Counting craters: A close-up of the terraced structure within a crater taken by the Japanese SELENE probe.
Credit: SELENE/JAXA
SYDNEY: New images of the far side of the Moon show that volcanoes continued to erupt there for much longer than previously thought.
The Moon is covered by large 'seas' of basalt, called mares. Most mares stopped forming three billion years ago, one billion years after the Moon formed from a collision between the Earth and another nascent planetoid.
Episodic volcanism
However, several mare deposits on the lunar farside (the side that always faces away from Earth) show a much younger age of around 2.5 billion years old, according to research published today in the U.S. journal Science.
These young ages indicate that mare volcanism on the Moon lasted longer than experts realised and may have occurred episodically, the authors write.
"Our results will lead to a reconsideration of the internal evolution of the Moon, because the lunar farside volcanism was considered to have ceased in early times," said lead author Junichi Haruyama, a planetary scientist from the Japanese Aerospace Exploration Agency (JAXA) in Kanagawa, Honshu.
He added that the new find contributes important constraints for modeling of the internal evolution of the Moon.
Surprising result
Because lunar samples are rare, and have only been obtained from the nearside of the Moon, the next best way to date geological deposits is by counting the number of craters in photographs of the surface. The older the deposit, the more impact craters there are.
For the new study, Haruyama's team counted craters on mares on the lunar farside using images taken from JAXA's SELENE (SELenological and ENgineering Explorer) lunar orbiter, launched in September 2007.
The satellite's Terrain Camera can resolve images to a scale of 10 m. They then used statistical methods to calculate the new dates.
