Volcanic
activity on the far side of the moon may have lasted longer than previously
thought, recent images from a Japanese lunar satellite suggest.
The
finding, detailed in the Nov. 7 issue of the journal Science, could help
shed light on the moon's
formation and evolution.
Scientists
think that the moon formed when a rogue planet about the size of Mars crashed
into Earth and ripped out a chunk of the planet's molten mantle. Some of the
material from that chunk began to
orbit Earth, gradually cooling over millions of years to form the moon.
The lunar
surface is dead now, but over the millions of intervening years since it
formed, it experienced bouts of volcanic
activity.
Scientists
have studied lunar volcanic features, the most common of which are mare (dark
"seas") basalts, from orbit to determine when they formed. Radiogenic
dating is the best way to date mineral deposits, but samples from the moon's
surface are limited, and come only from a few locations on the moon's nearside.
Another way
to estimate the age of volcanic features is to count the number of impact
craters they have: the younger the feature, the fewer the craters that mark its
surface.
A group of
researchers did just that for two areas on the far side of the moon, the South
Pole-Aitken (SPA) basin and Mare Moscoviense, with images from the Terrain
Camera aboard the Japanese polar lunar orbiter SELENE
(KAGUYA). The orbiter launched on Sept. 14, 2007, and began its mission observing
the moon's surface on Dec. 21.
Most mare
volcanism ceased on the moon's far side about 3 billion years ago, but at a few
locations, Junichi Haruyama of the Japan
Aerospace Exploration Agency and his colleagues estimated that mare deposits
were only about 2.5 billion years old, suggesting that some episodic volcanism
continued after the main thrust had ended.
Volcanism
also continued on the nearside, apparently lasting longer than on the far side,
the researchers found. In Oceanus Procellarum, for example, basalts have been
estimated to be a young 1 billion years old.
The
difference in the termination of volcanic activity on the two sides of the moon
could be related to a thicker crust on the far side, or fewer heat-producing
radioactive elements on the far side compared to the near side, the authors
said.
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