Iapetus, Saturn’s Strange Walnut Moon
[Writer] This is research news from U-I-C – the University of Illinois at Chicago. Today Andrew Dombard, associate professor of earth and environmental sciences, describes a hypothesis he and colleagues developed about how a peculiar mountain range was created around the equator of Saturn’s moon Iapetus. Collaborators include William McKinnon of Washington University in St. Louis, Andrew Cheng of the Applied Physics Laboratory at Johns Hopkins, and Jonathan Kay, a UIC graduate student.
Here’s Professor Dombard:
[Dombard] Spacecraft that have been exploring the solar system have revealed many different geological wonders out there, but one of the most peculiar things that we’ve seen can be found on the satellite Iapetus. Iapetus is a satellite of the giant planet Saturn. It’s a fairly modest-sized satellite – much smaller than the Earth. It’s only about 735 kilometers in radius, or about 450 miles.
Unlike Earth and the other terrestrial planets, this is a icy satellite, which means instead of being composed of rocky material, like the Earth, it’s primarily made out of ice.
Now the truly peculiar thing about Iapetus is it has a massive mountain range that sits on its surface. This mountain range is up to 200 kilometers wide – about 120 miles – and it’s about 20 kilometers tall – about 12 miles. And the really amazing thing about it is it sits perfectly straight and sits exactly on the equator of Iapetus. This is such a big feature on the surface that it’s clearly visible in global views of the whole satellite from space. In fact the ridge running right along the equator kind of gives Iapetus the look of a giant space walnut.
Now imagine standing at the base of this thing. You are confronted by a wall of rock that is 12 miles tall – far taller than any mountain we have here on Earth and it’s almost as tall as the largest mountain in the solar system, Olympus Mons, which is found on the planet Mars. Unlike Olympus Mons, this is a ramrod-straight wall of ice that runs off either direction to your left or right and you don’t see it end. Instead it just continues on, unabated, until it just disappears over the horizon over 100 kilometers away.
This is truly one of the most astounding geological features in our solar system. It was discovered about 5 years ago, but in that time, we have not figured out how this thing formed.
There are three critical observations that any hypothesis for the formation of the ridge has to be able to explain. First, it sits exactly on the equator, it’s only found on the equator of Iapetus, and it’s only found on Iaputus. It’s not found anywhere else in the solar system.
All of the models that have been proposed so far may explain one or two of these observations, but not all. My colleagues and I think we have a hypothesis that explains all three.
One of the things that we know happens in the early history of our solar system is giant impacts. The planets and satellites in our solar system formed through the process of accretion. You start off with a whole lot of small particles in orbit around the sun, and these small particles run into each other and grow into bigger things. These bigger things run into still other bigger things and eventually you start building up large bodies. The last thing that happened during this process is the largest thing in a particular orbital zone will run into the second largest thing, and when this happens you have a truly massive collision. Our Earth went through one of these collisions very early in the formation of our solar system 4.5 billion years ago when a body roughly the size of the planet Mars ran into the proto-Earth. This was a truly massive impact that stripped off the outer portion of our planet. A lot of that debris came back down on the Earth but some of it stayed in orbit and ended up making our Moon. Charon, the large satellite of Pluto, was similarly formed by a giant impact.
Well suppose we had one of these giant impacts on Iapetus. What would have happened to this debris? Well this debris may have formed a sub-satellite in orbit around Iapetus, which itself was in orbit around Saturn. And the gravitational tidal interactions between this sub-satellite and Iapetus would have caused this sub-satellite to spiral in over time scales of less than 1 million years to 1 billion years. Now this seems like a long time, but bear in mind we’re talking about something that happened 4.5 billion years ago. Eventually this sub-satellite would have gotten so close to Iapetus that the tides from Iapetus would have torn this sub-satellite apart. This debris would have ended up right over the equator of Iapetus for exactly the same reasons the rings of Saturn are right over Saturn’s equator. And this material would have eventually rained down on to the surface, slowly building up the ridge.
This model explains the three critical observations. First of all, it explains why the ridge is found exactly on the equator – that’s where the debris was coming down. It explains why it is only found on the equator – again, the debris was right over the equator and would not have come down anywhere else. And it may explain why this ridge which is only found on Iapetus and not found anywhere else in the solar system. That’s because Iapetus has the very (unusual) combination of being a fairly large satellite and also being fairly far away from its primary body – in this case, Saturn. As a consequence, it has the largest hill sphere of any major satellite in the outer solar system. The hill sphere is the zone around a satellite where the gravity of that satellite dominates over the gravity of the primary body. That means if you park something in orbit around a body within this hill sphere, it’s going to stay in orbit around that body, and not be dislodged by the larger primary body. Iapetus has the largest hill sphere, so as a consequence, only Iapetus has the orbital room to hold on to one of these sub-satellites.
Undoubtedly, the other satellites in the outer solar system had these giant impacts and may have formed sub-satellites, however these sub-satellites would have been dislodged from orbit around these satellites and lost to the primary body – Jupiter, Saturn, Uranus or Neptune.
Well we think we have a very compelling idea to explain the formation of this ridge on Iapetus – so compelling, in fact, that I almost believe it. Almost.
This is the way that science works. First we propose a hypothesis, then we have to test it. Right now we have a series of preliminary tests that demonstrate that our idea is plausible. But future tests are required.
My colleagues and I, including students here at UIC, are going to be performing these tests over the next couple of years to determine whether or not the ridge came from a giant impact.
[Writer] Andrew Dombard is associate professor of earth and environmental sciences.
For more information about this research, go to www.today.uic.edu, click on “news releases” and look for the release dated December 13, 2010.
This has been research news from U-I-C – the University of Illinois at Chicago.
