The Japanese probe Kaguya has created the first map of gravity differences on the far side of the Moon, which always points away from Earth. The gravity signatures of some craters suggests the far side might have been stiffer and cooler than expected (Illustration: Namiki et al/AAAS)
The first detailed map of the gravity fields on the Moon's far side shows that craters there are different than those on the near side. The results could reveal more about the Moon as it was billions of years ago, when magma flowed across its surface.
The motions of the three spacecraft, which are sensitive to variations in the Moon's gravity field, were measured by tracking their radio signals.
Crucially, while the main Kaguya spacecraft was on the far side of the Moon and therefore out of direct contact with Earth, one of the small probes relayed its signals to Earth.
The resulting map - the first detailed one completed of the Moon's far side - shows that craters on the far side have a markedly different gravity signature from those on the side that always faces Earth.
That suggests that billions of years ago, there might have been large differences in the temperature or thickness of the Moon's two halves.
"It's fabulous new data," says Walter Kiefer, a planetary geophysicist with the Lunar and Planetary Institute in Houston, Texas, who was not part of the study. "We haven't been able to get a good look at the far side until now."
Most of the large craters on the Moon formed more than 3.8 billion years ago. These were partly filled in by magma that flowed on the surface before the Moon cooled and its geological activity died down.
But a number of craters also seem to have been filled in from below. Researchers believe material from the mantle also rose up in craters, since these are sites where impacts had thinned the Moon's crust.
The new Kaguya measurements reveal some craters on the far side that seem to have been filled only with mantle. These craters have higher-than-normal gravity at the centre, surrounded by a thick ring of low gravity that closely matches the original low elevation of the crater.
It is not yet clear what these new crater measurements suggest about the early Moon. In order for these structures to survive, the lunar far side must have been too cool and stiff to allow the mantle at the craters' centres to smooth out much over time, says team leader Noriyuki Namiki, of Japan's Kyushu University. "The surface had to be very rigid to support these structures," Namiki says.
But Keifer says the low gravity rings could argue for the opposite scenario. The structures in the centres of the craters might be narrow because the top layer of the Moon's far side was too thin and warm to be able to hold up anything larger. Comparing the Kaguya observations with models could help settle the question, Kiefer says.
The Moon's two halves also show other striking differences. NASA's Lunar Prospector, which operated in the late 1990s, found that radioactive elements seem to be concentrated on the near side. The far side also shows less evidence of past volcanic activity.