An Asteroid’s Destruction May Have Given Mars Rings, Then Moons

Scientists simulated a situation that may offer an improved explanation for how the Red Planet ended up with small Phobos and tiny Deimos.

Something’s not quite right about the moons of Mars. They are too small — Phobos is 17 miles across, and Deimos is a mere nine miles in length. And they aren’t round, but lumpy, misshaped objects. Frankly, they don’t resemble moons at all.

“They look like asteroids, they smell like asteroids, as well as looking like potatoes,” said James O’Donoghue, a planetary astronomer at the University of Reading in England. Perhaps, then, astronomers have suggested, they are asteroids — two space rocks captured long ago by Mars’s gravity.

A study published Wednesday in the journal Icarus makes a case that the moons did indeed start out in asteroid form. But it’s not the genesis everyone was expecting. Using supercomputer-powered simulations, scientists describe a situation in which a large-enough asteroid was captured by Mars long ago and torn to shreds by the planet’s gravity, briefly forming a debris cloud — and possibly a ring system — around Mars that ultimately clumped together to form two moons.

“What they’ve got here is really compelling,” said Dr. O’Donoghue, who wasn’t involved with the study. “I’m sold.”

The notion that Phobos and Deimos may be captured asteroids has long come up against one major problem: Their orbits are too circular, and too neatly aligned around Mars’s equator. Asteroids approach planets at all sorts of angles, and if these moons were once asteroids, their orbits would be expected to be tilted, and perhaps be somewhat oval-shape.

That they aren’t seems suspicious, and supports the theory that they were forged another way. This is akin to the favored origin story of Earth’s own satellite, wherein a Mars-size object slammed into the nascent world, creating a spray of debris, which glued itself together to form our moon.