Apr 17, 2007
The South Pole-Aitken basin (circled) is the largest impact crater in the solar system. One edge lies on the Moon's south pole, while its centre lies about 30° away (Image: Clementine Project/LPI)
An enormous impact basin located near the lunar south pole may have caused the Moon to roll over early in its history, new research suggests.
The biggest, deepest impact crater in the solar system lies near the Moon's south pole. Called the South Pole-Aitken (SPA) basin, it is 2500 kilometres wide and 12 kilometres deep and is thought to have been created about 4 billion years ago.
Francis Nimmo of the University of California in Santa Cruz, US, believes the impact probably occurred near the Moon's equator. That is because the equator lies in the plane of most other objects in the solar system and therefore would more likely be in a hurtling space rock's 'line of fire'.
But he thinks the giant hole destabilised the Moon, so that within 100,000 to 1 million years of forming, the basin – a region of low mass – had rolled over to the south pole. The phenomenon is similar to how spinning bowling balls tend to stabilise with their finger holes – the area of least mass – aligned vertically along the spin axis.
"In my view this is almost certainly what happened," Nimmo told New Scientist.
He was led to this conclusion after studying Saturn's moon Enceladus, which may have reoriented in a similar way. In that case, a low-mass region of geysers may have rolled over to its south pole (see Did Saturn's volcanic moon roll with it? (http://space.newscientist.com/article/dn9248-did-saturns-volcanic-moon-roll-with-it.html)).
The SPA is not centred exactly on the Moon's south pole, however, but about 30° away. Nimmo thinks that is because the Moon has shifted its orientation slightly since the basin first rolled to the pole.
"Presumably the reason that it didn't go all the way to the pole is because other large basins will have also reoriented the Moon," he says. "Quite likely, the Moon was reoriented several times during the early period of large impacts."
Future missions, such as NASA's Lunar Reconnaissance Orbiter, may provide further evidence for the Moon having been rocked and rolled. Says Nimmo: "Reorientation causes big tectonic stresses, so maybe we will detect some new tectonic features."