[LINK] "Bizarre Bulge Found on Ganymede, Solar System's Largest Moon"

[rfmcdonald]

National Geographic's Nadia Drake reports on an unusual feature of Ganymede's surface that is, among other things, a hint about the existence of an ocean.

There’s a big, weird bulge on Ganymede, the largest moon in the solar system. Protruding from a spot on the moon’s equator, the bulge is about 375 miles (600 kilometers) across, about the area of Ecuador, and two miles (three kilometers) tall, about half the height of Mount Kilimanjaro.

It’s not at all what scientists expected to find on this moon of Jupiter.

“I found it a bit by accident while I was looking to complete the global mapping of Ganymede,” says planetary scientist Paul Schenk of the Lunar and Planetary Institute, in Houston. He reported the weird feature on March 20 at the Lunar and Planetary Science Conference.

The size and location of Ganymede’s bulge, which appears to be made of thick ice, suggest that once upon a time, the moon’s icy shell rotated atop the rest of the moon, like an interplanetary Magic 8 Ball.

First, Schenk thinks, the bulge began growing at one of the poles. Then, once the bulge grew big enough, its mass began to drag the shell into a different position. The shell slid atop the ocean, while the moon’s interior stayed in the same orientation. Eventually, the part of the shell that once capped the poles ended up at the equator.