Nearly 400 years after Galileo first 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.
Over the past few decades, lunar spacecraft have revealed that 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 hidden half can’t be tracked directly from Earth, researchers had lacked a detailed map of the farside’s distribution of matter.
The Japanese SELENE (“KAGUYA”) mission, launched in 2007, has now remedied that problem, scientists report in the Feb. 13 Science. As SELENE’s main satellite orbits the moon, the craft slows down while passing over lower density regions and speeds up while passing over higher density regions, all the while broadcasting radio waves. Shifts in the frequency of these radio waves, due to the motion of the craft, are recorded by a small companion satellite in a higher-altitude orbit. The companion satellite is positioned to relay the signals to Earth, where they are transformed into a gravity map.
Preliminary interpretation of the SELENE observations, which also include precision topographic measurements with a laser altimeter, “quantifies the asymmetry between the farside and the nearside, a phenomenon that is not yet well understood,” comments Maria Zuber of MIT. Understanding that asymmetry may shed light on the moon’s early evolution, she adds.
The gravity map includes sources or deficits of mass at the moon’s surface, such as mountains or basins, along with concentrations of mass underground. Noriyuki Namiki of Kyushu University in Fukuoka, Japan, and his colleagues removed the effect of surface features and still found relative highs in the moon’s gravitational field at the center of farside basins. That indicates the presence of dense material beneath the surface, he notes.
Since there is no evidence of lava flow in the farside basins, the dense material probably rose up from the moon’s mantle, says Namiki. That further suggests that the lunar crust and mantle are stiffer on the farside than the nearside, he says.
If the moon formed from the debris generated when a Mars-size body struck the young Earth, as is widely assumed, the nearside and farside were probably very 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 how much of that heating was related to impactors that preferentially struck one side of the moon, versus internal events, such as volcanic plumes or intrusions, “is still ahead of us,” she says.
Help is on the way. NASA’s Lunar Reconnaissance Orbiter mission, scheduled for launch in April, has an altimeter that will collect data about 140 times faster and with about ten times the accuracy as the instrument on SELENE, Zuber says. In 2011, the Gravity Recovery and Interior Laboratory mission will map the moon’s gravity at a resolution two to three times finer than SELENE.
