The formation of terrains antipodal to major impacts

Abstract

In 1975, Schultz and Gault suggested that seismic energy from a major impact could extensive could cause crystal fracturing and surface disruption on the part of the planet directly opposite the impact. Disrupted terrains of this type have been identified on Mercury antipodal to the Caloris impact basin and on the Moon antipodal to Imbrium. Large impact basins are found on many icy satellites: Herschel on Mimas, Valhalla on Callisto, Odysseus on Tethys, and others. Antipodal disruption from these impacts might appear as a fractured surface, or as a resurfaced area. The Simplified Arbitrary Lagrangian Eulerian (SALE) code was used to calculate the antipodal pressures generated by large impacts into stylized icy satellites and planets that have different compositions and core sizes. It was found that in a planet with a high-density core sorrounded by a lower density mantle the incident pressure wave is refracted away from the antipode by the core and the generated antipodal pressure will be low. In a planet with a low velocity core sorrounded by a mantle with a higher wave speed the core will focus the wavefront and the antipodal pressure will be high; in both cases (for equal impacts) the magnitude is dependent upon the size of the core relative to the planet. We also modeled specific impact basins on the Moon. Mercury, and several icy satellites to determine whether the impacts were capable of producing the observed antipodal surface features. We found surface accelerations and pressures which exceed the strength of surface materials for the Caloris impact on Mercury, Imbrium on the Moon, Odysseus on Tethys, and the Herschel impact on Mimas. Images of the surface antipodal to all of these impacts except Herschel show a morphology which could be interpreted as impact-induced, seismically disrupted terrain.