Interstellar Grains in Meteorites? Grains

Home , Carbonaceous chondrite, Orgueil (meteorite)

Nature Vol. 283 28 February 1980 813 might provide a sample of interstellar Interstellar grains in meteorites? grains. To this end the Chicago group used chemical separation techniques and the from R. Hutchison Bern group physical methods. Lewis et al. of Chicago (Astrophys. J. TODAY, the most favoured theory of the conclude that these inclusions crystallised Lett. 234, 165; 1979) used a sample of the origin of the Solar System entails the when 26AI was still extant. But, as D.D. water-bearing, CM2, carbonaceous collapse of a gas-dust cloud, perhaps Clayton has pointed out, there is no chondrite which fell at Murchison, initiated by a shock-wave from a nearby positive proof that the white inclusions Victoria, Australia in September, 1969. As supernova. Heating during the collapse crystallised within the Solar System (see for with Allende, scientists were fortunate to phase would have vaporised much, if not example Nature 257, 36; 1975). Because have been provided with about 500 kg of a all, of the pre-existing dust. The question isotopic anomalies of different elements, type of meteorite which had previously then arises: did any of the presolar dust but especially light and heavy elements, do been in short supply. The Chicago group grains survive? And if so, how might they not generally correlate with each other, it is dissolved away over 99. 70Jo of their sample, be recognised? Answers to these questions argued that crystallisation of the white using various reagents including HF, may be found in carbonaceous chondrite inclusions must predate the Solar System. HNO3, NaOH and H2O2• The residue was meteorites. Injection of newly synthesised nuclides split into three size-fractions and from Evidence that some materials are from a supernova into a presolar gas-dust aliquots of each the rare gases were enriched in the products of a supernova (or cloud should have caused complete mixing extracted by stepwise heating. Isotopic supernovae) was found in inclusions in the of the injected elements. Thus, any isotopic ratios were measured in the gas released at Allende meteorite. The scientific anomalies should be correlated within each each heating step. Ne-E was released from community was extremely lucky to receive white inclusion. Lack of such correlation the coarse (>3µm) fraction in two steps at two tonnes of specimens from the Allende may be interpreted as indicating that the 800 °C and 1,200 °C. Low-temperature meteorite shower that fell in northern white inclusions represent condensates (800 °C) release was not observed in an Mexico in February, 1969, making from the outer shells of the expanding aliquot of material which had been treated

available for study kilograms of samples of supernova itself, assuming that the shells with HC1O4 • The low temperature carrier a type which had previously been doled out are blown concentrically outwards with a destroyed by HC1O4 was thought to be an in gram-sized amounts. The Allende minimum of mixing between them. oxidisable, carbon-rich material. From material was found to contain whitish Last year, a presolar age for two Allende previous work and from its resistance to inclusions consisting of minerals rich in inclusions was given a boost. Jessberger attack, the high temperature carrier of Ne calcium, aluminium and titanium. Such and Dominik (Nature 277, 554; 1979) E was tentatively identified as spinel mineral assemblages were soon recognised found that two inclusions have 39Ar-40Ar (MgAIOJ. In addition to carriers of Ne-E, as those predicted by Lord in 1965 (Icarus ages of about 5.0 Ga. The technique uses a carrier of s-process xenon was identified 4, 279) as the first material to condense the "°K-"°Ar decay scheme, but 40K is as a carbon-rich phase stable at 1,400 °C. from a cooling gas of solar composition. estimated from the amount of 39 Ar Lewis et al. concluded that the two And in 1973, R.N. Clayton et al. (Science produced by neutron irradiation of 39K. If carbonaceous phases and spinel are 182,485) established that the oxygen of the the 40K/39K ratio throughout the inclusion 'extrasolar'. However, stability of spinel high temperature inclusions and had been uniformly high, it could have requires a more oxidising environment chondrules of Allende and related accounted for the apparently old age. than phases rich in elemental carbon. The carbonaceous chondrites is enriched in However, Stegmann and Begemann authors, therefore, speculated that the pure 160. It was argued that the pure 160 (Nature 282, 290; 1979) found the extrasolar minerals may be condensates was produced by nuclear processes in an potassium isotopic ratios to be normal. from red giants "at different stages of expanding outer shell of a supernova. Soon Thus the inclusions seem to have evolution". after the pure 160 was introduced into the crystallised 400 Ma before the birth of the The Bern group prepared nine density Solar System, condensation of refractory Solar System. But because other age fractions of the Orgueil, water-bearing, minerals trapped a portion of the pure determination techniques give a 'normal' carbonaceous chondrite, and stepwise isotope, the remainder having been mixed age of about 4.55 Ga for Allende and its heating was used on all but one, which was in with average Solar System oxygen, so inclusions (see for example Tatsumoto et too small (Astrophys. J. Lett. 834, 169; losing its identity. al. Geochim. cosmochim. Acta 40, 617; 1979). Ne-E was most highly concentrated Further studies revealed that the white, 1976 and Chen and Tilton, ibid.), the in the least dense fraction (<2.3 g cm·3), calcium, aluminium, titanium-rich 39Ar-40Ar age is still treated with suspicion. from which it was released below 900 °C. inclusions of Allende have anomalous Even so, there seems to be a good chance From three high density fractions (total isotopic abundances of various elements. that in some of the Allende inclusions we range, 2.9-3.6 g cm-3) Ne-E was mostly Potentially, the most important of these have presolar aggregates of refractory released at about 1,200 °C. Because anomalies is in 26Mg because this is the minerals up to about 1 cm in diameter. potential contaminants are of lower daughter isotope of 26 Al, of which the half Other approaches in the search for density, it was argued that the Ne-E carrier life is only 740,000 years. The 26Mg presolar, interstellar grains have been probably has a density of about 3.5 and is anomaly was discovered by Gray and taken by Anders' group at Chicago and probably spine!. Eberhardt et al. point out Compston (Nature 251,495; 1974) and by Eberhardt's at Bern. In 1969, Black and that the low density fraction has a low Lee and Papanastassiou (Geophys. Res. Pepin argued by a process of elimination content of cosmic ray-produced 21 Ne, Lett. 1, 227; 1974). Wasserburg's group than an anomalous neon component in which indicates a low abundance of the soon showed that in several Allende some meteorites is ofpresolarorigin (Earth target elements Mg, Al and Si, and is inclusions there is an internal isochron planet. Sci. Lett., 6, 395; 1969). This type consistent with a carbon-rich composition. relationship between 26Mg/24Mg and of neon, 'Ne-E', is characterised by low Furthermore, the composition argues 27Al/24Mg (Geophys. Res. Lett. 3, 109; 20Ne/22Ne and 21 Ne/22 Ne ratios; Ne-Eis, against the possibility of the in situ 1976). The isotope 27Al is the only stable therefore, highly enriched in 22Ne. Because production of pure 22Ne by particle isotope of the element, and 24Mg is the noble gases are released from mineral bombardment within the Solar System. A most abundant common isotope of grains at moderate temperatures, any presolar origin of one Ne-E carrier magnesium. Thus, in each mineral of some carrier of Ne-E could not have had a high therefore seems assured. D inclusions there is a positive correlation temperature history like that of the Allende R. Hutchison is in charge of the national between 26Mg and the aluminium to inclusions. So, if the carrier minerals ofNe collection of meteorites in the British Museum magnesium ratio. We can therefore E could be separated and identified, this (Natural History), London.