Isotopes as fingerprints

Atoms of the same element can have different numbers of neutrons. The ratios of these isotopes in a meteorite act like a fingerprint for the body it came from.

What isotopes are

Every atom of an element has the same number of protons, its atomic number. Isotopes of an element have different numbers of neutrons, so they have different mass numbers. Oxygen always has 8 protons, but it has three stable isotopes: oxygen-16 with 8 neutrons, oxygen-17 with 9 and oxygen-18 with 10.

Isotopes of an element have almost identical chemical properties, because chemistry depends on electrons, and all three have 8 electrons. Their masses differ slightly, though, and that is enough for sensitive instruments to tell them apart.

Relative atomic mass

The relative atomic mass, Aᵣ, of an element in the periodic table is the mean mass of its atoms, weighted by how common each isotope is. On Earth, oxygen is about 99.76% oxygen-16, 0.04% oxygen-17 and 0.20% oxygen-18, which is why Aᵣ for oxygen is 16.0.

Fingerprints from space

In meteorites, the proportions of the three oxygen isotopes are slightly different from Earth's, and they are different for different groups of meteorite. Rocks from the same parent body share the same oxygen signature, whatever minerals they contain. That is how scientists showed that the HED meteorites all come from one asteroid, that some meteorites come from Mars, and that lunar rocks have almost exactly the same oxygen as Earth, a clue to how the Moon formed.

The differences are tiny, often a few parts in a thousand or less, so they are measured with a mass spectrometer, which separates atoms or molecules by mass.

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