There are many odd things about Iapetus, the icy moon of Saturn. One of its most striking features is the strange two-tone coloration of its surface, likely due to the evaporation of water ice leaving behind various organic compounds. But another curious feature of the moon is its shape.
Iapetus is not – like most large satellites in the solar system – spherical or ellipsoid, but has been described as more “walnut-shaped”. This is compounded by the bizarre mountainous ridge that perfectly encircles about two-thirds of the moon around the equator. It now seems that this ridge may well be the remnants of a ring system that fell to the surface.
At 20 kilometers (12.4 miles) wide and up to 20 kilometers high in some places, the range contains some of the tallest mountains in the entire Solar System. Incredibly, if they were scaled to being on our own planet, it would result in a mountain range some 100 kilometers (62 miles) high.
It is not only the size of these mountains that is hard to explain, but more so the near-perfect band they form running around the moon's middle. One suggestion has been that the moon once rotated much quicker than it does today, and that during a rapid cooling event the mountains formed.
Another theory, however, suggests that the moon once had a ring that has over time fallen to the surface. This idea has been lent greater support by a new paper published in Icarus.
But wouldn’t falling debris simply smash into the moon and cause impact craters like those created by meteorites? Well, according to the researchers, not necessarily. Falling ring material behaves differently because it has already been captured by the moon’s gravity, meaning that it doesn’t rain down on the surface in the dramatic manner of asteroids and meteorites.
Instead, it would likely spiral around the moon, and eventually hit the surface after several full orbits. This means that the angle at which it touches down would be much shallower than if it had simple crashed down. Bearing this in mind, the researchers decided to run computer simulations to take into account this different projection.
What resulted was a similar topography to that currently seen on Iapetus. The shallow impact angles from falling ring material, made up of lumps of ice ranging in size from 1 meter to 1 kilometer (3.3 feet to 0.6 miles) in this simulation, rarely created deep impact sites. What is more, the fragments often survived break up and so began piling up on the surface, with more and more pieces knocking into them to form the distinctive ridge of material similar to that on the curious moon.
[H/T: New Scientist]