The Moon is quite an interesting piece of rock, and while it's the only planet-like object that we've explored by putting astronauts on it, there are still many things that have remained a mystery about the Moon, just as much as any other planet that we've explored with satellites. However, NASA says they've solved one big mystery particularly.
Ever since NASA sent satellites to the Moon to scout out landing spots for the Apollo missions, scientists have noticed that the gravitational pull on these satellites was extremely unstable, especially when they would travel over craters and impact basins. The satellites would periodically veer off course, plummeting toward the Moon's surface, but would eventually climb back up into the intended orbit.
It's quite obvious at this point to know these craters and basins have a stronger gravitational pull than other parts of the Moon, and scientists have suspected that it has to do with a surplus distribution of mass below the Moon's surface. However, it's been a mystery as to how this excess distribution of mass came to be, until now.
To find out more, NASA and researchers at MIT and Purdue University mapped out the gravitational fields of several lunar craters and impact basins to discover that the gravitational fields resemble a dartboard, where there's a small center of strong gravitational pull surrounded by alternating rings of strong and weak gravity. NASA used high-res imagery from their Gravity Recovery and Interior Laboratory (GRAIL) missions to map out the gravitational fields.
The agency discovered that this is caused by the way asteroids hit the Moon upon impact. When there's an impact, the asteroid sends debris flying outward, and the impact sends a shockwave through the moon’s interior that creates a counterwave that attracts dense material from the Moon's interior toward the surface of the crater at the center. This is what makes the bulls-eye, if you will. Pretty cool, right?