There are three stable (non-radioactive) isotopes of the element oxygen:
- 16O has 8 protons and 8 neutrons
- 17O has 8 protons and 9 neutrons
- 18O has 8 protons and 10 neutrons
All the oxygen in our solar system was forged in stars that existed before the birth of our Sun. The fusion processes that create oxygen from lighter elements require both high temperature and pressure. These conditions exist deep within a star. Different isotopes are created. A nucleus of an atom containing 8 protons identifies it as an oxygen atom, but it is the number of neutrons in the nucleus that determines which isotope it is. Not all isotopes are created in equal abundance.
When the solar system was forming, the oxygen in the “solar nebula” no doubt originally came from various progenitors. A supernova here or there, a planetary nebula somewhere else, and so on. As the solar nebula collapsed to form the Sun and planets, the relative abundance of oxygen to the other elements may or may not have been different in different parts of the solar nebula. Similarly, the relative abundances of the three stable isotopes of oxygen may also have been different in different parts of the solar nebula.
When we measure the relative amounts of the three oxygen isotopes in a terrestrial rock, ocean water, moon rocks, or the solar wind, it may tell us where the oxygen in those materials came from. It may also tell us something about the “life experiences” of the oxygen since the solar system formed. For example, water molecules containing 16O are more likely to evaporate than those water molecules containing the heavier isotopes 17O or 18O. Thus, ground water in the middle of a continent has a higher abundance of 16O than does water in the ocean.
When we look at the solar system today, we find significant differences in the relative abundances of the oxygen isotopes depending on where the material came from. On Earth, 99.75% of the oxygen atoms are of the 16O variety, 0.04% are 17O, and 0.21% are 18O, on average. We see very similar oxygen abundance ratios in moon rocks, indicating perhaps a common origin, but the oxygen abundance ratios in meteorites and solar wind particles are significantly different from this. For example, if you plot the 17O/16O ratio vs. the 18O/16O ratio for a bunch of terrestrial rocks, you get pretty much a straight line. Moon rocks fall along the same line. The calcium-aluminum-rich inclusions (CAI) and iron-magnesium-silicon chondrules in meteorites also form a straight line on this plot, but it has a distinctly different slope.
The solar wind samples collected by the Genesis spacecraft yielded abundances that fall along the same line as the CAIs and chondrules. Mars rocks fall on a line that parallels the Earth-Moon line, but is shifted upwards, indicating that for a given abundance of 18O, the Mars rocks will have a relatively higher abundance of 17O.
