Evidence for separation across a gray band on Europa

Thynia Linea, a gray band on Europa, is found to be a similar to 25 km wide and >900 km long region of lithospheric separation which has been infilled by relatively dark material. Roughly a dozen older features and the cuspate segments that form its outline appear to have been displaced across its width. Displacement azimuths indicate a best fit pole of opening near 79 degrees S, 200 degrees W. However, displacement magnitude decreases toward either end of the gray band, indicating that it is more akin to a ''tear'' in a nonrigid europan lithosphere. Opening was in response to NW-SE directed tensile stress, in accord with the stress predictions of nonsynchronous rotation. Observations of Thynia Linea are consistent with a laterally mobile brittle lithosphere, decoupled from ductile or liquid material below, as previously suggested to account for opening of wedge-shaped bands in Europa's antijovian region. Lithospheric separation and contemporaneous emplacement of new material offers a possible volcano-tectonic scenario for resurfacing Europa. If this process is ongoing, resurfacing can be accomplished on a time scale consistent with the satellite's surface age if one gray band or zone of wedge-shaped bands forms every similar to 10(3)-10(4) yr and becomes unrecognizable with age. This would imply that features on Europa brighten with age, as through continuous deposition of frost onto the surface. (C) 1996 Academic Press, Inc.