(Every day until Christmas, I’ll be posting a science-related image.)
Day 16
![A map of Earth's Moon, using gravitational data from the twin GRAIL spacecraft. The GRAIL mission ends today, when the spacecraft will be deliberately crashed into a lunar mountain. [Credit: NASA/JPL-Caltech/ IPGP]](https://sciencevspseudoscience.files.wordpress.com/2012/12/grail_topography.jpg?w=500&h=375)
A map of Earth’s Moon, using gravitational data from the twin GRAIL spacecraft. The GRAIL mission ends today, when the spacecraft will be deliberately crashed into a lunar mountain. [Credit: NASA/JPL-Caltech/ IPGP]
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.