About-face: the farside exposed: mission provides gravity map of moon's hidden half.

Nearly 400 years after Galileo viewed the moon's nearside through a telescope, scientists still know relatively little about the moon's hidden half- the hemisphere that always faces away from Earth. Now, researchers have for the first time mapped the gravitational field of the moon's farside.

The moon's two hemispheres show striking differences. The visible nearside is covered with smooth, dark seas of volcanic material, while the farside is more heavily cratered and consists of brighter, highland material. But because a lunar orbiter traversing the moon's farside can't be tracked directly from Earth, researchers had lacked a detailed map of that side's distribution of matter.

The Japanese SELENE (also known as Kaguya) mission, launched in 2007, has now remedied that problem, a team reports in the Feb. 13 Science. SELENE's main satellite orbits the moon and broadcasts radio waves to a small companion satellite in a higher-altitude orbit. The companion satellite relays the signals to Earth. The main satellite slows while passing over less dense regions of the moon and speeds up while passing over more dense regions, and these changes in motion result in changes in the frequency of the radio waves it broadcasts.

Preliminary interpretation of the SELENE observations "quantifies the asymmetry between the farside and the nearside, a phenomenon that is not yet well understood," comments planetary scientist Maria Zuber of MIT. Understanding that asymmetry may shed light on the moon's early evolution, she adds.

The new gravity maps indicate the presence of dense material beneath the farside's surface, says Noriyuki Namiki of Kyushu University in Fukuoka, Japan, a coauthor of the study. The dense material probably rose up from the moon's mantle, Namiki says, since there is no evidence of lava flow in the basins.

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If the moon formed from the debris generated when a Mars-sized body struck the young Earth, as is widely assumed, the nearside and farside were probably similar at birth. Pulses of heat, separated both in space and time, could have caused the two hemispheres to evolve differently, suggests Zuber. Sorting out the causes of that heating "is still ahead of us," she says.