Saturn’s flying saucers

Did you know that there are flying saucers around Saturn?

Okay, I confess, they’re not really flying saucers. They’re moons. But they sure look like flying saucers. At least, they look like the flying saucers from the space alien movies from the 1960s. You know, the ones that look like this:

The 2 moons in question are called Pan and Atlas. They’re really small; Atlas is only about 25 miles by 15 miles in size. That’s tiny compared to the size of Saturn’s biggest moon, Titan, which has an equatorial radius of 1600 miles. (Incidentally, that makes Titan larger than the planet Mercury, and much bigger than the ex-planet Pluto.)

These small planets have confused scientists ever since they were discovered by the Voyager 1 and 2 spacecraft in the early 1980s. Recently, the Cassini spacecraft has provided highly detailed images of the moons, showing us how oddly-shaped they really are. A research team from the Space Science Institute in Boulder, Colorado, believes it may have the answer for why these moons have such an odd shape.

Saturn’s rings lie in a flat disc that corresponds to the planet’s equator; the moons are actually embedded within those rings. Atlas lies within the A-ring, the outer of Saturn’s 2 brightest rings. Pan is within the Encke Gap of the A-ring. (The diagram here shows the naming convention of the A- and B-rings, as well as the Encke Gap.) Scientists had previously argued that the moons were made up solely of fragments left over from collisions of larger moons. However, scientists from a team led by Carolyn Porco have concluded that the moons are made up primarily of material just like the rings themselves – light, porous and icy. In fact, as much as two-thirds of the moons might consist of this matter, which is quite unlike the dense, massive material that would be the leftovers of other planetary bodies.

Porco’s research group believes that the cores of Pan and Atlas likely came from these planetary leftovers. But over the millennia, the moons may have drawn dust and ice slowly to themselves, building themselves up. Two important features of the moons are consistent with this model: the ridges around the equators of the moons are in the same plane as the rings of Saturn, and they are as thick as the vertical distance that the moons cover in the rings. So it makes sense that they could likely have picked up material as they orbited Saturn to make themselves larger. This accumulation would likely have occurred in several stages, the last of which would have resulted in the strange ridges that give them their flying saucer shape. That’s because the moons act as “accretion discs;” as matter falls towards a gravitational pull, it gradually builds up into discs. In fact, this is quite possibly comparable to how Saturn’s rings themselves were made.

However, it is also believed that Pan and Atlas have reached their final shape, and will not grow any larger. That’s because their orbits are thought to prevent any additional material from being able to stick stably to their surfaces.

But don’t worry. Despite appearances, Martians are not making their way towards Saturn in their flying saucers. I guess these are just another one of the bizarre things in our universe!

The following image was used in this entry:
Flying saucer: http://www.altfg.com/blog/biography-obit/bernard-gordon/