Published online 12 February 2009 | Nature | doi:10.1038/news.2009.100


The Moon reveals its weirder side

SELENE mission reports on gravity anomalies.

Gravity anomaly on the MoonGravity highs (red) and lows (blue) on the Moon (Lunar nearside right, farside left)Science

Results from the Japanese space agency's SELENE mission to the Moon are revealing details about why the lopsided lump of rock orbiting Earth is so unbalanced.

The SELENE (Selenological and Engineering Explorer, or Kaguya) mission was launched in September 2007 to gather detailed geological information about the Moon. The results are published in Science1,2,3,4.

Because the Moon has no atmosphere or weather to speak of, its geology has remained almost unchanged since it formed. So unpicking its structure could offer information about how the early Solar System — including Earth — developed. But the Moon has some unusual traits that have so far proved tricky to study: its gravity and the thickness of its crust vary from the near side that faces Earth to the far side.

Gravity from afar

The Moon spins on its axis once per Earth orbit, so the same side always faces Earth. To get information from the far side of the Moon relayed to Earth is difficult, because the Moon itself blocks the line of sight back to Earth. To get round this problem, SELENE has one satellite close to the Moon to collect data, and a companion sub-satellite in a much higher orbit to relay that information back to Earth.

A team led by Noriyuki Namiki from Kyushu University in Fukuoka, Japan, measured the Moon's variable gravity, in particular around impact craters on the far side. As the gravity changes, the main satellite is drawn closer or pushed farther away from the Moon. This in turn affects the frequency of the signal sent from the satellite to the sub-satellite.

Impact crater basins on the near side of the Moon, says Namiki, can display characteristic plateau-shaped areas of unusually high gravity ('positive anomalies') and denser rock, called mascons. "On the far side, we don't find mascon-like signatures. Instead, there are concentric rings of positive–negative–positive gravity anomalies over the basins," he says.

As the basins on the far side are not dense mascons, their regions of high gravity demand another explanation. Namiki thinks such areas have denser material just under their crust — suggesting that the mantle underneath has been lifted up and compressed. "We assume the crust-boundary is shallower [than was previously thought]," Namiki says.

This kind of mantle uplift would be seen now only if the far side of the early Moon contained a rigid outer layer (crust and mantle) to support the basin, the authors say. And this layer would only stay rigid if the far side was cooler than the near side, where basins are more deformed.

"That's very exciting," says Maria Zuber, a geophysicist from the Massachusetts Institute of Technology in Cambridge, adding that the question of why the far side is cooler needs more study.

Mapping the gravity field on the far side of the Moon for the first time is the "most transformative piece of information" that the SELENE mission has provided, Zuber adds. "What they have managed to find is that there are differences between the near and far side. It underscores and quantifies the asymmetry of the Moon."

Volcanic differences

In another SELENE study, Takayuki Ono and his colleagues from Tohoku University in Sendai probed the Moon's subsurface layers using radar. "As there is no water at the surface of the Moon, radio waves can penetrate into the upper part of the Moon crust, probably up to a few kilometres," says Yasushi Yamaguchi, a member of the team.

On the near side of the Moon, the team looked at a characteristic flat, dark basaltic plain, the Mare Serenitatis, or 'sea of serenity'. Several hundred metres below the surface the group spotted a thin layer of loose debris sitting between thick basaltic lava. The debris suggests that the Moon enjoyed a 'quiet period' in terms of volcanic activity, between 3.55 billion and 2.84 billion years ago, the researchers think.


In the 1970s the Apollo missions used a similar radar instrument and found thin layers down to a depth of kilometres in select regions. But SELENE's data did not reveal this deep layer, which Yamaguchi says may be down to the fuzzier resolution of Apollo's radar sounder.

SELENE has also taken pictures of areas with volcanic activity on the far side, and allowed these volcanoes to be dated by the number of impact craters seen there. The data suggest volcanic activity continued until 2.5 billion years ago.

Ben Bussey, a planetary scientist from the Applied Physics Laboratory at Johns Hopkins University in Laurel, Maryland, says that the topography mapping from SELENE, especially at the poles, will "be very useful in planning future lunar missions".

Zuber is principal investigator of NASA's Gravity Recovery and Interior Laboratory mission set to launch in 2011, which will map the Moon's gravity in greater detail. "We'll be able to go after fundamental questions, such as does the Moon have a solid core surrounded by liquid," she says. "This is a golden age of lunar exploration." 

  • References

    1. Araki, H. et al. Science 323, 897-900 (2009). | Article | ChemPort |
    2. Namiki, N. et al. Science 323, 900-905 (2009). | Article | ChemPort |
    3. Haruyama, J. et al. Science 323, 905-908, (2009). | Article | ChemPort |
    4. Ono, T. et al. Science 323, 909-912 (2009). | Article | ChemPort |
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