Tag: oxygen

Oxygen Speaks with an Accent

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.

Metallicity

No, it’s not the name of a rock band. Astronomers (unlike everybody else) consider all elements besides hydrogen and helium to be metals. For example, our Sun has a metallicity of at least 2% by mass (Vagnozzi 2016). That means as much as 98% of the mass of the Sun is hydrogen (~73%) and helium (~25%), with 2% being everything else.

Traditionally, elemental abundances in the Sun have been measured using spectroscopy of the Sun’s photosphere.  In principle, stronger spectral lines (usually absorption) of an element indicate a greater abundance of that element, but deriving the correct proportions from the cacophony of spectral lines is challenging.

A more direct approach to measuring the Sun’s elemental abundances is analyzing the composition of the solar wind, though the material blown away from the surface of the Sun that we measure near Earth’s orbit may be somewhat different from the actual photospheric composition.  The solar wind appears to best reflect the composition of the Sun’s photosphere in the solar polar regions near solar minimum.  The Ulysses spacecraft made solar wind measurements above both the Sun’s north and south polar regions during the 1994-1995 solar minimum.  Analysis of these Ulysses data indicate the most abundant elements are (after hydrogen and helium, in order of abundance): oxygen, carbon, nitrogen, magnesium, silicon, neon, iron, and sulfur—though one analysis of the data shows that neon is the third most abundant element (after carbon).

The elephant in the room is, of course, are the photospheric abundances we measure using spectroscopy or the collection of solar wind particles indicative of the Sun’s composition as a whole?  As it turns out, we do have ways to probe the interior of the Sun.  Both helioseismology and the flux of neutrinos emanating from the Sun are sensitive to metal abundances within the Sun.  Helioseismology is the study of the propagation of acoustic pressure waves (p-waves) within the Sun.  Neutrino flux is devilishly hard to measure since neutrinos so seldom interact with the matter in our instruments.  Our studies of the interior of the Sun (except for sophisticated computer models) are still in their infancy.

You might imagine that if measuring the metallicity of the Sun in our own front yard is this difficult, then measuring it for other stars presents an even more formidable challenge.

In practice, metallicity is usually expressed as the abundance of iron relative to hydrogen.  Even though iron is only the seventh most abundant metal (in the Sun, at least), it has 26 electrons, leading to the formation of many spectral lines corresponding to the various ionization states within a wide range of temperature and pressure regimes.  Of the metals having a higher abundance than iron, silicon has the largest number of electrons, only 14, and it does not form nearly as many spectral lines in the visible part of the spectrum as does iron.  Thus defined, the metallicity of the Sun [Fe/H] = 0.00 by definition.  It is a logarithmic scale: [Fe/H] = -1.0 indicates an abundance of iron relative to hydrogen just 1/10 that of the Sun.  [Fe/H] = +1.0 indicates an abundance of iron relative to hydrogen 10 times that of the Sun.

The relationship between stellar metallicity and the existence and nature of exoplanets is an active topic of research.  It is complicated by the fact that we can never say for certain that a star does not have planets, since our observational techniques are strongly biased towards detecting planets with an orbital plane near our line of sight to the star.

References
Vagnozzi, S. 2016, 51st Recontres de Moriond, Cosmology, At La Thuile