Planetary Scientists Create Global Geologic Map of Ganymede

U.S. Geological Survey researchers have produced the first complete global geologic map of the Jupiter’s largest moon, Ganymede.

Global geologic map of Ganymede. Image credit: Collins G.C. et al.

Global geologic map of Ganymede. Image credit: Collins G.C. et al.

Discovered in 1610, Ganymede has a diameter of 5,268 km, around 8 percent larger than that of Mercury and much larger than Pluto.

The history of this largest moon in our Solar System can be divided into three phases: an early phase dominated by impact cratering and mixing of non-ice materials in the icy crust; a phase marked by great tectonic upheaval; and a late phase characterized by a gradual drop in heat flow and further impact cratering.

The surface of Ganymede has been formed through a variety of processes and is a mix of two types of material: very old, highly cratered dark regions, and younger lighter regions.

“The dark/light distinction is based on sharp relative albedo contrasts at terrain boundaries, rather than on absolute albedo, because several other types of surface modification change the absolute albedo within these terrain classes. Dark materials cover 35 percent of Ganymede’s surface; almost the entire remainder of the surface is covered by light materials,” the researchers said in the description of the new map.

“Dark materials are heavily cratered, though not as heavily cratered as the surface of the neighboring satellite Callisto, suggesting that dark materials cannot be a primordial surface. At high resolution, dark materials are dominated by the downslope movement of loose dark regolith within impact craters and on the sides of bright ridges and hummocks. Observations suggest that dark materials are covered by a thin lag deposit of dark regolith derived by sublimation of a more ice-rich crust below. Dark materials commonly exhibit sets of concentric arcuate structures known as furrows. Furrows may be the remnants of ancient multi-ring impact basins, similar to intact impact basins on Callisto, such as Valhalla and Asgard.”

“Light materials crosscut dark materials and exhibit a lower impact crater density, demonstrating that they were formed later. Light materials are subdivided into an intricate patchwork of crosscutting lineaments called grooves, mixed with areas of relatively smooth terrain.”

“At high resolution, most light materials are dominated by extensional faulting. Even light materials that appear to be smooth at low resolution are marked at high resolution by sets of parallel lineaments of apparent tectonic origin.”

“There is an open question on the extent to which light terrain is formed by cryovolcanic flooding of dark material with brighter ice versus tectonic destruction of preexisting surface features and exposure of brighter subsurface ice in fault scarps; it is certainly possible that both of these processes play important roles in the formation of light materials.”

Ganymede. Image credit: U.S. Geological Survey.

Ganymede. Image credit: U.S. Geological Survey.

“Not all tectonic activity on Ganymede has led to the formation of light material – some dark material is cut by extensional faults without exhibiting a major change in albedo while reticulate material is cut by two sets of tectonic lineaments and is transitional in albedo between adjacent light and dark materials.”

“Other materials found on Ganymede were created by several types of impact features, ranging from impact craters to viscously relaxed impact features called palimpsests to the large impact basin, Gilgamesh, in the southern hemisphere.”

The new geological map of Ganymede, published yesterday by the U.S. Geological Survey, is based on a global image mosaic of Voyager and Galileo data assembled by researchers at a nominal resolution of 1 km/pixel.
Voyager was the first mission to fly through the Jupiter satellite system and passed by the icy surface of Ganymede in 1979. Those first images revealed a complex surface, segmented and fractured into dark and light terrain.

In 1995, the Galileo spacecraft was placed in orbit around Jupiter and began to return high-resolution images of the surface that help to understand many of the features seen at low-resolution by Voyager.

With its varied terrain and possible underground ocean, Ganymede is considered a prime target in the search for habitable environments in the Solar System, and the scientists hope the new map will aid in future exploration.

Prof Jim Head from Brown University, who is one of the map’s co-authors, said: “it is very rewarding to see the results of all of our efforts into this integrated global compilation that will now be used to plan the next phase of scientific exploration of the Galilean satellites.”

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Collins G.C. et al. 2013. Global geologic map of Ganymede: U.S. Geological Survey Scientific Investigations Map 3237, pamphlet 4 p., 1 sheet, scale 1:15,000,000; doi: 10.3133/sim3237