Pluto and its large moon Charon may have been bowled over when they were struck by wayward space rocks in the past, a new study suggests. If so, NASA's New Horizons spacecraft may find evidence of these rolls when it arrives at the distant worlds in 2015.
Jay Melosh of the University of Arizona in Tucson, US, first suggested about 30 years ago that the basins gouged out by impacts would redistribute the mass of planetary bodies, causing them to roll over to re-stabilise themselves.
This mechanism may have caused Earth's Moon– which boasts the biggest impact crater in the solar system – to roll over so that the crater moved from the equator to the south pole. Similarly, a low-mass region of geysers on Saturn's moon Enceladus may have caused that icy world to rotate, as well.
The two distant bodies may have been especially prone to this reorientation because they spin relatively slowly, rotating once every 6.4 days. That is slow compared to many other satellites, Saturn's moon Enceladus spins once every 1.4 days. Slow spinners are more unstable on their axes than fast spinners.Their surfaces have not yet been imaged in very good detail, so no impact craters are currently known on either body. But in the early solar system, before the planets settled into their present orbits, planetary building blocks are thought to have knocked into each other frequently – leaving behind large impact basins.
Assuming Pluto and Charon have basins as big as those on Saturn's moons Tethys and Rhea and Uranus's moon Titania, the researchers calculate that Pluto probably tipped over by 10° and Charon by 20°. Charon should have rotated farther because a given impact would have had a larger effect on the 1200-kilometre-wide body than on Pluto, which is about twice as wide.