The GRAIL mission conducts six science investigations:
1. Map the structure of the lithosphere.
The lithosphere is the portion of the crust and upper mantle with significant strength over a geological time scale. Since the strength of rock is highly dependent on temperature, the thickness of the lithosphere is directly related to thermal evolution. GRAIL will provide evidence of how rigid the lithosphere was -- and therefore what the thermal conditions were -- at various locations when features were formed.
2. Understand the Moon's asymmetric thermal evolution.
The thickness of the crust suggests the extent to which the surface was melted in its early history. The Moon's crust has a variable structure, thinner on the near side than on the far side, except for the the South Pole Aitken basin region, and thinned beneath the major impact basins. Scientists would like to know why.
3. Determine the subsurface structure of impact basins and the origin of mascons.
A surprising discovery was made in the 1960s, when it was found that the Moon's gravity was unexpectedly strong over certain impact basins, and that this is what had been pulling lunar spacecraft off course. It was deduced that these impact basins have large mass concentrations (or "mascons" for short) underneath them. GRAIL will provide more information about their nature.
4. Ascertain the temporal evolution of crustal brecciation and magmatism.
Analyses of a set of lunar craters indicates that those which formed less than 3.2 billion years ago show less gravity than the surrounding plain, while those formed earlier show about the same gravity as the surrounding plain. GRAIL will help to explain the reasons, including the roles played by brecciation (the formation of new rocks by cementing together fragments of older rocks, often found beneath impact craters on Earth), magmatism (the movement of molten rock inside the Moon), and isostatic compensation (a process by which a planetary body evens out the distribution of its mass -- for example, by distorting the boundary between the crust and mantle beneath a large impact basin).
5. Constrain deep interior structure from tides.
Earth's gravity creates tides in the solid Moon just as the Moon creates tides in Earth's oceans. Of course, the Moon's rocky surface doesn't bulge as much as water would -- it only deforms about 9 cm in response to Earth's gravity from one part of the lunar orbit to another. But what's significant to GRAIL is that the Moon responds to Earth's gravity all the way down to the core, and different internal structures would produce differences in the way the Moon's gravitational field deforms. By analyzing how the field deforms at various parts of the Moon's orbit around Earth, scientists will be able to deduce information about the core and other deep features.
6. Place limits on the size of a possible solid inner core.
GRAIL scientists will search for evidence of a solid core within the liquid core the Moon is believed to have, and place limits on how big that solid core could be.