Structure of Icy Worlds

The moons of Jupiter are some of the most confounding bodies in the Solar System. Ganymede and Callisto, for example, are massive icy worlds believed to contain subsurface oceans, but the thicknesses of their outer ice shells and putative subsurface oceans are poorly constrained because current spacecraft measurements are limited to observations of their surface features and, as a result, cannot directly measure interior properties. Consequently, knowledge of the internal structures of these icy bodies depends on reliable interpretations of specific classes of surface features which depend to some extent on characteristics of the subsurface. One such type of surface feature is impact craters, whose size and morphology are directly related to the geologic conditions and materials of the planet on which the impact occurred. Fortunately, both Ganymede and Callisto have both impact craters and impact basins, and studying the morphologies of these impact structures can provide insights into their internal conditions.

Impact craters exist on nearly every solid planetary surface in the Solar System and can provide clues about the interiors of the planets and moons on which they formed. This is especially true in the case of multiring basins, which are the largest impact basins and are associated with circumferential ring faults that extend beyond the rim of the original crater. Ring tectonic theory is the prevailing theory of multiring basin formation and articulates how the presence of a low- strength layer sandwiched between the strong lithosphere and basement layers facilitates ring formation during the collapse of the transient crater (Melosh & McKinnon 1978). Because multiring basins are the largest impact structures, the transient craters penetrate deeper into the planetary body than the transient craters of other crater morphologies. This deep penetration makes them more sensitive than smaller craters to any rheologic stratification in the subsurface.

Multiring basins fall into two broad categories, loosely correlated with the type of planetary surface on which they are observed. These types of multiring basins are “rock-like” and “ice-like”. A basin that has only two or three ring faults and shows normal displacement towards the basin center is characterized as “rock-like.” Although relatively few in number, these ring faults can extend far beyond the original crater dimensions, as seen at Orientale Basin on the Moon whose rings extend up to nearly 300 km. “Ice-like” multiring basins, also referred to as Valhalla-class multiring basins, have similar circumferential ring faults spreading far beyond the original crater dimensions but these icy ring faults are more numerous, have much tighter spacing, and show normal fault displacement dipping away from the basin center (McKinnon & Melosh 1980; Melosh 1989).

By modeling multiring basins under the principles of ring tectonic theory to recreate basins observed on the icy surfaces of Ganymede and Callisto, we are providing unparalleled insight into both internal layering and total thickness of their outer ice shells as well as the thicknesses of the subsurface oceans, with implications for geologic histories of these icy bodies of the outer Solar System.

Collaborators:  This work is led by Brown DEEPS graduate student Evan Bjonnes in collaboration with Brandon. C. Johnson and Alexander J. Evans.