New Observations to Shed Light on Moon Mystery

Newobservations from the Japanese SELENE (Kaguya) lunar mission are shedding lighton the perplexing geological differences between the near and far sides of the moon.

For nowthere are no breakthroughs, but the new data is expected to provide researcherswith lots of clues.

Not untilthe dawn of the Space Age were humans able to get a glimpse at the moon's farside, which is perpetually kept from Earth's view by the satellite's synchronousrotation ? it rotates about its axis in about the same time it takes it toorbit the Earth.

Oncespacecraft were launched that could take in the long-hidden view, scientistsfound that the moon was two-faced: the nearside was covered with smooth, dark volcanic maria(solidified pools of ancient lava), while the far side almost completely lacksthese features and is instead covered by more heavily cratered bright material.

Thesedifferences in topography and composition imply that the two sides evolveddifferently during the more than 4 billion years of the moon's existence.

SELENE'sinstruments have provided the most detailed topographic map of the moon todate, as well as measurements of the variations of gravity across its surfaceand a look at what lies beneath. The observationsare detailed in a series of studies presented in the Feb. 13 issue of thejournal Science.


Theprevailing theory of howthe moon formed and evolved is that a Mars-sized body collided with theEarth shortly after the solar system began to form about 4.5 billion years agoand rent out a chunk from the Earth's mantle that eventually fell into orbitaround Earth and cooled over millions of years, coalescing into the moon.

The moonwas initially covered by a deep magma ocean that gradually cooled and hardenedinto crust. Space rocks continuously bombarded the lunar surface until about3.8 billion years ago.

The decayof some elements in the lunar mantle produced heat that melted the surroundingrock and fueled volcanism that formed the maria (which means "seas") thatappear as dark splotches on the lunar surface.

Some typeof imbalance arose that caused the near and far sides to develop differently,with far more maria on the near side. Whether that imbalance was the result ofa giant impact or some internal change wasn't known.

The SELENEresults have helped resolve that issue.


SELENE wasable to measure the gravity anomalies across the far side of the moon, andresearchers could compare then to those of the near side.

Thegravitational patterns of the far side confirmed that it had a harder, coolerlithosphere (the outermost shell of any rocky moon or planet) than did the nearside during the period of major impacts, said Gregory Neumann of NASA's GoddardSpace Flight Center in Greenbelt, Md. and who is not part of the SELENE team.

Having ahard, cool lithosphere that far back in the history of the moon points to aninternal cause as the most likely differentiating factor that caused one sideto be warmer and more pliable than the other.

Thespacecraft also used its laser altimeter to make the most highly-resolvedtopographic map of the moon to date.

"It'sstunning," Neumann told

The mapalso suggested that the crust on the far side was rigid and may therefore lackwater and other evaporating compounds, which are part of what makes Earth'scrust so bendable.


SELENE'sobservations also shed some light on volcanism in the moon's early history. Aradar instrument that can penetrate the lunar surface indicated that there weredebris layers between some of the volcanic basalt flows, which suggests thatthe volcanism stopped and started a bit in the moon's early history.

A fourthstudy, also detailedearlier in an online version of Science, suggested that volcanism onthe far side of the moon lasted longer than previously thought, though not aslong as it did on the near side.

Modelerswill be able to use all of this data gleaned from the SELENE studies to build abetter picture of how the moon formed and evolved.

"Themodelers are going to have a field day with this," Neumann said.

Though thisSELENE data adds to the picture of the geology of the moon, "we stillhaven't got any huge headlines out of the moon," such as icehidden in lunar craters, Neumann said.

But othermissions, such as China's Chang'e-1 and India's Chandrayaan-1, and futuremissions, such as the upcoming NASA LunarReconnaissance Orbiter mission (of which Neumann is a team member), willall shed more light on enigmatic history of our closest planetary neighbor,Neumann said.

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Andrea Thompson

Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.