To Make a Big Moon, Start with a Small Planet
Excerpt: In a contest of which planet in the solar system has the most relatively massive moon, Earth takes the prize. The Moon is roughly 1% of Earth’s mass, whereas the moons of all the remaining moon-bearing planets—that’s all of them save for Mercury and Venus—are less than one ten thousandth their planets’ masses. ...“We think that a giant impact is a very efficient way to form fractionally large moons,” said Miki Nakajima, a planetary scientist at the University of Rochester in New York. Large collisions are thought to be a common occurrence in the chaos of a still-forming solar system, but if all giant impacts formed fractionally large moons, our solar system would be rife with them. ...Using computer simulations, Nakajima and her colleagues explored what happens when rocky or icy would-be planets of various sizes collide. The researchers found that after such a major impact, only rocky planets less than 6 times Earth’s mass and icy planets less than 1 Earth mass can form a fractionally large moon out of the collision debris. They published these results in Nature Communications on 1 February.…