Theory Says Mars Moons Are Pieces of a Larger One

The two moons of Mars -- Phobos and Deimos -- may have been formed by the catastrophic breakup of a much larger moon billions of years ago.

“It has always been controversial what formed the moons of Mars,” said Geoffrey Landis, a physicist at the NASA Glenn Research Center in Cleveland.

“They are very different from our moon, which is huge. The Martian moons are very small and they look like asteroids.”

Because the two moons look similar to the hundreds of asteroids that circle the sun in the region between Mars and Jupiter, there have been a variety of theories proposing that the Martian moons are actually captured asteroids.

But according to University of Virginia astrophysicist S. Fred Singer, those ideas violate the laws of physics.

Singer said the chances are slim that Mars could have captured two small asteroids. He said it was also unlikely that asteroids would end up in the equatorial orbits of Phobos and Deimos (“fear” and “panic” in Greek).

Calculations show that the Martian moons, which are each only about 10 miles across, used to share an almost identical orbit.

At the Sixth International Conference on Mars in Pasadena in July, Singer presented a theory that he said relies on only one assumption: that billions of years ago a large planetary body about 1% the size of Mars came too close to the Red Planet and was captured.

According to Singer’s theory, the close passage of this moon to Mars caused it to fracture into many pieces, ringing the planet with several smaller moons.

“If dinosaurs had invented telescopes,” Singer joked, “they would have seen those satellites.”

He said most of those moons spiraled down to Mars, a fate that Singer said awaits Phobos in a few million years. Deimos, which is further from Mars, is spiraling away from the planet.