In 1671, Italian astronomer Giovanni Cassini discovered Iapetus, Saturn’s most distant moon, and correctly surmised that it was tidally locked to the planet, always showing one light and one dark face. Now, as the Cassini space mission to Saturn returns more data, it appears Iapetus is showing a new feature – a ridge covering 75% of its equatorial surface.
The ridge is over 1300 kilometers long, 20 kilometers wide and between 13 and 20 kilometers in height. It is broken periodically by gaps and single peaks, but almost precisely follows the equator around the moon, giving it a distinct “walnut” shape.
While other bodies in the solar system show similar features that are of comparable age and cratering, none of them have a ridge that sticks so closely to the equator.
In the world of intra-solar system science, this is much like the “how its leopard got its spots” debate – everyone can see it, but no one can explain it.
Now, a Washington University team thinks they may have the answer – another moon.
According to the Washington U team, Iapetus itself once had a moon, one that spiraled in toward the planet’s surface over the course of billions of years. On Earth, our moon is slowly retreating as the planet loses the gravitational battle with acceleration, but if it were closer, say in the zone where geo-synchronous satellites orbit, it too would slowly descend toward the planet.
Due to tidal forces, the moon of either Iapetus or Earth would break up into small pieces as it approached the atmosphere, and would naturally gravitate to the lowest-energy state around the plant – above the equator.
Once there, the small pieces would pick up a significant amount of speed – 400 meters per second or so – and would crash into the surface of the planet, initially form craters in the surface. Over time, the continual bombardment would create a ridge-like effect, much like the ones seen on nutty-looking Iapetus.
Currently, not enough data or testing exists to confirm the hypothesis, but other options such as the idea that the ridge resulted from volcanic activity suffer from the problem that they cannot explain a) why along the equator b) why only on the equator and c) why only on Iapetus, and the Washington team believes their explanation is the best one to fit the bill.
Hopefully if they are correct they won’t…moon the competition.