Clark R. Chapman, W. Merline, B. Bierhaus, J. Keller, S. Brooks (Southwest Research Inst., #426, 1050 Walnut, Boulder CO 80302 USA); J. Head, R. Pappalardo (D. Geol. Sci., Brown U., Providence RI 02912 USA)
Craters and pits in medium-resolution images of Europa obtained early in the Galileo orbital mission show that Europa's surface is relatively very youthful. There is a spatially non-uniform population of pits (first seen by Voyager and hundreds seen in Galileo's G1 orbit images), having diameters within a factor of 2 of 8 km. The mono-modal size distribution and a spatial avoidance of pits and lineaments imply that most pits are of endogenic origin (e.g. collapse pits). A C3-orbit image centered on wedge-like features (with 0.4 km resolution) shows only two impact craters >2 km diameter in a 50,000 sq. km region. Unless this region is very anomalous, the two craters cannot be due to a plausible projectile population that would have produced the pits seen in G1 images, again implying that the pits are not impact craters. If the relatively youthful Uruk Sulcus region on Ganymede dates from after a heavy early bombardment period, then the nearly 2-orders-of-magnitude fewer craters in the wedge image implies a crater retention age younger than 30 m.y. Despite major uncertainties in the size-distribution and impact flux history of impactors that make 2 km size craters, Europa's surface is clearly very young and resurfacing is probably an ongoing process.