Space.com (September 27, 2010)
"One of the two moons of Mars most likely formed from rubble catapulted into space after a comet or meteorite slammed into the Red Planet, a new study finds.
"The moon, Phobos, looks a lot like an asteroid: It's lumpy, potato-shaped and very small. It has an average radius of just 11 kilometers (6.8 miles).
"Scientists have long wondered about the origin of Phobos — is it merely a captured asteroid, the leftovers from Mars' formation or evidence of a cosmic Martian hit-and-run with another object?
"The new study found that the moon's composition and density strongly indicate that, like the leading theory for Earth's own moon, Phobos is the result of a catastrophic impact with its parent planet...."
If Phobos is essentially a loose rubble pile, with open spaces left over from its formation, that would help explain why the Stickney Crater impact didn't shatter it. Hit a solid rock with a hammer - you may end with several smaller rocks. Hit a pile of gravel with a hammer - you'll get a pile of gravel with a dent in it.
Another indication that Phobos isn't from the Asteroid Belt is that it's chemically similar to the surface of Mars - including minerals that form when there's liquid water around.
We may know more, if the Russian Phobos-Grunt ("grunt" is "soil" in Russian) mission successfully scoops up some of the surface of that moon of Mars.
- "Earth and Moon May Be Younger Than Thought - But Not By Much"
(June 12, 2010)
- "Over the Moons of Mars"
(March 17, 2010)