Galileo's Pendulum

(Every day until Christmas, I’ll be posting a science-related image.)

Day 16

Maps come in many forms, depending on the role they play. Political maps are concerned with boundaries, the arbitrary borders we humans draw across our beautiful world in juvenile pretense that we’re inherently different from each other. Topographic maps attempt in two dimensions to characterize the three-dimensional peaks and valleys of the planet. The image above is a map drawn from gravity, marking the fluctuations in strength of the Moon’s gravitational field. The map was created by the twin GRAIL (GRavity Recovery And Interior Laboratory) spacecraft, orbiting Earth’s Moon since the beginning of this year.

The Moon’s surface only tells part of the story of its composition and history. Understanding the details of the Moon’s interior requires more indirect measurements, including the seismometers left by Apollo astronauts. GRAIL consists of two spacecraft flying in the same very low orbit (only 55 kilometers above the surface!). By precise measurements of the distance between the craft, researchers could determine minute fluctuations in the Moon’s gravitational field, which in turn reveal variations in the composition of the rocks below. The discoveries were striking: the lunar crust turns out to be thinner than expected by about 12%, and some of the impact craters were so deep that the meteorite that created them punctured all the way through into the underlying mantle.

GRAIL’s successful mission ends today: December 17, 2012. Such low and precise orbits are difficult to maintain, and use up fuel rapidly. So, today at 5 PM Eastern United States time, the probes will be crashed deliberately into the side of a mountain near the Moon’s north pole.