Scientists zero in on the origins of Saturns moons

Images of Saturns moons orbiting near the planets outer rings, taken by NASA, have shed light on their origin.

Washington, Dec 7 : Images of Saturn's moons orbiting near the planet's outer rings, taken by NASA, have shed light on their origin.

It has long been suspected that Saturn's rings formed in the disintegration of one or several large icy bodies, perhaps pre-existing moons, by giant impacts.

The resulting debris quickly spread and settled into the equatorial plane to form a thin disk surrounding the planet. And the small, irregularly shaped ring-region moons were believed to be the leftover pieces from this breakup.

Now, several years' worth of images of Saturn's 14 known small moons have been used to derive the sizes and shapes of most of them, and in about half the cases, even masses and densities.

Evidence for the theory regarding the origin of the moons comes from the very low density of the inner moons, about half of that of pure water ice, and sizes and shapes that suggest they have grown by the accumulation of ring material.

But the contradiction here is that these moons are within and near the rings, where it is not possible for small particles to gravitationally fuse together.

"We think the only way these moons could have reached the sizes they are now, in the ring environment, was to start off with a massive core to which the smaller, more porous ring particles could easily become bound," said Carolyn Porco, Cassini imaging team leader from the Space Science Institute in Boulder.

Simple calculations and more complicated computer simulations have shown that ring particles will readily become bound to a larger seed having the density of water ice. By this process, a moon will grow even if it is relatively close to Saturn. The result is a ring-region moon about two to three times the size of its dense ice core, covered with a thick shell of porous, icy ring material."The core may in fact be one of the remnants from the original ring-forming event," said co-author Derek Richardson, professor of astronomy at the University of Maryland, College Park. "It might have been left intact all this time and protected from additional collisional breakup by the mantle of ring particles around it," he added."Just exactly when the rings formed is not known. But it is not out of the question that the moons date back to the time of ring formation," said Porco.

ANI