A lunar crater. Like those on Mars and Mercury they are evidence of the period of intense meteoritic bombardment which occurred during the first billion years of the history of the solar system.
A fragment of a meteorite -
possibly part of the asteroid Vesta -which fell in western Australia in 1960.
* Most chemical elements have one or more isotopes; that is, elements with the same number of protons and electrons but a different number of neutrons, giving them a different atomic mass.
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