A. V. Sankaran

On the night of 20 June 1996, a large meteorite crashed through the earth’s atmosphere, watched by many villagers, and broke into two pieces upon impact at a place near Piplia Kalan, a village in the Pali District of Rajasthan^1,2. Dust-sized extraterrestrial objects called micrometeorites, rain almost daily over our planet while falls of larger ones, a few meters wide, are rare. Rarer still are the kilometer-sized objects, which invariably receive widest attention because of their potential to trigger global catastrophes in the wake of their impact, like the one which crashed 65 m.y. ago, wiping a large segment of life. The Pipliya Kalan meteorite that crashed in Rajasthan, unlike many collected so far on earth, is a re-melted product of a large asteroid. Though relatively small, weighing hardly 50 kg, it has yet created excitement among the planetary scientists following studies by Srinivasan et al.² at the Physical Research Laboratory (PRL), Ahmedabad. They have been able to nab evidence to confirm earlier presence of the radioelement aluminium-26 that had evaded years of global search in this class of meteorites. Detection of this element in such meteorites was vital to explain the chemical differentiation process observed in some larger asteroids or mini-planets, the parent bodies of aerolites, eucrites and other meteorites that fall on the earth from time to time.

Interstellar and interplanetary materials hitting the earth as meteorites are the main objects available for direct investigations and they have enhanced our understanding of the early solar system and the evolution of planets. Accepted ideas, evolved over the past few decades, assume that the interstellar gas formed the ‘basic mix’ for the pre-solar nebula or cloud which condensed to form the sun and the planets – large ones like Venus, Earth, Mars and others, and small ones – the planetessimals and asteroids. Compounds of aluminum, calcium, titanium oxides and silicates were some of the earliest to condense out of the primitive cloud when temperatures were still high (1800–1200 K). At a much lower temperature, the compounds of sulphur and iron were formed and at still lower temperatures, water, ammonia and methane appeared³. Thus the early-formed materials can range from compounds of metal (alloys) to stony irons. Among these are a variety of stony meteorites – carbona-
ceous chondrites that have not been subsequently re-melted and differentiated. Consequently, they retain their pristine nature and thus serve as excellent materials to probe the pre-solar and early solar history. However, not all of these primitive bodies remain undifferentiated; a few are known to have undergone re-melting followed by igneous differentiation to form compositionally distinct zones like core, mantle and crust. Among the various kinds of meteorites that fall on the earth, a small number belong to this class and they are actually chunks broken off from large differentiated asteroids. The Pipliya Kalan meteorite is believed to be ripped off from the crust of the well-known giant asteroid 4-Vesta.

The differentiated asteroids have been engaging the attention of several planetary scientists for a long time, especially to find an answer to the heat source for magma production in them. Heat produced by accretionary process (com-pressional forces) is unlikely to generate adequate molten material for igneous differentiation in bodies of width 500 km and less⁴; instead, decay of some of the radioactive elements present in them were suspected to be the main heat source. Way back in 1955, Harold Urey had suggested that the short-lived nuclide ²⁶Al may be the heat source, but he could not confirm it^5,6. While undoubtedly it could be the prime source, scientists had to establish first the possibilities for the formation of such nuclides, their enrichment and distribution in the pre-solar and solar nebulae for incorporation in objects condensed out. Vigorous studies undertaken, particularly during the post-1970s, derived theoretical models for production of ²⁶Al and a few other short-lived radionuclides which are capable of generating magma within their lifetime in their host asteroid bodies. Next, confirmation was needed for their presence in mineral phases in such early-formed objects. Carbonaceous chondrites are such objects and hence the obvious choice to look for the nuclides in mineral phases such as the Ca–Al rich inclusions, the CAIs for short⁷.

