B. R. Tufts

Does Europa have a subsurface ocean? Evaluation of the geological evidence

It has been proposed that Jupiters satellite Europa currently possesses a global subsurface ocean of liquid water. Galileo gravity data verify that the satellite is differentiated into an outer H2O layer about 100 km thick but cannot determine the current physical state of this layer (liquid or solid). Here we summarize the geological evidence regarding an extant subsurface ocean, concentrating on Galileo imaging data. We describe and assess nine pertinent lines of geological evidence: impact morphologies, lenticulae, cryovolcanic features, pull-apart bands, chaos, ridges, surface frosts, topography, and global tectonics. An internal ocean would be a simple and comprehensive explanation for a broad range of observations; however, we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm, soft ice with only localized or partial melting. Two different models of impact flux imply very different surface ages for Europa; the model favored here indicates an average age of ∼50 Myr. Searches for evidence of current geological activity on Europa, such as plumes or surface changes, have yielded negative results to date. The current existence of a global subsurface ocean, while attractive in explaining the observations, remains inconclusive. Future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within Europa today.

Pits and uplifts on Europa

Abstract A survey of depression and uplift features on Europa, based on Galileo regional mapping images, shows that these features come in a wide range of sizes, with numbers increasing greatly with decreasing size, down to the limits of resolution. Size distributions are similar in the northern leading and southern trailing hemispheres, where they are distinctly different from the southern leading and northern trailing hemispheres, suggesting an oblique, antipodal symmetry pattern, similar to that of chaotic and tectonic terrain. This pattern is suggestive of polar wander. Uplifts are usually polygonal or irregular in shape and rarely are cracked. Patches of chaotic terrain, which we had surveyed earlier, are not included in the current study because their topography is generally unclear, and because there is no a priori known genetic linkage with the pits and uplifts. These results contradict generalizations based on the earlier “pits, spots, and domes” (PSD) taxonomy. Most of the type examples for PSDs were simply patches of chaotic terrain selected from a limited portion of their full size range. The use of the term lenticula to collectively describe PSDs is inconsistent with the IAU definition of lenticula: a small dark spot seen at low resolution. Pits and uplifts do not correlate with lenticulae, although chaos often does. Properties of PSDs that have been widely cited as primary evidence for convective upwelling in thick ice (e.g., that uplifts are generally dome-shaped and often cracked; that pits and domes are regularly spaced; that there is a typical diameter of ∼10 km) were premature and not supported by subsequent data. Most pits and uplifts are less than 10 km across so, if they formed by diapirism or convective upwelling, the sources must have been very shallow, less than 5 km deep. How they actually formed remains unknown.

The rotation of Europa

Theoretical predictions of non-synchronous rotation and of polar wander on Europa have been tested by comparing tectonic features observed in Voyager and Galileo spacecraft images with tidal stresses. Evidence for non-synchronous rotation comes from studying changes in global scale lineaments formed over time, from the character of strike-slip faults, and from comparison of distinctively shaped cycloidal cracks with the longitudes at which such shapes should have formed, in theory. The study of cycloids constrains the rotation period (relative to the direction of Jupiter) to less than 250 000 years, while direct comparison of the orientation of Europa in Voyager and Galileo images shows the rotation is slow, with a period of >12 000 years. Comparison of strike-slip faults with their theoretical locations of formation provides evidence for substantial polar wander, supported by the distribution of various thermally produced features.