The following quoted section is from Earth: Inside and Out, Edmond A. Mathez, Editor, 2001:

"Dr. Ranu's jaw dropped when he saw the data. Contrary to what he had expected, these new, high-precision numbers showed no difference between the composition of the Earth's oxygen and the Moon's oxygen....

The data compiled by Dr. Noom is called oxygen isotope data, and it is arrived at by studying the atomic composition of oxygen in rocks using a machine called a mass spectrometer. An element is defined by the number of protons it contains, and its atomic weight is determined by the sum of protons and neutrons in its nucleus. Isotopes are atoms with the same number of protons and different numbers of neutrons....

When rocks from the Moon were returned and analyzed, it was found that the isotopes of oxygen in lunar rocks were indistinguishable from the isotopes of oxygen found in rocks from Earth.

Why is this similarity between terrestrial and lunar oxygen important? Because evidence from meteorites shows that the oxygen composition of extraterrestrial materials depends upon where in the solar system they formed. This conclusion has recently been confirmed by analyzing Martian rocks hurled into space by asteroid impacts that eventually fell to Earth as meteorites. These Martian meteorites all have an oxygen isotopic composition distinct from that of the Earth--and the Moon."


This book, by the way, is terrific.