A number of radionuclides with short half-lives ranging from 0.1 to 16.7 m.y. were considered as potential heat suppliers. Since they are extinct now, investigators had to rely on indirect evidence for their earlier existence by checking on the build-up of their decay products, i.e. their daughter isotopes. For example, presence of the parent ²⁶Al with a half-life of 0.73 m.y. was inferred in Allende meteorite and a few other primitive chondrites through detection of excess build-up of its daughter isotope ²⁶Mg in the early-formed Ca–Al inclusions^7,8. Similarly, studies on Efremovka meteorite, another early solar system object indicated excess ⁴¹K isotope which confirmed the existence of parent nuclide ⁴¹Ca (mean life ~0.15 m.y.) within a million years of condensation of the solar nebula⁹. In these investigations it was necessary to ensure that the samples studied had incorporated the parent nuclide ‘live’ and had ‘locked up’ their daughter nuclides in the mineral phases studied with no contribution from daughters of ‘fossil’ parentage^10,11; further, these minerals must not have been subjected to post-crystallization thermal episodes that could redistribute the excess build-up². All these studies, however, assumed that the short-lived nuclide investigated was homogeneously distri-buted at the site of differentiated meteorite in the solar system^2,12. Searches, in this manner, yielded the following short-lived parents which had decayed to their respective daughter nuclides: ⁴¹Ca ®  ⁴¹K; ²⁶Al ®  ²⁶Mg; ⁶⁰Fe ®  ⁶⁰Ni; ⁵³Mn ®  ⁵³Cr; ¹⁰⁷Pd ®  ¹⁰⁷Ag; ¹⁸²Hf ® ¹⁸²W; ¹²⁹I ®  ¹²⁹Xe (refs 8, 9, 11, 13–19). Although these results were obtained from refractory CAIs, which are present only as minor constituents in the chondrites, it was found that even in the more abundant non-refractory phases (e.g. in Semarkona chondrite) presence of short-lived ²⁶Al nuclide could be established²⁰.

The chemical zonation, i.e. differentiation process in the primitive meteorites, therefore, requires magma generation first, an event that has to take place subsequent to CAI formation, within a very short time. Though ²⁶Al with shortest mean life (1 m.y.) will meet the requirements for initiating this process, none of the several differentiated meteorites examined so far confirmed the presence of this nuclide. Unfortunately, not many of the latter type of meteorites are found among those falling on the earth and searches so far in eucrites (a calcium plagioclase rich variety), augite achondrites or angrite (essentially made up of a Ca, Ti-pyroxene augite) and stony irons (mesosiderites) were negative.

Justifiably, in the prevailing scenario, the excess ²⁶Mg decayed from ²⁶Al observed by Srinivasan et al.² in the differentiated Pipliya Kalan meteorite assumes importance. Their studies have confirmed that ²⁶Al was the plausible heat source for the production of magma in differentiated meteorites and planetessimals. Petrographically, this meteorite belongs to the class eucrite, which usually is rich in Ca-plagioclase (anorthite), Ca, Mg-pyroxene (pigeo-nite) with accessories like, chromite and ilmenite. Dated to be 4570 ±  23 m.y. (Sm–Nd age), this specimen is older than similar types reported elsewhere. Large presence of relatively coarse grains of plagioclase (65%)¹ with high Al/Mg values, was favourable for the investigations. Since the plagioclase grains they studied carried little or no magnesium, they were considered highly suitable for estimating excess ²⁶Mg decayed from ²⁶Al. Their estimation indicated that the ²⁶Mg levels were higher by about 3% over the normal terrestrial plagioclases, which is quite significant as this clearly establishes that ²⁶Al was the primary heat source in the differentiated meteorites.

All mineralogical aspects about the minerals they probed confirmed in situ decay of the parent ²⁶Al undisturbed by thermal metamorphism. They noted that the difference between the formation of earliest objects CAIs and the formation of eucritic crust in the differentiated parent asteroid (4-Vesta) was 4.2 ±  0.1 m.y., a time gap supported by similar estimate obtained from studies on ⁵³Mn abundance in eucrites elsewhere². The results, according to the authors, would imply that ‘the accretion heating, melting differentiation and subsequent crust formation on the parent body of eucrites took place very rapidly, within 5 m.y. of formation of solar system’².

The excitement among planetary scientists is understandable after the confirmation by Srinivasan and colleagues on the role of ²⁶Al in the differentiation process in small asteroids or mini-planets. PRL group’s findings in the Pipliya Kalan meteorite, besides putting at rest the on-going somewhat frustrating efforts to pin down the elusive ²⁶Al nuclide, will go a long way to evolve a better understanding of planetary accretion and differentiation.

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A. V. Sankaran lives at 10, P and T Colony, I Cross, II Block, R.T. Nagar, Bangalore 560 032, India.