid water is essential et al. 1989, Hallikainen and Winebrenner 1992, Kovacs and Morey 1992). Terrestrial ice-covered lakes (Oswald and for life as we know it, the possible existence of a second liquid water ocean in the Solar System is of great Robin 1973, Oswald 1975), such as Lake Vostok in Antarc- tica (Kapitsa et al. 1996), have been detected and character- exobiological interest (Reynolds et al. 1983, 1987, Chyba and McDonald 1995). In addition, a subsurface Europan ized beneath as much as 4 km of polar ice. It is easier to sound colder (polar) ice than warmer (temperate glacier or sea) ice, in part because attenuation in ice decreases 1 Present address: SETI Institute, 2035 Landings Drive, Mountain View, CA 94043, and Department of Geological and Environmental exponentially with decreasing temperature (Evans 1965, Sciences, Stanford University, Palo Alto, CA 94305-2115. Hobbs 1974). This suggests that, all else being equal, it 292 0019-1035/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.