Geology and mapping of dark terrain on Ganymede and implications for grooved terrain formation

Abstract

Abstract. Geological mapping of regional and high-resolution Galileo images reveals a variety of units and structures within Ganymede's dark terrain. We have made a detailed study of areas within Galileo, Marius, and Nicholson Regiones in order to investigate the style of tectonic deformation experienced by dark terrain adjacent to swaths of grooved terrain. Dark terrain appears to become fractured as a precursor to grooved terrain formation; in places, dark fractured swaths are recognized which have similar characteristics to brighter grooved terrain swaths. Tectonic deformation may be, but is not always, focused through preexisting weaknesses caused by impact craters and furrows. A prominent groove lane, Anshar Sulcus, is inferred to have formed by the process of hanging wall rollover, accompanied by a small amount of right-lateral horizontal shear offset resulting from NE-SW extension.

Formation of Grooved Terrain on Ganymede: Extensional Instability Mediated by Cold, Superplastic Creep

“…This paper updates our earlier results, incorporating nominally final, published flow laws for the creep of ice at low stresses Kohlstedt 1997a,b, 2001) as well as revised parameters for the power-law creep of very cold ice . We conclude with a detailed discussion of our results in light of the new Galileo images and interpretations (Collins et al 1998a, Patel et al 1999, Prockter et al 2000, comparing the structural form predicted by this model with what is actually observed, the implications for Ganymede's heat flow and internal evolution at the time of grooved terrain formation, and requirements for further progress.…”

“…7) and estimated total extension in excess of 50-60%. Similarly large strains (∼25%, ∼35%, and >100%) are indicated by three faulted (if not grooved) and distorted craters elsewhere , Patel et al 1999, Prockter et al 2000. Such large strains naturally are problematic for Ganymede as a whole, given the present lack of evidence for extensive compression or consumption (subduction) of crust (Collins et al 1998a).…”

“…Moreover, because of the sharp contacts between the dark and bright terrains, the nearly universal lack of matching topographic features on either side of a given bright terrain lane, and evidence for impact excavation of subjacent dark terrain material, bright terrain formation has been thought to be a product of the creation of wide rift zones and subsequent filling with at most a few kilometers thickness of bright terrain material, as water, slush, or hot "glacial" ice (for post-Voyager reviews of the geology and tectonics of Ganymede, see Shoemaker et al 1982, McKinnon andParmentier (1986), and Squyres and Croft (1986)). The dearth of obvious cryovolcanic features in Galileo imagery, however, has lead to the hypothesis that some regions of bright terrain were "tectonically resurfaced," with no or limited cryovolcanic activity (Head et al 1997, Collins et al 1998a, Prockter et al 2000, whereas discovery of potential contact matches across bright terrain swaths has led to the proposal that some lanes of bright terrain were created by crustal spreading (e.g., Collins et al 2001).…”

Mass anomalies on Ganymede

“…Several geological units are common to Galileo, Nicholson, and Marius Regiones, implying similar modes of formation and modification throughout Ganymede's dark terrain (Prockter et al, 2000). Therefore, the addition of meteoritic contaminant to surface ice results over time in a veneer of dark material on Ganymede's surface.…”

The tidal response of Ganymede and Callisto with and without liquid water oceans

“…The surface of Callisto, the outermost satellite, appears to have been unmodified by internal processes since shortly after its formation 4.5 billion years ago (Greeley et al, 2000). Ganymede, on the other hand, shows several distinct phases of surface modification, the most recent occurring perhaps 2 billion years ago (Prockter et al, 2000). These surficial clues to internal structure were supplemented by the radio science and magnetometer experiments on the Galileo spacecraft, which revealed that the interiors of the two bodies are also very different.…”