I recently wrote about Iapetus, an outer moon of Saturn that's 1,500 km wide, and has a huge ridge that runs all the way around its equator. While they find this method works for inner moons, it could work for some large outer moons as well. Credit: NASA/JPL-Caltech/Space Science Institute/University of Bern When that happens, the two collide, merge, and kinda sploosh together like slushballs, forming a ridge around them as the material gets squished out (like the ice cream part of an ice cream sandwich if the ice cream is too warm, and these food analogies are killing me)Ĭassini image of the walnut-shaped moon Iapetus (left), and a model of it based on collisions between smaller moons, reproducing the weird and huge equatorial ridge. The collisions have to be head-on, or nearly so, and occur at speeds of some dozens of meters per second (roughly up to twice as fast as highway speeds). Using sophisticated computer models of how objects like these behave when they collide, they found an astonishing thing: When they accounted for the objects' mass, texture (they tend to be porous as opposed to solid), and the tides from gravity, they were able to reproduce the shapes of Pan and Atlas pretty well. This process continues, with each moon growing as it moves through and away from the rings, and you wind up with a series of moons that get bigger the farther out from Saturn they are this growth process is called the pyramidal regime.īut what happens after? The new work looks at slow speed collisions between these moons, to see what shapes they take on. A second, smaller moon then forms in the spot where the first one formed, and it too moves out. First, a decently sized moonlet forms in the outer rings and, due to interactions with the rings, moves out from Saturn. In this scenario, the moons do grow by small particles colliding and sticking together, but the process changes with time, as bigger objects form. They don't, so something else must be going on.Ī team of planetary scientists came up with another idea that could explain these shapes: Slow speed collisions. If this were the case, though, the moons should form ellipsoidal shapes (like a rugby ball) due to the tides from Saturn's gravity. If the gravity of the moons could attract the icy ring particles to them, they could accumulate along the moons' equators, and in the moons' weak gravity form these bizarre structures. Pan orbits inside this gap, and Atlas just outside the sharp outer edge of the A ring. The broad outer ring of Saturn is called the A ring, and it has a gap in it called the Encke Gap, which is about 325 km wide. Both moons orbit Saturn in or very near the ring system. The first thought had to do with location. What could cause them to have these wide rims? That image shows the small (~35 kilometer wide) moons Atlas and Pan. Credit: NASA/JPL-Caltech/Space Science Institute Saturn’s moons Atlas (left) and Pan (right), both of which have large flattened rims around them, making them look like ravioli.
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