New observations from the Japanese SELENE (Kaguya) lunar mission are shedding light on the perplexing geological differences between the near and far sides of the moon.
For now there are no breakthroughs, but the new data is expected to provide researchers with lots of clues.
Not until the dawn of the Space Age were humans able to get a glimpse at the moon's far side, which is perpetually kept from Earth's view by the satellite's synchronous rotation ? it rotates about its axis in about the same time it takes it to orbit the Earth.
Once spacecraft were launched that could take in the long-hidden view, scientists found 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 lacks these features and is instead covered by more heavily cratered bright material.
These differences in topography and composition imply that the two sides evolved differently during the more than 4 billion years of the moon's existence.
SELENE's instruments have provided the most detailed topographic map of the moon to date, as well as measurements of the variations of gravity across its surface and a look at what lies beneath. The observations are detailed in a series of studies presented in the Feb. 13 issue of the journal Science.
The prevailing theory of how the moon formed and evolved is that a Mars-sized body collided with the Earth shortly after the solar system began to form about 4.5 billion years ago and rent out a chunk from the Earth's mantle that eventually fell into orbit around Earth and cooled over millions of years, coalescing into the moon.
The moon was initially covered by a deep magma ocean that gradually cooled and hardened into crust. Space rocks continuously bombarded the lunar surface until about 3.8 billion years ago.
The decay of some elements in the lunar mantle produced heat that melted the surrounding rock and fueled volcanism that formed the maria (which means "seas") that appear as dark splotches on the lunar surface.
Some type of 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 of a giant impact or some internal change wasn't known.
The SELENE results have helped resolve that issue.
SELENE was able to measure the gravity anomalies across the far side of the moon, and researchers could compare then to those of the near side.
The gravitational patterns of the far side confirmed that it had a harder, cooler lithosphere (the outermost shell of any rocky moon or planet) than did the near side during the period of major impacts, said Gregory Neumann of NASA's Goddard Space Flight Center in Greenbelt, Md. and who is not part of the SELENE team.
Having a hard, cool lithosphere that far back in the history of the moon points to an internal cause as the most likely differentiating factor that caused one side to be warmer and more pliable than the other.
The spacecraft also used its laser altimeter to make the most highly-resolved topographic map of the moon to date.
"It's stunning," Neumann told Space.com.
The map also suggested that the crust on the far side was rigid and may therefore lack water and other evaporating compounds, which are part of what makes Earth's crust so bendable.
SELENE's observations also shed some light on volcanism in the moon's early history. A radar instrument that can penetrate the lunar surface indicated that there were debris layers between some of the volcanic basalt flows, which suggests that the volcanism stopped and started a bit in the moon's early history.
A fourth study, also detailed earlier in an online version of Science, suggested that volcanism on the far side of the moon lasted longer than previously thought, though not as long as it did on the near side.
Modelers will be able to use all of this data gleaned from the SELENE studies to build a better picture of how the moon formed and evolved.
"The modelers are going to have a field day with this," Neumann said.
Though this SELENE data adds to the picture of the geology of the moon, "we still haven't got any huge headlines out of the moon," such as ice hidden in lunar craters, Neumann said.
But other missions, such as China's Chang'e-1 and India's Chandrayaan-1, and future missions, such as the upcoming NASA Lunar Reconnaissance Orbiter mission (of which Neumann is a team member), will all shed more light on enigmatic history of our closest planetary neighbor, Neumann said.
- Video: How the Moon Was Made
- Video: Kaguya Captures Full Earth-rise
- Top 10 Cool Moon Facts