A follow-on from this question, if meteorites can come from Mars, it seems they could have come from Earth and ended up back here (though I'm guessing that to be less likely). Do we have any evidence of this having happened? Would there be a way of verifying that they left and came back, other than direct observation of them falling from space?
This somewhat depends on your definition.
It is reasonably likely that a huge impact on Earth could knock rocks into a loose orbit around us, or at least around the sun but coming close to us. It seems likely that some of these would eventually fall back to the Earth, being Earth-origin meteorites.
The second possibility, that they landed on another world and made their way back, is extremely unlikely. Earth has a very large gravity well, and it is thus much harder to escape than Mars. We are relatively certain that there are significant amounts of Earth on the moon and Mars, in the amount of approximately 2kg/square km (See this Science@NASA article). That's not very much at all. As we've only had a very few confirmed Mars rocks found on Earth, or even Moon Rocks, it's even less likely.
The last thing to consider is how would we know that such rocks came from the Earth? Here's some common methods of identifying meteorites, in the context of this discovery.
- Meteorites tend to have more Iron than most Earth material, and thus tend to be magnetic. This wouldn't work for an earth based rock, as it came from Earth.
- Meteorites show signs of very high temperatures and pressures from their reentry into Earth. This is a possible way to identify meteorites, but it could easily be either volcanic in nature, or else produced from cosmic collisions, ie, Earth knocked loose by an impact from a small asteroid would have similar effects.
- If a rock is found on top of a glacier, it could not have formed there. It could have come from volcanic activity, been moved there, or it could be a meteorite. This is how an Earth meteorite would have to be found, but it would be difficult to tell among the other possibilities. (See the Antarctic Meteorite Program)
- Spectral analysis is used to look at the ratios of isotopes, to identify the location from which a rock came from. The answer for such a rock would be Earth, but it would be difficult to tell from other rocks.
So, if you are walking in Antarctica, and see a rock on a huge glacier that shows signs of reentry, and has the spectral analysis of Earth, it might be an Earth based meteorite. As far as I know, no one has identified a rock as such, but it very well could happen.
There are a class of small objects known as tektites http://en.wikipedia.org/wiki/Tektites which are thought to be melted rock that was thrown up by impact events. These objects are thought to have been liquid rock in suborbital trajectories that fell back to earth. Unless you want the term meteorite to imply that the object must have once been not gravitaionally bound to the earth, these should qualify for your O.P.
There is only a relative handful of meteorites on Earth known to come from Mars. The odds of any chunk of rock making two trips and being discovered/recovered by humans are incredibly long.
If that somehow did happen nevertheless, it would be pretty unambiguous to verify. Earth rocks have distinctive chemical and geological (and even more distinctive biological) characteristics, and so finding an Earth rock on Earth with evidence of Martian dust, reentry and impact on/in it would be conclusive.