The Gravity Recovery and Interior Laboratory
Image Credit: NASA / JPL / MIT

GRAIL is one of those nifty experiments. Relatively small, GRAIL consists of two spacecraft weighing less than 500kg each. Orbiting the Moon, the two craft will piece together an extremely detailed map of the Moon’s gravitational field. This will allow scientists to construct a model of the interior structure of the Moon from crust to core.

The two GRAIL spacecraft will be launched aboard a Delta II rocket during a 26 day window, which opens on 8 September 2011. Lasting only 90 days, the mission is designed to avoid the lunar eclipses of 10 December 2011 and 4 June 2012.

The GRAIL mission’s principal investigator is Maria Zuber, who is chair of MIT’s Department of Earth, Atmospheric and Planetary Science. In 2008, she was selected by NASA as the first woman ever to head a major planetary robotic research mission.

The GRAIL spacecraft design and their components are derived from Lockheed Martin’s XSS-11 (pdf) spacecraft heritage. GRAIL’s science team includes NASA Goddard Space Flight Center, JPL, the Carnegie Institution of Washington, the University of Arizona, the University of Paris and the Southwest Research Institute.

Four of the Science Team members (Frank Lemoine, Greg Neumann, Dave Smith, Maria Zuber) have ties to current lunar missions (LRO and Kaguya/SELENE)

The GRAIL spacecraft is designed to conduct the following measurements:

  • Map the structure of the crust & lithosphere
  • Understand the Moon’s asymmetric thermal evolution
  • Determine the subsurface structure of impact basins and the origin of mascons
  • Ascertain the temporal evolution of crustal brecciation and magmatism
  • Constrain deep interior structure from tides
  • Place limits on the size of the possible inner core

The GRAIL website describes the mission objectives this way:

The Grail Spacecraft
Image Credit: NASA / JPL / MIT

The Moon is the most accessible and best studied of rocky, or “terrestrial”, bodies beyond Earth. Unlike Earth, however, the Moon’s surface geology preserves the record of nearly the entirety of 4.5 billion years of solar system history. Orbital observations combined with samples of surface rocks returned to Earth, show that no other body preserves the record of geological history so clearly as the Moon.

The structure and composition of the lunar interior (and by inference the nature and timing of internal melting and heat loss) hold the key to reconstructing this history. Longstanding questions such the origin of the maria, the reason for the nearside-farside asymmetry in crustal thickness, and the explanation for the puzzling magnetization of crustal rocks, all require a greatly improved understanding of the Moon’s interior. Deciphering the structure of the interior will bring understanding of the evolution of the Moon itself, and also extend knowledge of the origin and thermal evolution of the Moon to other bodies in the inner solar system. For example, while the Moon was once thought to be unique in developing a “magma ocean” shortly after accretion, and now such a phenomenon has now been credibly proposed for Mars as well.

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