In 1968, just a year before the historic Apollo 11 Moon landing, NASA scientists discovered something that could have sent astronauts Neil Armstrong, Buzz Aldrin and Michael Collins plunging to their deaths: an unexpected gravitational force—one so strong it caused the unmanned Lunar Orbiter spacecraft to violently shake up and down as it orbited Earth’s neighbor.
The cause, NASA determined, was the presence of “mascons,” or mass concentrations of especially dense rock just below the surface of the Moon, with much stronger pulls than the rock that surrounds them. Scientists adjusted accordingly to land the Apollo. But for decades, a pressing question lingered: how could these mascons—not found anywhere on Earth—even exist in the first place?
Today, as published in Science, we finally have an answer. In short: blame the asteroids—and the make-up of the Moon itself.
For the rest of the story: http://science.time.com/2013/05/30/revealed-the-awesome-explanation-for-the-moons-extra-gravity/