GRAIL's primary objectives are to help scientists determine the structure of the lunar interior, from crust to core, and to advance understanding of the Moon's thermal evolution.
Why thermal evolution? A planetary body such as the Moon, Earth, and the other terrestrial planets starts as a molten ball, heated to the melting point by meteoroids smashing into it and by radioactive elements within it. Then, over the eons, it cools off by radiating its heat into space. While it's still molten, heavy materials sink down toward the core and lighter materials float on top, ultimately forming a crust. The story of how a particular planetary body processes its heat and how, when, and where heat is replenished by meteorite impacts and radioactive elements is the story of how its structure came to be. So understanding the Moon's thermal evolution is essential to understanding the Moon's origin and development.
The mission's secondary objective is to extend the knowledge it will gain about the Moon to inform our understanding of the development of the other planetary bodies in the inner solar system: Mercury, Venus, Earth, and Mars. As the most accessible planetary body besides Earth, and as one that is thought to have changed little since its initial development (unlike Earth, Mars, and Venus), the Moon offers a unique look into the distant past of planetary evolution.
To achieve its primary objectives, the GRAIL mission will conduct six investigations using gravity science as the chief investigative tool.