Europa's rate of rotation derived from the tectonic sequence in the Astypalaea region

The most recent visible tectonic features in the Astypalaea region in the s outhern hemisphere of Europa are a set of cycloidal ridges, three of which have cross-cutting relationships that define a time sequence for their form ation. The longitudes at which each of these features formed, which may be different from the current location due to rotation of Europa, are constrai ned by models of their formation. Reconciling the time sequence with the in ferred longitudes of formation appears to require that < 1 cycloidal crack formed in this region per 180 degrees of rotation relative to the direction of Jupiter. Given that limit on the rate of crack formation, and the impli cation from the paucity of craters that much of the region has been resurfa ced by tectonics within the past 50 million years, Europa's nonsynchronous rotation period must be < 250,000 years, consistent with the lower limit of 12,000 years demonstrated earlier. A rotation rate in this range would be fast enough to be directly observed in the not-too-distant future. (C) 2001 Academic Press.