G.K. GILBERT AWARDEE: HOW WAS GANYMEDE RESURFACED?

Ganymede, the Solar System’s largest satellite, underwent a profound resurfacing event in the geologic past, transforming 2/3 of its ancient, dark, heavily cratered terrain into much brighter, presumably more ice-rich, smooth and grooved terrains. These latter descriptors are Voyager-era, and in those days Ganymede geology took center stage as the fundamental icy satellite problem to be understood (Voyager images of Europa were far poorer). The Galileo mission changed all that, with its focus on Europa and with a limited amount of new Ganymede imaging contributing to a reassessment of resurfacing mechanisms. Seemingly scrapped were concepts of rifting, infill by water, slush, or warm ice, and further tectonic imbrication. The latter alone was deemed sufficient to turn darkness into light, and indeed, examples of rifted and extended dark terrain exist, although whether they are sufficiently “bright” is unclear. All clear examples of grooved terrain have well defined bounding faults, which may be extensional breakaway margins, although the specific conditions and material parameters that lead to this tectonic expression are unclear (numerical work is ongoing). Particularly enigmatic are the smooth (or “subdued” grooved) terrains. We argue that not only is it difficult to make smooth bright terrain from rough dark terrain by tectonic resurfacing alone, it is simply impossible. We are left with two alternatives: 1) smooth terrains are the product rift infilling after all, with very modest subsequent tectonic extension, or 2) they represent Europa-style crustal separation and band formation. Limited evidence for the first comes from topographic studies, embayment relationships, and the structural associations of smooth terrains and caldera-like features (themselves certainly not the product of tectonic resurfacing) (e.g., at Sippar Sulcus). Limited evidence for the second comes from Arbela Sulcus and other “band-like” features on Ganymede (e.g., at Sippar Sulcus as well), although this structural interpretation has been questioned. Europan bands are topographically subdued, yet lineated, so with age and mass wasting could very easily resemble Ganymede smooth terrains. Nor should we be surprised that Ganymede, in its high heat flow days, could resemble Europa in terms of tectonics or eruptions to its surface.