Technical Comments

Response to Comment on "Ancient Asteroids Enriched in Refractory Inclusions"

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Science  14 Nov 2008:
Vol. 322, Issue 5904, pp. 1050
DOI: 10.1126/science.1161585

Abstract

Although the exact abundance of phases in carbonaceous chondrites remains debatable, a potentially lower absolute abundance of calcium- and aluminum-rich inclusions (CAIs) in the Allende meteorite does not change our fundamental conclusion. In a relative comparison, CAI-rich asteroids contain two to three times as many CAIs as the most CAI-rich meteorites. These asteroids are therefore greatly enriched in the earliest solar system materials and remain enticing targets for future exploration.

Hezel and Russell (1) argue for an abundance of calcium- and aluminum-rich inclusions (CAIs) in the Allende meteorite of ∼3 volume %, far below the ∼10 volume % of CV chondrites that we cited in our work (2). They correctly note that previous workers have not distinguished CAIs and amoeboid olivine aggregates (AOAs), grouping both under the heading of refractory inclusions. Indeed, the abundance of CAIs in Allende remains a subject of intense study. Recent work by Ebel et al. (3) suggests CAI abundances in Allende that range from ∼3.3 to 5 volume %. Yet, Hezel and Russell acknowledge that if Al is concentrated only in CAIs, Allende could have as much as 9.3 volume % CAIs. If Ir is contained only in CAIs—amore likely assumption than that for Al—a bulk Ir value of 0.785 parts per million (ppm) (4) coupled with an average mean Ir of 8.16 ± 1.36 ppm in CAIs (5) yields 9.6 ± 2% CAIs in Allende. This number is in excellent agreement with McSween (6), who reported 9.4 volume % CAIs in Allende in addition to 3.2 volume % AOAs. Although our paper included references for the abundance of <10% CAIs for CV chondrites (7), the value for the actual meteorite we examined, Allende, is widely taken to be 9.4 volume % as reported by McSween (6). This value is indistinguishable from the estimate derived from spectral modeling of ∼10%, an abundance we reported as an approximate value with appropriately large uncertainties in (2).

Recognizing that the modal mineralogy of carbonaceous chondrites in general and Allende in particular remains a subject of debate, a potentially lower abundance of CAIs in Allende does not change the underlying foundation of our work. Although the same techniques were used, the values determined from spectral modeling of Allende (∼10% CAIs) and the CAI-rich asteroids (∼30 ± 10% CAIs) are independent of one another (i.e., we did not assume a value of ∼10% for Allende in order to infer 30 ± 10% for the asteroids). A simple comparison of the spectra of bulk Allende and the asteroids reveals a higher albedo and more pronounced spectral features in the asteroids, which supports the relative enrichment of spinel-rich CAIs in the asteroids. Thus, our inference that the CAI-rich asteroids contain two to three times more CAIs than even the most CAI-rich meteorites is particularly robust.

We find it puzzling that Hezel and Russell stand by estimates of CAI abundances on these asteroids derived by Burbine et al. (8), which seem to yield a result (∼5 to 10% CAIs) that they find more agreeable. The difference between our work and that of Burbine et al. (5), however, has nothing to do with initial assumptions of CAI abundances in CV chondrites but rather reflects a greatly improved understanding of the nature of modern asteroid regoliths (i.e., they are well mixed) and the modeling thereof, as well as higher-quality spectra of both the asteroids and the CAIs. These advances were acknowledged by Burbine in a commentary (9) that accompanied our study. In (2), we argued that the asteroids we studied likely formed earlier in the history of the solar system, when CAIs were forming before the injection of 26Al into the solar nebula, thus preventing melting. If substantiated, a lower absolute abundance of CAIs might mitigate the need for early formation, as would heterogeneous distribution of 26Al, incorporation of ice, or an anomalous thermal history, all of which were possibilities we mentioned. Nonetheless, we stand by our conclusion that these asteroids are greatly enriched in the earliest solar system materials compared with the suite of meteorites available in the world's collections and that they therefore remain enticing targets for future spacecraft exploration.

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