Ocean of molten rock covered Moon's entire surface
Washington, Mar 12 : Molten rock may have been present on the Moon more recently and for longer periods than previously thought, a new study has found.
As the lunar magma ocean cooled over millions of years, it differentiated to form the Moon's crust and mantle.
But according to the new analysis by planetary scientists from Brown University, this wasn't the last time the Moon's surface was melted on a massive scale.
The research, led by graduate student William Vaughan, shows that the impact event that formed the Orientale basin on the Moon's western edge and far side produced a sea of melted rock 220 miles across and at least six miles deep.
Similar seas of impact melt were probably present at various times in at least 30 other large impact basins on the Moon.
Vaughan and his colleagues show that as these melt seas cooled, they differentiated in a way that was similar to the lunar magma ocean.
As a result, rocks formed in melt seas could be mistaken for "pristine" rocks formed very early in the Moon's history, the researchers say.
"This work adds the concept of impact melt magma seas to the lexicon of lunar rock-forming processes," planetary geologist James W. Head III, the Scherck Distinguished Professor of Geological Sciences and the senior researcher involved in the study said.
"It emphasizes that one must consider the detailed point of origin of the rocks in order to interpret them correctly," her said.
That includes rocks brought back during the Apollo program and Russia's Luna missions.
The researchers said that it's quite possible that impact melt material is present in lunar samples thought to be representative of the early formation of the lunar crust.
The amount of rock formed in melt seas is far from trivial.
Vaughan and his colleagues estimated that impacts forming the Moon's 30 large basins produced 100 million cubic kilometers of melt, enough to make up 5 percent of the Moon's crust.
If lunar samples do include melt material, it would help to explain some puzzling findings from lunar samples.
For example, in 2011 an analysis of a sample assumed to have originated in the early lunar crust suggested that the sample was 200 million years younger than the estimated time when the lunar magma ocean solidified.
That led some researchers to conclude either that the Moon is younger than previously estimated or that the lunar magma ocean theory was flawed.
But if that sample actually originated from a melt sea, its young age could be explained without rewriting the history of the Moon.
The research is published in the journal Icarus.