... for it was founded upon a rock.--Matthew 7:25
Galactic Chemical Evolution and the Isotopes of the Solar System
In the course of their lives, stars fuse light nuclei to generate the energy that sustains their pressure and supports them against collapse due to their self gravity. The consequence is that, when these stars die and expel their outer layers, they enrich the interstellar medium with nuclei heavier than those from which they formed. In this way, then, over the billions of years of Galactic history, millions of stars have forged the atoms we find in our Solar System from the hydrogen and helium left over from the Big Bang.
What then is the composition of the Solar System, that is, the Solar Abundance Distribution, or SAD, that arose from the nuclear processing of these many generations of stars? The question is harder to answer than one might first expect. It is certainly always possible to imagine such a distribution as the number of all atoms of, say, 16O relative to the number of all atoms of 1H. The difficulty is that we cannot sample all atoms in the solar system to do this counting! Instead, we must focus on a more limited sample of atoms and infer the SAD from them.
It is difficult to overemphasize the importance of a proper accounting for the abundances of the isotopes in the Solar System. The SAD underlies many efforts in astronomy and many, if not most, studies in cosmochemistry. It is the standard against which the compositions of other stars are measured and reported. It is also the baseline against which the isotopic composition of rocks and meteorites are compared.
Difficulties in Determining the SAD
A natural guess as to the proper approach to determining the SAD is to use spectroscopy to measure the abundances in the photosphere of the Sun. The Sun contains most of the atoms in the Solar System, and we expect those atoms to be fairly well mixed in the Sun's atmosphere; thus, the photospheric abundances should give excellent estimates of the Solar System's composition. The problem is that spectroscopy gives good information about the abundances of the elements, but with a few exceptions, little information about the isotopic abundances. Moreover, the evidence for low-abundance elements in the Sun's spectrum is often swamped by Doppler broadening and other spectral line effects from more abundant species.
Due to these difficulties, attempts to determine the SAD turn to cosmochemical samples, namely, rocks and minerals, whose isotopic abundances can be determined with great accuracy in Earth-based laboratories. Such studies have their own difficulties, however. The first is that the atoms in the Solar System are a mix of the debris from so many stars. It is reasonable to wonder whether the distribution of the various species is uniform in the Solar System. Different stars produce different proportions of the isotopes, so, if the contributions from the different stars are not well mixed together in the Solar System, we might expect strong variations in the abundances of the different atomic species.
It is now clear that the isotopes from different stars were not completely mixed in the Solar System. The evidence is from presolar grains, which are tiny dust particles recovered from meteorites. These grains condensed in the outflows from individual stars, and, because the grains have remained little altered in the journey from their parent stars, their isotopes directly reflect the composition of the material from which they condensed rather than that of the Solar System. The isotopic abundances in presolar grains can be hundreds of times enriched or depleted compared to the composition of average Solar System minerals. With the exception of the presolar grains, however, the isotopes in the Solar System seem to have been fairly well mixed. Abundance deviations of isotopes in a variety of meteorite and planetary samples show uniformity typically to within parts per thousand. Evidently the mixing of the isotopes that occurred in the interstellar medium prior to the formation of the Sun and in the Solar System as the planets accreted did a good job in erasing much of the inhomogeneity in the Solar System's isotopic composition.
A second difficulty in determining the SAD is in the chemical and geological processes occurring during formation of the planets. As a planetary body heated up due to energy deposited by radioactive decay of the unstable nuclei in its interior and to bombardment by other planetesimals, certain elements, know as the siderophiles, followed iron as it sank to the planet's core in the process of planetary differentiation. Other elements tended to remain in the planet's crust:, atmosphere, or, if they were sufficiently volatile, escaped the planet altogether. For this reason, the abundances of a geologically active object like the Earth are not representative of the Solar System's overall composition.
The Solar Abundance Distribution
It is clear that determination of the SAD must be done with care and judgment. One seeks cosmochemical samples from bodies that have experienced little geological activity. The preferred samples are the carbonaceous chondrite meteorites, which apparently were ejected during collisions of small asteroids. The evidence from the minerals in the carbonaceous chondrites is that the parent asteroids were too small to experience much heating; therefore, their compositions are good representatives of the SAD.
Confirmation of this supposition lies in the excellent agreement between the abundances of elements in the carbonaceous chondrites and available abundances from the Solar photosphere. We now have detailed tables of the SAD derived from careful combined studies of carbonaceous chondrite and photospheric abundances. One of the best known is that of Anders and Grevesse. New tables are being produced, including a Cosmochemical Periodic Table of the elements by Dr. Katharina Lodders. These new tables reflect the revised abundance of oxygen based on new analyses of the Solar spectrum and new models of the Solar atmosphere.
The Solar Abundances Tool
Tables in archived journals are the standard for recording the SAD. In many cases, however, one may seek a more interactive forum for viewing the composition of the Solar System. In addition, since improvements in spectroscopy are leading to revisions of the SAD, the interactive forum should allow users to upload their own SAD.
The purpose of the Solar Abundances Tool is to provide such a forum. The default abundance distribution is from the well-known study by Anders and Grevesse, but the user can upload his or her own table of abundances. Once the default table is chosen, or a proper table is uploaded, the user can compute fractional abundances and mass fractions and can sort and graph the results. The user can also download tables in html or ascii format.
The best way to start using the tool is to try out the tutorials. Help links in the tool also aid the user in determining the correct input. More background and other related information is available from the Papers & Links sidebar link. We hope you enjoy the tool, and, as always, we welcome feedback.
