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.

This need to understand the internal structure in order to reconstruct planetary evolution motivates the GRAIL primary science objectives, which are to determine the structure of the lunar interior from crust to core and to further the understanding of the thermal evolution of the Moon. The GRAIL mission will accomplish these goals by performing global, regional and local high-resolution (30x30 km), high-accuracy (<10 mGal*) gravity field measurements with twin, low-altitude (50 km) polar-orbiting spacecraft using a Ka-band ranging instrument.

The GRAIL Science Team will conduct six lunar science investigations to:

• 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.

In the figure to the right, the top panel shows the current gravity field uncertainty in mGal from a gravity model by GRAIL co-investigator Alex Konopliv. The bottom part of the figure shows the gravity field uncertainty in mGals anticipated from GRAIL for the mission’s science floor. GRAIL uncertainties (bottom) are a globally-uniform 0.1 mGal with an improvement factor of >100 for the nearside and ~1000 for the farside compared to the state of knowledge today.