The second finding is that there is a high correlation between variations in the Moon’s gravitational field and its topography. “This, to me, is one of the most surprising things we found,” says Zuber. The result suggests that the gravitational field of the Moon is dominated by its deep craters rather than the structure of its interior. This relationship between topography and gravity was closer for the Moon than for other terrestrial bodies such as Mars, Venus and Earth, Zuber says. “It actually makes you wonder how well we understand the other terrestrial planets.”

Bradley Jolliff, a planetary scientist at Washington University in St Louis, Missouri, says that this high correlation ”will spark a good bit of debate”. He adds that he also is confident that the full GRAIL data set will include some regions where the correlation is weaker — revealing important density variations in the Moon's subsurface.

Third, Zuber reported that GRAIL has not been able to confirm some of the larger and older impact basins hypothesized to exist on the Moon’s surface. Lunar scientists examining the Moon’s surface have tried to unravel the history of asteroid impacts — which would illuminate the evolution of not only the Moon but also other bodies of the inner Solar System — by defining areas where large impact basins might underlie more visible and obvious craters. But GRAIL hasn’t found many of the hypothetical basins, Zuber says.

Zuber did not discuss one key goal of GRAIL: understanding the Moon’s deep interior structure and core. Clive Neal, a lunar geologist at the University of Notre Dame in Indiana, says that it will take more time to make these more complicated interpretations from the data. Zuber says the data from GRAIL’s primary mission have already been processed and analyzed, which puts the mission a year ahead of schedule.

This article is reproduced with permission from the magazine Nature. The article was first published on September 14, 2012.

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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