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Meteoritics

Stars in stones

Silicate minerals that predate the Solar System have been detected inside primitive stony meteorites. Isotopic analysis suggests that the silicates probably condensed around dying ancient stars.

Meteorites that date from around the time of the formation of the Solar System — a little over four and a half billion years ago — are testament to the events that occurred before and during planet formation. Most of the interstellar dust that went into forming planetary precursors was melted, vaporized, shocked and, once incorporated into asteroids, further heated and damaged. This has caused the chemistry and isotopic composition of minerals from meteorites to become more homogeneous. But a few mineral survivors predate these events. These presolar grains originated around stars that were the predecessors of our own, and made up part of the interstellar medium before collapsing into our Solar System. Several carbonaceous and oxide presolar grains have been identified in meteorite samples. Nagashima et al.1 have now uncovered presolar specimens of silicates, the most common rock-forming minerals (page 921 of this issue).

This discovery is impressive, because presolar silicates are much more difficult to find than presolar carbonaceous and oxide grains. The latter are resistant to acid and can be separated out of a meteorite by dissolving away the major components — silicates and metal. The solid residue that survives can then be examined for grains of interest. This technique has been compared (by Edward Anders) to burning down a haystack to find the needle, and is more than a little distressing for meteorite curators. Nevertheless, it is a relatively straightforward way for researchers to find presolar gems.

The presolar grains identified so far are all chemically resilient enough to have survived this acid processing: silicon carbide, graphite, aluminium oxide and spinel, at levels of up to a few parts per million. Diamonds, which make up to 0.1% of some meteorites, might also be presolar, but their carbon-isotope composition and variable relative abundance in ancient objects has raised some doubt about this2. Because these gems condensed around ancient stars, they offer unique insight into how stars synthesize isotopes, how easily different parts of the star mix together and how grains condense in the relatively cool circumstellar region. They also provide a snapshot of the interstellar medium several billion years ago, so we can judge how the composition of our Galaxy has evolved since before the Sun came into existence.

As well as studying presolar grains in meteorites, we have recently learnt a lot about mineral grains in space by characterizing them remotely. For example, one of the most surprising findings of the Infrared Space Observatory mission was the great variety of types of star around which fine-grained silicates crystallize. It thus seemed certain that the young Solar System would also have contained interstellar silicate dust, as well as the other known grains, and there should be silicates from a variety of stellar environments in our meteorite collections.

Silicates make up the bulk of chondritic meteorites, however, so searching for the presolar variety requires a more subtle approach than for carbonaceous and oxide grains — akin to inspecting the haystack straw by straw. It requires both admirable patience and an ingenious analytical technique. Nagashima's group has developed a micro-imaging technique3 that uses an ion microscope to detect different isotopes (such as those of oxygen — 16O, 17O and 18O). In the images produced, any region of the meteorite that does not match isotopically the overall composition of the meteorite — and hence might be presolar in origin — shows up as a ‘hotspot’. Nagashima et al.1 have found hotspots in situ in the meteorites Acfer 094 and NWA 530: one micrometre-sized presolar grain made of the silicate olivine, plus five clusters of very fine-grained silicate that contain at least one presolar component.

This technique is remarkable in that it has managed to compete with the new-generation ion probe, the ‘NanoSIMS’, developed specifically for isotope-mapping over very small areas and hence perfect for the interstellar silicate search. In a parallel study, Nguyen and Zinner4 have also reported presolar silicates in a sample of Acfer 094, captured in exquisite detail in NanoSIMS images. But these authors worked with a disaggregated sample of the meteorite: Nagashima and colleagues' detection has the advantage of being an in situ measurement.

The presolar silicates identified by Nagashima et al.1 have a higher ratio of the oxygen isotope 17O relative to the two other stable isotopes of oxygen, 16O and 18O, than does the bulk of the material in the Solar System. The silicon isotope composition of the grains is, however, close to normal. These nuclides formed inside stars and their isotopic abundances reflect the composition of the star, its size and its evolutionary stage; the grains that eventually took up these isotopes effectively bear a fingerprint that identifies the kind of star in which they evolved. The isotopic make-up of the presolar silicates suggests that they formed around red giants1 — stars nearing the end of their lifetime and losing mass into space.

Nagashima et al. put the abundance of presolar silicates at between 3 and 30 parts per million, making them perhaps the most abundant type of presolar grain known (with the possible exception of diamonds). This abundance is very high for meteoritic presolar material, but it is about 100 times lower than that of the presolar silicates detected in interplanetary dust particles collected in the stratosphere5. The reason for the substantial difference in these values might be that the meteorites studied here originated in asteroids in the inner Solar System, whereas at least some interplanetary dust particles come from comets, which originate much farther from the Sun. Dust from the asteroidal regions might have experienced higher temperatures, at least intermittently, than dust farther out; or the presolar cloud might have been heterogeneous. Other presolar grains such as silicon carbide do not seem to be so depleted in meteorites, compared with their abundance in dust particles.

A comparative study of different meteorites should provide insight into how presolar grains were mixed and processed as the Solar System formed, and perhaps into the thermal profile of the early Solar System. If more silicate grains can be found, they should also help to answer the question of whether the inner Solar System once hosted presolar silicates that had formed in different astronomical environments.

References

  1. 1

    Nagashima, K., Krot, A. N. & Yurimoto, H. Nature 428, 921–924 (2004).

  2. 2

    Dai, Z. R. et al. Nature 418, 157–159 (2002).

  3. 3

    Yurimoto, H., Nagashima, K. & Kunihiro, T. Appl. Surf. Sci. 203–204, 793–797 (2003).

  4. 4

    Nguyen, A. N. & Zinner, E. Science 303, 1496–1499 (2004).

  5. 5

    Messenger, S., Keller, L. P., Stadermann, F. J., Walker, R. M. & Zinner, E. Science 300, 105–108 (2003).

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