Geology of Lofn Crater, Callisto

Lofn crater is a 180-km-diameter impact structure in the southern cratered plains of Callisto and is among the youngest features seen on the surface. The Lofn area was imaged by the Galileo spacecraft at regional-scale resolu tions (875 m/pixel), which enable the general geology to be investigated. T he morphology of Lofn crater suggests that (1) it is a class of impact stru cture intermediate between complex craters and palimpsests or (2) it formed by the impact of a projectile which fragmented before reaching the surface , resulting in a shallow crater (even for Callisto). The asymmetric pattern of the rim and ejecta deposits suggests that the impactor entered at a low angle from the northwest. The albedo and other characteristics of the ejec ta deposits from Lofn also provide insight into the properties of the icy l ithosphere and subsurface configuration at the time of impact. The "target" for the Lofn impact is inferred to have included layered materials associa ted with the Adlinda multiring structure northwest of Lofn and ejecta depos its from the Heimdall crater area to the southeast. The Lofn impact might h ave penetrated through these materials into a viscous substrate of ductile ice or possibly liquid water. This interpretation is consistent with models of the current interior of Callisto based on geophysical information obtai ned from the Galileo spacecraft.