How twin GRAILs use gravity to map the Moon's interior

Ars Technica » Scientific Method 2012-12-05

The twin GRAIL spacecraft sitting side-by-side in the cleanroom, in preparation for their mission to measure the composition of the Moon's interior.

Rocky planetary bodies aren't uniform: different parts of them have different densities due to variations in composition. Mountains composed of granite will not be the same density as ones built of basalt, and both will be more dense than an ice sheet.

This variation is reflected in the gravitational pull exerted by these mountains, though you'd never notice the difference. The difference in the thickness of tectonic plates produce a variation in Earth's gravitational field, for example, but Earth-orbiting satellites typically don't need to take that into account.

However, measuring these fluctuations can reveal a lot about the interior of a rocky world. The GRAIL mission (Gravity Recovery And Interior Laboratory), launched in 2011, is designed to map the details of the Moon's composition with an eye toward understanding both its interior and its history. GRAIL's design is based on an earlier Earth-orbiting mission known as GRACE (Gravity Recovery And Climate Experiment), a collaboration between the United States and Germany in 2002.

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