Were Martian moons blown off planet’s surface?

EDMONTON — New research suggests Mars’ moons were once part of the planet, blasted into space by some cataclysmic collision long ago.

Until now, the most common theory was that Deimos and Phobos were once asteroids, captured into orbit by Mars’ gravitational field.

“They kind of look like asteroids,” said Chris Herd, a planetary geologist at the University of Alberta and a co-author of a new paper published in the Journal of Geophysical Research.

“They’re really pockmarked with craters and they have those characteristics that make them look like asteroids we’ve seen farther out.” Still, doubts remained. Herd and his colleagues analyzed light recorded from one of the moons by the Mars Global Surveyor mission that orbited the planet in 1997. They then compared that analysis with a similar look at a meteorite known to have come from the asteroid belt — the Tagish Lake meteorite from northwestern British Columbia.

They didn’t look like each other at all.

“It was not a match,” said Herd. “The best match is groundup basalt, the kind of common rock that Mars is made of.”

The most likely conclusion is that Deimos and Phobos are chunks of rock blown off the surface of the planet, perhaps by a collision with some other heavenly body far back in the history of the solar system.

The theory may help to explain another puzzling feature of a planet that has fascinated skywatchers for centuries. Mars’ northern hemisphere has a far lower elevation than its southern half. The difference is large — several kilometres.

Scientists have long wondered if the difference is the result of some long-ago impact.