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Mars Had an Ancient Impact Like Earth
quixote9 writes "The BBC reports on a set of Nature articles showing that Mars had an impact about four billion years ago by a huge asteroid. This was about the same time that a much bigger object slammed into the Earth, throwing material into orbit around our infant planet. This material is thought to have coalesced to form the Moon. 'It happened probably right at the end of the formation of the four terrestrial planets — Mercury, Venus, Earth and Mars,' said Craig Agnor, a co-author on the Francis Nimmo study. 'In terms of the process of the planets sweeping up the last bits of debris, this could have been one of the last big bits of debris.' There's a theory that having a big moon is important to the development of life, because the much bigger tides create a bigger intertidal zone, but people used to think having a huge Moon like ours was a once-in-a-universe event."
The two colliding bodies generally merge. IE, "Earth" as we know it didn't really collide with some foreign object that went skipping off into space again. "Earth Mark I" was a somewhat different planet whose remains are still here with us, and it collided with a very large object (essentially a planet in it's own right) called Theia (or sometimes aka Orpheus). Those two would have merged and the collision throw up the debris that formed the moon (which is also made up of parts of both)
So there is no search for the object that hit Earth to form the moon. It's still here with us as part of the new Earth, and Theia/Orpheus is essentially our original planet as well; it's just easier to call the bigger planet involved in that collision "Earth" because, well, it was the bigger of the two.
The same is likely true of Mars - the object that struck it probably merged with it.
As an interesting note, it's also thought that a large impact must have struck Venus as well, and must have hit it hard enough to "flip" the planet, as Venus, compared to every other planet in the solar system, rotates "backwards". There were some BIG things floating around and colliding in the early Solar system.
"People who think they know everything are very annoying to those of us who do."-Mark Twain
I seem to recall that a mechanism had been proposed whereby the material for the colliding body collected at a Trojan point, from which is was dislodged once it reached sufficient size - after which collision with the proto-earth would be just about inevitable. If the mars collider formed in the same way then collision with a large body could be pretty much normal in our sort of system and the incidence of large moons might be very high indeed.