16 December 2010

Strange landscapes of the outer giants’ moons

Posted by mohi

Photomosaic of Cassini images, showing that Iapetus sports a strange equatorial ridge. Image courtesy of NASA

Kansas might be flatter than a pancake, but some of the moons circling Jupiter and Saturn are not.

The relatively tiny, icy, rocky moons of the outer solar system host a gigantic array of interesting surface features. The topographical menu on four moons alone is enough to feed the work of scientists for decades–even the slick, formerly-known-as-smooth surface of Jupiter’s Europa hosts a number of offerings.

Some features are fairly routine, like gently sloping depressions or plains. Others are more exotic, like the measured, undulating ridges on Ganymede and the cratered ruins blanketing Callisto, both moons of Jupiter. And some make the leap to the truly bizarre: enormous mountains encircling the equator of Saturn’s Iapetus, the likes of which have not been observed elsewhere.

Iapetus is sporting a biker-chic, spiked belt.

These topographical textures were the focus of Wednesday morning’s session P31D, The Shape of Things to Come: Using Topography to Investigate the Evolution of Outer Solar System Satellites II. Scientists modeled the formation of the exotic landmarks and attempted to understand the geological basis for their formation.

The take-home message? Each of these bodies is a dynamic, shifting world–not a dead, dry chunk of rock slowly revolving around a host planet. Without tectonic motion, temperature variations, and the remodeling force of liquids, the observed surface features would be absent.

Jeffrey Moore, from NASA Ames, discussed the craters on Callisto. Unlike the sharply defined pits in the Earth’s surface, Callisto’s craters are shallower, flatter, and much more numerous. Moore explained that frost formation on a crater’s rim promotes erosion of surrounding bedrock into the crater’s belly, which reduces the depth of the crater and gives it a ruined appearance. “This is unusual erosion,” he said. “The craters seem to be dissolving.”

Ganymede was the object of Michael Bland’s reflections. Bland, from Washington University in St. Louis, described the evenly spaced undulations in Ganymede’s surface, and suggested that heat was needed to create the observable grooves. When the moon’s icy crust expanded, it did so unstably, resulting in the alternate formation of pinched and swollen areas–hence the ridges.


A full view of Iapetus, showing its equatorial ridge. Is this a remnant of an orbiting smaller moon? Image courtesy of NASA/Cassini

Paul Schenk, from the Lunar and Planetary Institute, said that Europa is not as smooth as assumed. In fact, it has mountainous structures and plateaus, some of which rise as high as 1000 meters above the icy ocean.  Europa also has deep pits. “The topography is more diverse than we thought,” Schenk said. Schenk suggested that Europa’s topographical diversity makes it unlikely that the icy world is wrapped in a thin, 5-kilometer thick shell, as had been commonly thought. He said the moon’s upper crust is likely to be much thicker.

And then there’s Iapetus, with its 20-kilometer high, 200-kilometer long mountainous ridge sitting directly on the equator. And only on the equator. “Imagine standing at the base of this thing,” said Andrew Dombard, from the University of Illinois at Chicago. “It’s a wall of ice, taller than Mount Everest.”

Dombard said that scientists used to think the ridge resulted from tectonic activity in the moon’s interior. But those models failed to describe the observed location and absence of other similar features. Instead, he suggested the ridge originated from a “giant impact” during the latter stages of Iapetus’s formation. In theory, the collision ejected a large chunk of matter that began to orbit Iapetus. Eventually, the chunk spiraled inward, raining pieces onto the moon’s surface. Those pieces became the mountainous ridge–a sort of breadcrumb-trail for the failed ring’s orbit.

These four moons are only a fraction of the satellites orbiting our solar system’s large, outer planets. What might be waiting for us to discover on the others?

–Nadia Drake is a science communication graduate student at UC Santa Cruz