Since it arrived at the ringed planet, NASA's Cassini orbiter has flown past Saturn's cloud-enshrouded moon Titan nearly two dozen times. And with each encounter, scientists come closer to piecing together the complex story of the Saturnian system's largest satellite.
Titan exhibits a variety of features, such as giant equatorial sand dunes, polar lakes, and methane-soaked mud flats. But the surface topography rarely varies more than 500 meters (1,600 feet) from the global average. So far only one mountain ridge has been spotted, and it is only about a kilometer from top to bottom.
In the 15% of the moon's surface mapped by Cassini's radar, astronomers have found just three impact features — the smallest about 30 km wide. The dearth of midsize divots around 20 km across indicates a young and active surface. There is convincing evidence for at least one volcano, and several other spots suggest flows and calderas.
But the key to understanding how Titan ticks seems to be the moon's "hydrological" cycle of methane clouds, rainstorms, rivers, and lakes. "Besides Earth, we really haven't seen fluvial erosion of an active nature anywhere else in the solar system," says Jason Barnes (University of Arizona).
This cycle runs at a net loss, however, methane is destroyed in the upper atmosphere by ultraviolet sunlight, so there must be some source of replenishment. Methane-spewing volcanoes could be it.
North of 75° latitude, radar images from several recent flybys reveal a well-populated lake district with more evidence for recent fluvial activity. "There are lakes with very well-defined perimeters. There are lakes with islands in them. There are channels between the lakes. There are small channels that feed the lakes," says Steve Wall (NASA/Jet Propulsion Laboratory). Similar lakes probably exist at high southern latitudes, but Cassini's radar has yet to map this region.
Widespread sand dunes near the equator are nearly all shaped the same way, indicating a global wind blowing from west to east. Also, according to Caitlin Griffith (University of Arizona), many of Titan's clouds are convective, forming (as on Earth) when there is an upwelling of air. Mysteriously, some clouds seem to hover over particular locations. "Perhaps we are seeing volcanic outgassing," says Griffith.