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Morrison (2006) Failure and how to avoid it

Morrison (2006) Failure and how to avoid it - N EWS VIEWS...

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NEWS & VIEWS NATURE | Vol 440 | 6 April 2006 752 from most other Solar System bodies (Fig. 1). This implied that the Sun itself is 16 O-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. The main difference between the two studies is the choice of sample. Hashizume and Chaussidon 2 used a sample of ancient lunar regolith that they had previously shown contained carbon enriched in 13 C (ref. 6) and nitrogen depleted in 15 N (ref. 7), results they ascribed to the effect of the solar wind. Their metal grains contained a thick oxide layer that compromised the normal solar- wind implantation profile, but the authors identified 16 O-rich oxygen they found inside the grains as so-called solar energetic par- ticles. These particles travel at much higher speeds than normal solar-wind particles, and are therefore much more deeply embedded in the grains. Ireland and colleagues 1 used a sample of lunar soil only recently exposed to the solar wind, but which had one of the highest expo- sures known. Their metal grains had only a very thin oxide layer, and the 16 O-poor oxygen was found at a depth consistent with normal implanted solar wind. The two studies 1,2 thus measured solar particles of different ages and, apparently, from different energy regimes. Yet even taking these facts into account, such divergent results are hard to understand. Processes within the Sun, and those that accelerate solar-wind and solar energetic par- ticles away from its surface, would be expected to affect isotopic composition in a mass- dependent manner. Thus, samples would simply move up or down a line of slope 0.5 on the three-isotope plot. To move away from such a line, either oxygen of a different com- position must be added or a process that is mass independent must be invoked. Although the outer layer of the Sun could have changed through the addition of new material over two billion years, there is no known reservoir of material in the Solar System that has a compo- sition extreme enough or a mass great enough to shift the composition of this layer by the required amount. A process called self-shielding can also alter oxygen isotopic composition. When certain compounds such as carbon monoxide are exposed to intense ultraviolet radiation, mol- ecules containing the highly abundant 16 O can exhaust the supply of photons with the correct energy to break them up, although there are still plenty of photons that can disrupt mol- ecules containing 17 O and 18 O. A reservoir of oxygen with a composition different from the starting carbon monoxide can be created if the released oxygen becomes trapped in a differ- ent molecule, such as water 5 . But there is no obvious way to apply this mechanism to solar- wind or solar energetic particles.
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