The best way to date is through detailed computer simulations. Astronomers encode the rules of physics into a model that includes characteristics of a planet’s composition, such as rock or ice; the effects of gravity; the sizes, speed, and masses of planet and impactor; and many other parameters. Then they set up the collision and watch what happens.
In 2001, Robin Canup (Southwest Research Institute, Boulder, Colorado) and Erik Asphaug (University of California at Santa Cruz) performed new, high-resolution simulations. In them, they show that an oblique impact by an object with 10 percent of Earth’s mass can throw enough iron-free material into orbit to make the Moon, while also leaving Earth with its final mass and correct initial rotation rate.
“It is now known that giant collisions are a common aspect of planet formation, and the different types of outcomes from these last big impacts might go a long way toward explaining the puzzling diversity observed among planets,” Asphaug says.
In 2005, Canup used the same techniques to simulate impacts in the early Kuiper Belt. She showed that Pluto’s moon Charon also likely formed when Pluto collided with an object around 10 percent of Pluto’s mass.
“This work suggests that despite their many differences, our Earth and the tiny, distant Pluto may share a key element in their formation histories,” Canup says. And it provides further support for the emerging view that random impact events may have played an important role in shaping planetary properties in the early solar system.
Play the movies and see for yourself.