London, Oct 6 : A big impact possibly caused Pluto to tilt over by 10 degrees, University of California Santa Cruz astronomers have said.

They have said Pluto and its large moon Charon were bowled over when they were struck by awkward space rocks in the past, possibly billions of years ago.

But if one happened to strike within the past few million years, Pluto might still be wobbling on its axis today, and NASA’s NASA spacecraft may find evidence of these rolls when it arrives at the distant worlds in 2015, they said.

Francis Nimmo of the University of California in Santa Cruz, US, who led the Moon and Enceladus studies, has said that impacts might have also caused Pluto and Charon to flip over on their sides.

He said the two distant bodies might have been especially prone to this reorientation because their spin is relatively slow, once in every 6.4 days.

“That is slow compared to many other satellites. Slow spinners are more unstable on their axes than fast spinners,” said Nimmo, adding that Saturn's moon Enceladus spins once every 1.4 days.

Jay Melosh of the University of Arizona in Tucson, US, first suggested 30 years back that the basins gouged out by impacts would redistribute the mass of planetary bodies, causing them to roll over to re-stabilise themselves.

Melosh had said this mechanism might have also 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, he said.

Nimmo said assuming that Pluto and Charon still had basins as big as those on Saturn’s moon Tethys and Rhea and Uranus's moon Titania, Pluto probably tipped over by 10 degrees and Charon by 20 degrees.

Nimmo said while observations till date have ruled out the presence of impact craters on either body, 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 and Uranus’ moons, Nimmo believes the rocky bodies will have preserved evidence of their tumultuous pasts.

“One prediction is that there will be a network of tectonic fractures caused by the satellites rolling over,” said Nimmo, adding that their 'equatorial bulges' – a widening at their equators due to their rotations – would create stresses when their equators shift position.

“A second is that the leading face – the region facing forwards [in their orbits] – of Pluto and Charon will not be as heavily cratered as one would expect because before they tilted over, the leading face was somewhere else.

“A third is that the gravity field of Pluto and Charon will be bumpy,” he said.

“New Horizons should certainly be able to detect a global network of tectonic features and look at the distribution of impact craters when it arrives at Pluto in 2015. Measuring the gravity field is more difficult but might be possible,” New Scientist magazine quoted him as saying. (ANI)