[thaumasiotes]

> given the frequency of rock transfer between planets

What is this frequency, exactly? My impression was that all planets are constantly bombarded with rocks from space (not from other planets), but that it would be extremely surprising for a rock to e.g. leave Earth and end up on Mercury.

[danieltillett]

Link 0 suggests that 150 kg of rock ends up on Mars every year from Earth. It is not too hard to find rocks from Mars here on Earth.