Saturn’s moon Iapetus has fascinated astronomers since Cassini discovered in 1671 that it seemed to appear and disappear in orbit around its parent planet. This little walnut-shaped beaten and battered moon’s later claim to fame would be that it appeared to be bright on one side and dark on the other, which is why he could only see it on the western side of the planet and then it would seemingly vanish.
How it got that way would cause a lot of speculation down through the centuries.
In February of 2009 a trio of astronomers at The University of Virginia in Charlottesville lobbied for the use of NASA’s infrared Spitzer Space Telescope to investigate the long-standing theory that Iapetus was being “painted” with soot from another of Saturn’s moons; namely Phoebe. What team leader Anne Verbiscer and her colleagues Douglas Hamilton and Michael Skrutskie discovered would write a new chapter in astronomic history.
They found that over a span of millions of years, meteorite impacts on Phoebe had indeed kicked up enough dust to paint Iapetus, which orbits Saturn like our own moon with one hemisphere always facing the planet. Since the two possibly captured moons orbit in opposite directions of each other only one side is being “painted.” When they looked closer an immense ring of thinly spaced soot-like powder 20 times the thickness of Saturn was revealed in orbit far out in space.
So fine were the particles that they can only be detected by their faint thermal radiation signature in orbit. Deep black in color, they individually collect just barely enough heat from the distant sun to distinguish the ring from the surrounding cold of space. According to Verbiscer, "The particles are very, very tiny, so the ring is very, very tenuous — and actually if you were standing in the ring itself, you wouldn't even know it. In a cubic kilometer of space there are all of 10 to 20 particles."