Stone Meteorites from Moon and Mars
Do we really find lunar and martian rocks on the surface of our own planet? The answer is yes, but they are extremely rare. About one hundred different lunar meteorites (lunaites) and approximately thirty Martian meteorites (SNCs) have been discovered on earth, and they all belong to the achondrite group. Impacts on the lunar and Martian surfaces by other meteorites fired fragments into space and some of those fragments eventually fell on earth. In financial terms lunar and Martian specimens are among the most valuable meteorites, often selling on the collectors' market for up to $1,000 per gram, making them worth many times their weight in gold. [1]
Small rocks on the surface of the Moon and in orbit around the Sun or Earth are exposed to cosmic rays. The cosmic rays are so energetic that they cause nuclear reactions in the meteoroids that change one nuclide (isotope) into another. Some of those nuclides produced are radioactive. As soon as they fall to Earth, production stops because the Earth's atmosphere absorbs nearly all cosmic rays. The radionuclides decay on Earth with no further production. The most well-know such isotope is 14C (carbon 14), which is produced from oxygen atoms in the meteoroid. Other important radionuclides produced by cosmic-ray exposure are 10Be, 26Al, 36Cl, and 41Ca. Because the various radionuclides all have different half-lives, it is often possible to tell how long a rock was exposed on or near the surface of the Moon, how long it took to travel to Earth, and how long ago it fell.
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