GEOLOGICALLY YOUNG TROUGHS ON EUROPA: THE CASE FOR RECENT TECTONIC ACTIVITY

Previous work has identified six types of troughs on Europa that formed in response to a tectonic stress field, be it regional or local. Troughs are geologically recent tension fractures that form subtle, linear depressions across the surface. They lack the raised crack margins that typify ridges (the most common tectonic feature), superpose most other surface features, and are unlikely to penetrate the entire ice shell. The youngest troughs ostensibly indicate the most recent period of tectonic activity; however, it is unknown if they imply active tectonics. Troughs occur everywhere on Europa and may thus be a response to a global stress field caused by diurnal tides and long-term nonsynchronous rotation (NSR). Diurnal tidal stresses produce a maximum tension of ~0.1 MPa and vary over a period of ~3.5 Earth days. In contrast, NSR stress may be ~3MPa, but likely builds up over a time scale on the order of 100,000 years before fracturing is initiated. Our detailed fracture mapping has identified systematic orientations defined by multiple trough sets in numerous locations. One region, in Galileo mosaic 15ESREGMAP01 (220 m/pixel and centered at 220° W 40° N) contains 46 tectonic troughs distributed between at least four distinct systematic fracture sets. Some of these sets are mutually orthogonal, indicating at least a 90° difference in the orientation of the stress field over the time scale of these fracturing events. These observations indicate that trough orientations are not random, but indicate discrete periods of tectonic activity during a critical state of stress for fracturing. Future work will directly compare trough set orientations with the contemporary global stress field to examine if they are compatible and perhaps indicate active tectonics on Europa.