![]() "But it is not out of the question that the moons date back to the time of ring formation," said Porco. Just exactly when the rings formed is not known. "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, "which might have been left intact all this time and protected from additional collisional breakup by the mantle of ring particles around it." Where did such large cores come from? And when did this all take place? To make a 30-kilometer moon (19 miles) requires a seed of about 10 kilometers (6 miles). 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. By this process, a moon will grow even if it is relatively close to Saturn. 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. Image credit: NASA/JPL/Space Science Institute Janus appears above Saturn's rings and Prometheus, more elongated, is below the rings. Porco is the lead author of the first of two related articles published in this week's issue of Science. ![]() "We think the only way these moons could have reached the sizes they are now, in the ring environment as we now know it to be, 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, Colo. So how did they do it? They got a jump start. The trouble was, these moons are within and near the rings, where it is not possible for small particles to fuse together gravitationally. The tip-off was the very low density of the inner moons, about half that of pure water ice, and sizes and shapes that suggested they have grown by the accumulation of ring material. 7 issue of the journal Science, has led to new insights into how some of these moons may have formed. Now, several years' worth of cosmic 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. And the small, irregularly shaped ring-region moons were believed to be the leftover pieces from this breakup. The resulting debris quickly spread and settled into the equatorial plane to form a thin disk surrounding the planet. 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 moons began as leftover shards from larger bodies that broke apart and filled out their "figures" with the debris that made the rings. Imaging scientists on NASA's Cassini mission are telling a tale of how the small moons orbiting near the outer rings of Saturn came to be. The highest resolution images of Pan (right) and Atlas (left) reveal distinctive "flying saucer" shapes created by prominent equatorial ridges not seen on the other small moons of Saturn.
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