Weird moons orbiting the ringed planet might have been forged from head-on collisions
Images: JPL-Caltech/NASA, Space Science Institute; Simulations: A. Leleu, M. Jutzi and M. Rubin/Nature Astronomy 2018
A space ravioli. A planetary baguette. A cosmic Kaiser roll. Some of Saturn’s moons have shapes that are strangely reminiscent of culinary concoctions.
Images of the oddball moons, mostly from the now-defunct Cassini spacecraft (SN Online: 9/15/17), got planetary scientists wondering how these satellites ended up with such strange shapes. Now, researchers suggest that collisions between young moonlets could have done the job, according to a study published online May 21 in Nature Astronomy.
Adrien Leleu , a planetary scientist at the University of Bern in Switzerland, and colleagues developed computer simulations that let the scientists virtually smack together similar-sized moonlets at various speeds and angles. The team found that, at low angles and relative speeds of tens of meters per second (roughly equal to a car on country roads), impacts can create offbeat shapes that look like the misfits around Saturn.
Head-on collisions result in a flattened moon like Pan, which resembles an empanada (SN Online: 3/10/17). An impact angle of just a few degrees leads to an elongated satellite such as Prometheus, which looks like a French loaf.
A variety of moon shapes could result from two moonlets coming together at different angles and speeds, according to computer simulations. Dark areas are original moonlet surfaces, whereas in the light areas the surface has been ejected or contorted. Speeds are relative to the escape velocity, the speed at which ejected material escapes the gravity of the combined moonlets.
Not all run-ins create a weird looking moon. At higher angles, for example, moonlets might hit and run. Or they could form highly elongated rotating moons that subsequently break apart.
Leleu and collaborators focused on the smaller moons of Saturn that orbit within the planet’s rings. But the team also found that a similar collision between two larger moonlets could also account for the odd shape of Iapetus (SN Online: 4/21/14), a more distant walnut-shaped moon with a pronounced ridge along its equator that has puzzled scientists since the belt’s discovery. Other speculative origins for the ridge include volcanoes, plate tectonics or ring debris that rained down on the moon.
A. Leleu , M. Jutzi and M. Rubin. The peculiar shapes of Saturn’s small inner moons as evidence of mergers of similar-sized moonlets. Nature Astronomy. Published online May 21, 2018. doi:10.1038/s41550-018-0471-7.
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