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The Riddle Moon

by Perry Pezzolanella

Several moons in the Solar System are bizarre, but there is one orbiting Saturn that is different from all others and it was known to be so as long ago as the late 1600’s. Iapetus is the third largest moon of Saturn with a diameter of 905 miles. It orbits much farther way from Saturn than any other major moon, three times farther away than Titan and about ten times farther away than the other moderate sized icy moons. It is also the only major moon of Saturn that is inclined 15º away from the ring plane. Iapetus is the only moon where someone standing on its surface could see Saturn’s rings open enough to see them for what they are worth.

The oddness of Iapetus is its brightness as it shines at magnitude +10.2 on the western side of Saturn and dims to +11.9 on the eastern side. Iapetus was discovered by Giovanni Cassini on October 25, 1671 when it was at its brightest. He tried to follow it to determine its orbit, but it dimmed too much to follow. He finally followed Iapetus successfully in 1705 with a better telescope and determined that it kept the same face towards Saturn. He correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere. The dark hemisphere was later named Cassini Regio in his honor.

Iapetus is composed mainly of ice with only about 20% rocky materials. It is walnut-shaped with a huge ridge wrapping nearly all the way around the equator. The most dramatic feature is the sharp dichotomy of the dark and bright sides. The dark side hardly reflects 4% of the sunlight hitting it while the bright side reflects up to 60%. The dividing line between the regions is dramatic with no gray transitional zone. The surface goes from nearly blackish brown with a reddish tint to brilliant white. Near the transition zone there are patches of white on the dark surface of the pole facing slopes of craters and dark material filling low-lying regions in the bright areas.

Scientists believe that the dark material is a residue from the sublimation, or evaporation, of water ice on the surface, possibly darkened further by exposure to sunlight. Because Iapetus has a slow rotation of 79.3 days, it can have the warmest daytime and coldest nighttime temperature in the Saturn system. It is as warm as -227ºF and as cold as -256ºF. A uniformly ice-covered surface would gradually separate as the ice sublimates from the warmer hemisphere and condenses on the colder areas. As this progresses over the course of a billion years or so, one hemisphere grows darker and ice free and the other grows icier and brighter. This further darkening and brightening creates a thermal runaway feedback where the dark areas grow warmer and sublimate faster and grow into a totally darkened hemisphere. During this course of time Iapetus loses about 60 feet of ice to sublimation in the dark hemisphere. Iapetus has weak gravity that also aids in the ice re-distributing itself effectively onto the colder hemisphere and poles. Dark material blasted off the small outer moons, especially Phoebe, has dusted the leading edge of Iapetus, further aiding in the sublimation process.

Another riddle to ponder is the gigantic equatorial ridge that wraps nearly all the way around Iapetus. It is about 12 miles wide and 9 miles high with a few peaks up to 12 miles. This is an impressive height for a small world. It is not known how it formed, but it is ancient because it is heavily cratered, probably having been formed early in the creation of the Solar System when the rate of cratering was much higher. This ridge could be considered a second riddle because it either formed from within Iapetus or externally, possibly from an ancient debris ring that once surrounded it.

Iapetus is bizarre in that it is unusually flattened at the poles like Jupiter and Saturn, which enhances the walnut appearance. It also has an excessive amount of large impact basins to add to the enigma of the ridge and contrasting bright-dark surface. The Cassini spacecraft discovered the ridge when it flew by Iapetus on December 31, 2004. The moon has a fairly smooth, round shape that is interrupted right at the equator by the long ridge, which is triangular in cross-section. The ridge is prominent on the part of Iapetus that faces its direction of travel in its orbit around Saturn, the darker hemisphere, and breaks up a bit on the trailing, bright hemisphere. Since there are no troughs on the sides of the ridge, it had to form when Iapetus was no longer warm from its creation. It formed late though still ancient, sits exactly on the equator and there are no other ridges on Iapetus. This feature is unique to Iapetus.

One theory suggests the cause of the ridge is due to the slowing down of Iapetus’ spin from a 16.5-hour day to a very long 79 days where it keeps the same side facing Saturn. It is possible that it was spinning so fast that its rotation was on the verge of instability, which flattened the poles and pushed out material along its equator. The problem is Iapetus is large enough that such rotation would give it a football shape, not walnut. The other theory is that a debris ring collapsed onto Iapetus. If this is true, how come other moons do not have ridges? Iapetus is far enough from Saturn that a moon or ring could briefly exist. A small moon could eventually get too close to Iapetus and break up into a ring, or huge impacts could throw debris up from the surface of Iapetus to form a ring. Over the course of several billion years as the rotation of Iapetus slowed down the ring collapsed onto the surface along the equator. The ring may have surrounded Iapetus in an elliptical, which could explain why the ridge does not go perfectly around it. The only way to possibly solve this riddle is to land on the ridge, determine its composition and density, and compare it to the rest of Iapetus. Such a mission is far beyond our lifetimes, probably several hundred years away.

Iapetus is a world of riddles, but the big riddle of whether it is “dark on bright” or “bright on dark” is finally settled, thanks to the Cassini spacecraft. Cassini flew by Iapetus for the last time on September 10, 2007, never to return as it orbits 2.2 million miles away from Saturn, too far to approach again. The images of Iapetus that we have today will probably remain the very best we will ever have for the rest of our lives.