Maria Zuber often starts her presentations with a Maya Angelou quote: “There is no greater agony than bearing an untold story inside you.” If so, then the man in the Moon must be writhing in pain.
At an LPSC session on Tuesday, Zuber, of MIT, flashed a cross section showing what is known about the structure of the Moon’s interior. The answer is: not much. “There are a lot of question marks,” she says. “We don’t actually know that the Moon has a core.” She’s fairly sure the GRAIL gravity mapping mission will change all that.
Zuber described the mission in the first public presentation since NASA awarded her the Discovery class mission in December. Later we went to a nearby Chinese restaurant (not so bad for League City, really) to further discuss how GRAIL would, umm, unearth the secrets below the Moon’s surface.
GRAIL would send two orbiters to the Moon. The two satellites, in constant communication via radio beacons with each other and the Earth, would circle the Moon in a polar orbit, 50 kilometres above the surface, separated by about 200 kilometres. The Moon would precess slowly below, so over the course of a month, GRAIL could map the whole planet.
Whenever the first satellite approaches a surface feature, like a crater rim, or a dense body hidden underneath, the tiny extra bit of gravity associated with the feature would tug the first probe ahead ever so slightly. The radio beacon would capture this minute change in distance between the probes, until the trailing probe, arriving at the surface feature, is tugged back into place. “These two spacecraft essentially chase each other around the Moon,” Zuber says.
The measurements are so precise that the science team will have to filter out the gravitational effects of every other planet in the solar system. They even have to subtract the slight motions of the tectonic plates that underlie the tracking stations in California, Spain and Australia.
Zuber says the instruments are two or three orders of magnitude more sensitive than the gravity instrument on the Japanese Kaguya (SELENE) orbiter that’s currently in operation, and GRAIL will be able to map squares down to resolutions of 20 kilometres, four times better than Kaguya.
Once she has the complete surface of equal gravitational potential – called the selenoid – she can compare that to the precision topographic maps that are going to be made by orbiters such as the Lunar Reconaissance Orbiter, scheduled to launch later this year. Zuber will subtract the gravitational effects of the topography. And that would leave her with the signal associated with an inner core, outer core and mantle. She’ll be able to tell which parts are still molten, if at all.
She credits a perfect storm of two overlapping events for allowing her to win the mission. GRACE, a mission to map the Earth’s gravity field, was proven to work well using a similar approach. And then Lockheed Martin came along, willing to declassify a satellite technology that they had been testing for the US Air Force.
Zuber has such high hopes for GRAIL, she’s pretty sure that, afterwards, there won’t be many moony questions to work on, and she’ll have to move on to another planet. “The kind of experiments I love are the kind that put me out of business.” GRAIL is planned for launch in September of 2011. Here’s Maria polishing off her hot and sour soup:
