The moons of Mars aren’t much more than big boulders that look a bit like potatoes. Phobos is about 17 miles long; Deimos, only about nine miles. But they’ve been the subject of a big scientific debate for decades — a debate about their origins.

One possibility says they’re asteroids that were captured by Mars long ago. Both bodies look like asteroids. And some of the minerals on one of them are the same as those found in some meteorites that probably came from asteroids.

The other possibility says they formed after a giant impact. A large body could have rammed into Mars early in its history. That blasted out material from both Mars and the impactor, forming a ring of debris around Mars. Phobos and Deimos formed from that debris. That idea seems to explain the moons’ orbits, but not necessarily their composition. But a recent study may have found a way to make it work.

It says that the impacting body was made mainly of frozen water. Such a body would have blasted out more debris than one made mainly of rock. And the vaporized water would have kept the debris cool enough to allow the formation of some of the minerals on the moons. Ice-rich bodies could have been fairly common in the early solar system — providing a way to create the moons of Mars.

Bright orange Mars is close to our own moon the next two mornings. It’s directly below the Moon at dawn tomorrow, and to the upper right of the Moon on Tuesday. More tomorrow.

Script by Damond Benningfield