The ‘S-complex’ asteroids are not easily identified as the source of the most common meteorites reaching Earth. Their relationship might be disguised, however, by the effects of space weathering.
Meteorites that fall to Earth come primarily from asteroids — those minor planets that orbit the Sun mainly between Mars and Jupiter. We have meteorites from approximately 100 different asteroid parent bodies, and the great challenge in meteorite–asteroid studies is to determine which meteorites come from which asteroids. A basic correlation has been established between types of meteorites and types of asteroids based on their ‘colour’ — the amount of light reflected at one wavelength compared with the amount reflected at another. Telescopic observations of asteroid colours and laboratory measurements of meteorite colours suggest relations between them, and have also revealed much about the environment of the early Solar System in which the asteroids formed.
It is therefore terribly perplexing that there is abundant other evidence supporting a link between the most common meteorite type, the ordinary chondrites, and the most common asteroid type in the inner main belt, the S-complex asteroids — yet their colours don't match! One suggested solution to the problem is that the colour of these asteroids may have changed as their surfaces became weathered in the space environment. Other solutions have been proposed, but they create more problems than they solve; colour change would be Occam's choice. Now Jedicke et al.1 demonstrate an interesting correlation between the age of an asteroid's surface and its colour (page 275 of this issue), and use it to argue in support of the space-weathering process.
It is clear that, over time, the exposure of airless bodies to the environment of space will change the optical properties of their surfaces. Most pertinent evidence comes from studies of lunar soils2,3, backed up by results from spacecraft observations4 and from experiments with meteorites5. The surfaces of asteroids are subject to impacts, ion-implantation from the solar wind, sputtering and micrometeorite bombardment — effects that collectively have come to be known as space weathering (Fig. 1). But whether this is the explanation for the colour mismatch between the ordinary chondrites and the S-complex asteroids is controversial.
Jedicke et al.1 have found a way to address the conundrum, based on the age of asteroid families. The orbital characteristics of asteroids within a family are so similar that the family members are inferred to be pieces of a once-single object that broke apart through collisions. This disruption would presumably have brought fresh bedrock material to the surface of each fragment. Then the asteroid family would gradually change colour through space weathering, starting from the time of its disruption. Jedicke et al. have shown that the age of an asteroid family is directly correlated with average colour in exactly the sense predicted by models of space weathering.
What else could explain this correlation? It is possible that the dating technique is erroneous, or that the strong correlation seen for 11 families (out of 12 studied) arises because their mineralogical composition is such that their colours just happen to vary in proportion to their age. But this is grasping at straws. Nevertheless, it is somewhat troubling that there is no significant concomitant correlation of albedo with asteroid-family age. Albedo is the fraction of light reflected by the object, and space-weathering effects are clearly associated with a decrease in albedo.
In any case, Jedicke et al.1 present an interesting interpretation of the correlation, and estimate the rate of space weathering for asteroids. This rate turns out to be about 100 times longer than any other estimate on the books. Furthermore, when they extrapolate back to a time shortly after the formation of each asteroid family, the predicted colour of the asteroid parent body matches that of ordinary chondrites: the implication is that these meteorites are chips off the blocks of S-complex asteroids. Certainly, this work will cause a stir in the trenches of asteroid science.
Although other meteorite–asteroid links are affected by colour changes, the link between ordinary chondrites and S-complex asteroids is the single most important example of the problem because of their great abundance: ordinary chondrites account for more than 80% of meteorites; S-complex asteroids make up more than 80% of some parts of the main belt.
A definite link between the ordinary chondrites and the S-complex asteroids would have important implications for meteorite and asteroid science, and for studies of the early Solar System. If these asteroids are made of ordinary-chondrite material, then such material must have been very common indeed and might even have been the starting material from which the terrestrial planets formed. If S-complex asteroids are not made of ordinary-chondrite material, then dynamicists have a lot to do to explain how the process that brings meteorites from the asteroid belt to Earth could have overwhelmingly over-sampled a rare type of asteroid.
I suspect that, until a spacecraft can scoop up a surface sample from an S-type asteroid and return it to Earth, the debate will continue, with each new piece of evidence adding fuel to the fire.
Jedicke, R. Nesvorný, D., Whiteley, R., Ivezić, Ž. & Jurić, M. Nature 429, 275–277 (2004).
Hapke, B. J. Geophys. Res. 106, No. E5, 10039–10073 (2001).
Pieters, C. M. et al. Meteor. Planet. Sci. 35, 1101–1107 (2000).
Clark, B. E. et al. Meteor. Planet. Sci. 36, 1617–1638 (2001).
Sasaki, S., Nakamura, K., Hamabe, Y., Kurahashi, E. & Hiroi, T. Nature 410, 555–557 (2001).