Morningfox
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We have found about 132 meteorites from Mars. They are identified by elements and isotopes they have, similar to the rocks and atmosphere of Mars. Argon and oxygen are particularly different in Mars meteorites.
We have one rock that *might* be from Mercury. None from Venus, maybe because the thick atmosphere slows down incoming rocks so much that ejected rocks don't have the energy to leave.
Erik
The type of rock it is, what minerals are contained in it, also the impact at which it hot the earth could indicate such.