By Eugenie Samuel Reich of Nature magazine

A sneak peek at the first results from a NASA mission to measure the Moon’s gravitational field hints at a lunar crust that is only half as thick as once thought.

There were a few gasps among scientists in the audience at a 13 September seminar at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, as they took in the data revealed by Maria Zuber, principal investigator for NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission. Zuber, a planetary scientist at the Massachusetts Institute of Technology in Cambridge, showed a crisp, high-resolution gravitational map made with data collected by GRAIL’s twin spacecraft between March and June of this year.

“We are three to four times better in resolution compared to Kaguya and Lunar Prospector,” said Zuber, referring to two previous missions that mapped the Moon's gravitational field. GRAIL’s results have not yet been published or released publicly by NASA, and Zuber was not at liberty to give an interview.

Yet her talk, and the thrilled reactions from those present at the seminar and others interviewed by Nature, suggest that GRAIL is poised to have a profound effect on scientists’ understanding of the origins and early evolution of the Moon when its results are released in the coming weeks.

GRAIL’s two probes, named Ebb and Flow by schoolchildren in a NASA competition, were launched in September 2011 (see ‘Twins to Probe Moon’s Heart’). The first probe began orbiting the Moon on 31 December 2011, with the second joining the next day. By March, they had begun detailed mapping. The two spacecraft exchange radio signals, recording fluctuations in their relative positions that are then used to reveal tiny accelerations and decelerations caused by variations in the Moon’s gravitational field. The average altitude of the primary mission was 55 kilometers — much lower than the orbit used by the Gravity Recovery and Climate Experiment (GRACE), a similar gravity-mapping mission for Earth that has to fly higher to avoid atmospheric friction. Occasionally, the GRAIL operations team brought the craft lower than 20 kilometers to further improve the resolution of the data. “Nothing beats flying low,” says Zuber.

Zuber gave the packed auditorium a heads-up on three science results. The first is that the Moon’s crust seems to be thinner than thought. When lunar geologists first estimated the thickness of the Moon’s crust, using data from seismometers placed by the Apollo astronauts, they concluded that it was around 60 kilometers thick. Subsequent re-analyses of those data brought the estimate down to around 45 kilometers. Now, GRAIL’s results suggest that the crust’s average thickness is only 30 kilometers, says Zuber.

That piques the interest of Linda Elkins-Tanton, a planetary scientist at the Carnegie Institution for Science in Washington DC. The Moon’s crust is thought to have formed when the mineral anorthosite crystallized and floated out of the ocean of magma on the body’s surface soon after it was formed out of a collision between the Earth and a giant asteroid. “The crustal thickness is an amazingly evocative clue about the origin of the Moon,” says Elkins-Tanton.

She adds that researchers are more likely to be interested in the variations in crustal thickness seen by GRAIL, because lunar scientists are already challenged in explaining why its crust is much thicker on its far side than on its near side. Given an average thickness of 30 kilometers, the crust in some regions on the near side may be unexpectedly thin, which will be particularly difficult to model.

6 Comments

1. 1. Postman1 07:58 PM 9/17/12

   What is directly under the crust? Is it molten, or just a different layer? I look forward to reading a lot more from and about the surveys.

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2. 2. jtdwyer in reply to Postman1 06:50 AM 9/18/12

   That's a good enough question that it might should have been anticipated and answered in the article. According to the Wikipedia entry for the moon, "Internal structure" section, the deep solid mantle is "geochemically distinct" from the thin crust. Please see:
   http://en.wikipedia.org/wiki/Moon
   
   I'm not sure how well planetary scientists understand terrestial planets, but several of the questions raised seem to have rather straightforward solutions.
   
   The question of why the "result suggests that the gravitational field of the Moon is dominated by its deep craters rather than the structure of its interior" may simply be the product of the Moon's deep solid mantle of rather homogeneous density and its relatively small iron core. In this configuration of mass, significant topological features should be expected to provide the greatest source of density variation.
   
   The question of why the crust is thinner on the Earth side may be revealed in the following overview from the Wikipedia entry:
   "The Moon is the second densest satellite after Io, a satellite of Jupiter. It is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters."
   
   From this it could be suggested that the near side of the much nearer early Moon, with a molten mantle and crust, would have been subjected to deformative tidal influences from the Earth's gravitation. I'd expect then that the near side of the Moon would not only have a thinner crust but also deviate from a spherically symmetrical configuration by protruding somewhat towards the Earth.
   
   I'm just guessing, of course, not being a planetary scientist...

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3. 3. vinodkumarsehgal 10:09 AM 9/18/12

   Moon has a radius of about 1500 KMs. and now scientists are estimating crust to be about 30 KMs thick. NASA placed first Human person on the surface way back in 1969. Is it not an odd and surprising that in 43 years, scientists have no fair knowledge as to what lies in a large depth beneath crust of 30 Kms?

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4. 4. BillR 08:38 AM 9/19/12

   I wonder why they were surprised about the concentrations of mass in the craters? Unlike Earth, there is nothing happening on the moon that would mix the pot so to speak. Where ever a mass hit n the moon, there it will stay.
   
   On Earth, there is plate tectonics, weathering, volcanoes, etc that will eventually subsume and mix in the materials, spreading the mass out. Even so, the mass distribution on Earth itself is not uniform either because there have been fairly recent impacts that have not had sufficient time to "smear out" yet.
   
   What I would expect to see is this tool being used to identify the best places to mine the impactors.

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5. 5. G. Karst 10:35 AM 9/19/12

   60km Vs 30km is a significant error. I see no real explanation for the size of this error. GK

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6. 6. R.Blakely 05:12 AM 9/23/12

   The Moon has been ignored because Mars has become the target, but the Moon is much easier to inhabit. We will make tunnels in the Moon and inhabit those tunnels, in the future. Crust thickness is useful to predict good mining locations, since mineral treasures will pay for tunnels in the Moon, tunnels that will become habitats for humans.

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