The Story of Iapetus’s Spots
At this week’s annual meeting of the Division of Planetary Sciences of the American Astronomical Society in Orlando, a special session was devoted to Saturn’s moon Iapetus and its dramatic terrain marked by an equatorial mountain ridge and alternating regions of bright white ice and carbon black. It has long been thought that the explanation for the dark material covering roughly half of Iapetus is that dust knocked off more distant moons of Saturn is swept up by Iapetus as it orbits Saturn. One problem with this hypothesis, though, is that the distribution of dark material does not cover just the leading hemisphere of the moon. The pattern looks more like that of a baseball, with the bright water ice wrapping around the poles and the dark carbon-rich material wrapping around the equator.
Image Credit: NASA/JPL/SSI. Click the image for image details.
John Spencer, a scientist on the Cassini Infrared Spectrometer (CIRS), and colleagues have developed a thermal segregation model that nicely explains the dark/bright pattern on Iapetus. In a nutshell, the idea is that you start by having Iapetus plow through the dark dust migrating inward from the dark outer moons and coating the leading hemisphere of Iapetus. The dark terrain gets hotter than the clean bright water ice because it absorbs more radiation from the Sun, just as dark asphalt gets hotter than concrete. Because it is warmer, water ice underneath that dark material evaporates through the thin coating of dust and recondenses on cooler parts of the moon. This transports ice to cold areas which are shielded from the incoming dust and keeps them very bright and removes bright ice from areas that have been darkened by dust. The result is the dramatic black and white landscape seen in the most recent pictures returned by Cassini. Cold, bright areas collect bright water ice from the warm, dark areas, and dark areas lose water ice to the bright areas. There are, therefore, no shades of gray.
Image Credit: NASA/JPL/SSI. Click the image for the full resolution image and caption.
Image Credit: NASA/JPL/SSI. Click the image for the full resolution image and caption.
One of the cool things about the thermal segregation model described above is that it can be simulated on a computer and the migration of bright material can be visualized. Starting with an assumed initial distribution of dust on the moon, Spencer calculates that it would take about 100 million years for the surface to get to its current distribution of bright and dark terrain. If more time passes, the dark area spreads further than is observed. This age is probably related to the rate at which meteoroids “garden” the surface of Iapetus by punching through the thin (< 1 meter) layer of dark material and mixing it in with ice underneath. So the distribution of the bright and dark terrain give a clock that helps us also understand the cratering history of Iapetus.
There remains a puzzle about getting the dark material to Iapetus in the right amounts from the outer irregular moons of Saturn. These moons, such as Phoebe, are presumed to be captured by Saturn rather than having formed with Saturn, like Iapetus did. They may have formed further from the Sun where the composition had a greater proportion of carbon-rich compounds leading to their darker color.