ScienceWeek

SOLAR SYSTEM: A CONTROVERSY ABOUT SOLAR OXYGEN ISOTOPES

The following points are made by Gary R. Huss (Nature 2006 440:751):

1) New work [1] reports investigations of lunar soil from which is inferred that compared with other Solar System bodies the Sun is depleted in the naturally most plentiful oxygen isotope, 16O. Just a year ago, another study [2] of soils on the Moon concluded exactly the reverse. So who is right? Oxygen is the third most abundant element in the Solar System (the Sun's huge stores of hydrogen and helium claim first and second place), and a principal constituent of the rocks, ices, and atmospheres that make up the planets. Its three naturally occurring isotopes --16O, 17O and 18O -- are found in relative abundances of about 2,700:1:5, respectively, although the physical and chemical processes occurring in different environments of the Solar System can cause shifts in these abundances of up to a few per cent.

2) Such deviations can be depicted on an oxygen three-isotope plot. All samples from Earth fall along a single line with a slope of about 0.5, indicating that the ratio 18O/16O shifts by twice as much as the ratio 17O/16O. This is mass-dependent behavior, since the difference in mass between 18O and 16O is twice that between 17O and 16O. Oxygen from other planets or asteroids -- represented on Earth by different classes of meteorite -- show similar mass-dependent fractionations, but as the underlying oxygen composition of each body is different, the data fall on different lines on the three-isotope plot. Oxygen isotopic composition has therefore become a crucial parameter in classifying meteorites.

3) How this oxygen-isotope variability arose, however, is not understood. Does it represent a heterogeneity inherited from the raw materials that made up the Solar System? Or is it the result of physical or chemical processes in the early Solar System [3-5]? The initial oxygen isotopic composition of the dust and gas from which our Solar System formed is not known. The Sun contains most of the matter in the Solar System, so its oxygen isotopic composition effectively defines the Solar System's oxygen composition. Models that generate the compositions of other bodies from an 16O-rich composition are very different from those that start with an 16O-poor composition. But measuring the chemical and isotopic composition of the Sun directly is difficult; the best data come from measurements of the stream of charged particles emitted by the Sun known as the solar wind.

4) One of the best places to measure the solar wind is in lunar soils. The Moon has no atmosphere and no magnetic field, so solar-wind particles are implanted directly into its surface. Measurements of matter from the solar wind have so far concentrated on elements such as the noble gases, nitrogen and carbon, which are not themselves significant components of minerals in lunar soil. Oxygen, by contrast, is found in many minerals. Therefore, solar-wind oxygen in lunar soils must be studied in minerals such as iron metal, which are by nature oxygen-free. Hashizume and Chaussidon [2] were the first to separate iron-metal particles from lunar soils and measure their oxygen content using an ion microprobe. They found that oxygen in a few grains of their soil, which had been exposed to the solar wind between one billion and two billion years ago, was enriched in 16O compared with the oxygen on Earth and that from most other Solar System bodies. This implied that the Sun itself is 16O-rich, just like the calcium-aluminium-rich inclusions (CAIs) that are found in meteorites and are believed to be among the oldest solid bodies in the Solar System. Now, Ireland et al [1] report results from a contemporary lunar soil that support the opposite conclusion.

References (abridged):

1. Ireland, T. R. , Holden, P. , Norman, M. D. & Clark, J. Nature 440, 776-778 (2006)

2. Hashizume, K. & Chaussidon, M. Nature 434, 619-622 (2005)

3. Clayton, R. N. , Grossman, L. & Mayeda, T. K. Science 182, 485-488 (1973)

4. Thiemens, M. H. & Heidenreich, J. E. III Science 219, 1073-1075 (1983)

5. Clayton, R. N. Nature 415, 460-461 (2002)

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