The solar system's largest moon has put on some weight around its middle, hosting a bulge that stands out from the otherwise fairly flat surface. The reason is unknown, but one theory hints at exciting developments beneath.
Presenting at the recent Lunar and Planetary Science Conference, Dr. Paul Schenk of the Lunar and Planetary Institute compares Ganymede's spare tire to Europa's “crop circles.”
The bulge is about 3 kilometers (2 miles) high, an unspectacular distance on Earth, but a significant feature on a world with a radius two-fifths that of Earth and a generally flat topography. The bulge is about 600 kilometers (375 miles) across.
“I found it a bit by accident while I was looking to complete the global mapping of Ganymede,” Schenk told National Geographic. He was looking to see if Ganymede had anything similar to the depressions seen on Europa.
Schenk writes, “Concentric arcuate (curved) trough-like depressions roughly 25-40 km (15-25 miles) across and 500 or more km (310 miles) long have previously been identified on Europa.”
These troughs reach 1.5 km deep, a huge amount for the notoriously smooth Europa; they form two structures—nicknamed "crop circles" by planetary scientists—located 10° north and south of the Europan equator, on roughly opposite sides of the planet.
These troughs reach 1.5 km (0.9 miles) deep, a huge amount for the notoriously smooth Europa. These troughs form two structures compared to crop circles located 10° north and south of the Europan equator, on roughly opposite sides of the planet. They are thought to be a product of the strain on Europa's ice shell from its rotation.
Credit: Schenk et al. Image (left) and topographic map (right) for Europa's troughs (T) and depressions (S).
While nothing similar was detectable on Ganymede, the bulge could be just as significant. Ganymede's surface is a mix of rock and ice, but the bulge is thought to be thick ice.
Schenk's theory is that the bulge originally grew at either the north or south pole. “The only place you can get a large mass like that—that’s not related to geology that we know of—is at the cold poles,” Schenk says. “The poles are permanently cold, so it leads to a significant amount of thickening of the ice shell.”
Ganymede's rotation and interaction with Jupiter's gravity tugged the extra mass towards the equator. For such an epic journey to take place, however, it would require an almost complete lack of friction. This is what one would expect if Ganymede has an internal ocean, as recent research has suggested. Indeed, it may even have more than one.
If this theory is correct, Schenk expects a matching bulge on the other side of the moon, but the Galileo probe has not mapped Ganymede in sufficient detail to observe it.
Schenk's co-author Bill McKinnon of Washington University, St. Louis, says even if the bulge's origins are known, it remains a mystery how the bulge is supported, rather than collapsing under its own weight. McKinnon noted, “We've never seen anything like it before; we don't know what it is.”
Therefore, the battle of the bulge is not yet over, as more research is needed to determine the cause of Ganymede’s waistline protuberance.