[thedoc (OP)]

Mike Greenland  asked the Naked Scientists:
   
Just listened to a man from NASA say meteorites have landed on Earth from Mars.

This provoked a few questions:

How to we know it was from Mars? How did it get here.... More importantly, how did it achieve escape velocity from Mars?

What do you think?

[Janus]

It was noticed that a certain type of meteorite had a similar make up to samples of Mars' surface made by the Viking probe. Later, some of these meteorites were found to contain trapped gas pockets and they matched the what was found in the Martian atmosphere.

They were most likely thrown free from Mars in a collision with an asteroid or comet.