Volcanoes Rocked Dark Side of the Moon
for National Geographic News
|November 6, 2008|
Volcanoes shook up the far side of the moon for far longer than scientists thought.
New images from the Japanese lunar satellite KAGUYA (formerly SELENE) reveal dark "seas" of volcanic rock that are as young as 2.5 million years old.
Until recently, the prevailing belief was that lunar volcanism started soon after the moon formed, about 4.5 billion years ago, and ended about 3 billion years ago.
Scientists can determine the age of a lunar landscape by counting the craters that have been blasted into its surface by meteors. The older a region, the more craters it has.
There are fewer craters on the far side's lunar maria, or basalt seas, than expected, meaning they're younger than presumed.
The finding will lead the scientific community to reconsider the the early geology of the moon, said lead study author Jun'ichi Haruyama of the Japan Aerospace Exploration Agency, whose study appears in this week's issue of the journal Science.
Scientists believe that early in the moon's formation—probably caused when a Mars-sized planet hit the Earth—light minerals floated to the top of a magma, or molten-rock, ocean, forming a harder crust.
Even after the crust had been fully formed, by about 3.2 billion years ago, the mantle melted occasionally and lava flowed on the lunar surface. Sometimes, meteor hits could trigger eruptions.
Most of the volcanism occurred on the near side in several phases, said Carle Pieters, a geologist at Brown University and a study co-author.
But there are relatively few basalts—glassy rocks formed by cooling magma—on the far side of the moon, so it was thought that volcanic activity had ended early in that hemisphere.
The volcanism that formed the few maria on the far side "lasted longer than previously considered and may have occurred episodically," the study says.
Pieters said, "The thermal history of the moon is certainly more complex than originally thought."
Craters as Clocks
Researchers have had easy visual access to the near side of the moon with Earth-based telescopes. The far side has been more difficult to observe.
KAGUYA, which was launched and began orbiting the moon in the fall of 2007, has sent back some of the first high-resolution images of the moon's dark side.
Using these images, the research team was able to manually count craters in several regions.
Michelle Kirchoff of the Lunar and Planetary Institute in Houston, Texas, who was not involved with the study, said crater counting is generally a reliable method for estimating the ages of lunar landscapes. "But without samples to constrain the calculations, they are just estimates."
Sampling lunar rocks "would be a great follow-up," Kirchoff said, "but this would require a whole other lunar mission and that's something that may not occur for a while."
More results will arrive soon to help solve to the lunar puzzle.
Brown University's Pieters pointed out that Japan, China, and India all have modern instruments orbiting the moon, and the United States will launch the Lunar Reconnaissance Orbiter next April.
(Read more about moon exploration.)
"After a great data famine," Pieters said, "this feast of quality new information about the moon will open a renaissance of scientific exploration and new understanding of Earth's nearest neighbor."
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