Reading yours answers and WSJ interview about the interplanetary exchange of rocks, could you give an example of what types of events could cause this effect?  Are there ‘modern day’ occurrences where this has been observed or is this logical explanation of material found on earth?

Thanks Thanks

Rocks are exchanged between planets as a result of large crater-forming impacts.  Impacts of objects several kilometers across into solid rock can accelerate some fragments of the local rocks to escape velocity without melting them.  Some of the ejected rocks will go into orbits around the Sun that will eventually lead to a collision with another planet.  Mostly this process takes millions of years, but there could be a few rocks that are launched on trajectories that provide short travel times between the source and the target.  Quite a few martian and lunar rocks have been found on Earth that came here by this process.  Some of these Mars rocks and Moon rocks are privately owned, and small pieces of them are for sale.  I own two very small fragments, one each from Moon and Mars.  Transport from Earth to Mars is more difficult, because the gravity of Earth is greater.  I have heard some speculation, however, that the end-Cretaceous impact 65 million years ago might have launched some Earth-rocks to Mars.

David Morrison

NLSI Interim Director

March 13, 2008

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