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Response to Comment on "Ancient Asteroids Enriched in Refractory Inclusions"
J. M. Sunshine,1*H. C. Connolly, Jr.,2,3,4T. J. McCoy,5S. J. Bus,6L. M. La Croix5,7
Although the exact abundance of phases in carbonaceous chondritesremains debatable, a potentially lower absolute abundance ofcalcium- and aluminum-rich inclusions (CAIs) in the Allendemeteorite does not change our fundamental conclusion. In a relativecomparison, CAI-rich asteroids contain two to three times asmany CAIs as the most CAI-rich meteorites. These asteroids aretherefore greatly enriched in the earliest solar system materialsand remain enticing targets for future exploration.
1 Department of Astronomy, University of Maryland, College Park, MD 20742, USA. 2 Department of Physical Sciences, Kingsborough Community College of the City University of New York, Brooklyn, NY 11235, USA. 3 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA. 4 Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 11024, USA. 5 Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA. 6 Institute for Astronomy, University of Hawaii, Hilo, HI 96720, USA. 7 Department of Geological Sciences and Engineering, University of Nevada, Reno, NV 89557, USA.
* To whom correspondence should be addressed. E-mail: jess{at}astro.umd.edu
Hezel and Russell (1) argue for an abundance of calcium- andaluminum-rich inclusions (CAIs) in the Allende meteorite of3 volume %, far below the 10 volume % of CV chondrites thatwe cited in our work (2). They correctly note that previousworkers 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 ofintense study. Recent work by Ebel et al. (3) suggests CAI abundancesin Allende that range from 3.3 to 5 volume %. Yet, Hezel andRussell acknowledge that if Al is concentrated only in CAIs,Allende could have as much as 9.3 volume % CAIs. If Ir is containedonly in CAIs—amore likely assumption than that for Al—abulk Ir value of 0.785 parts per million (ppm) (4) coupled withan average mean Ir of 8.16 ± 1.36 ppm in CAIs (5) yields9.6 ± 2% CAIs in Allende. This number is in excellentagreement with McSween (6), who reported 9.4 volume % CAIs inAllende in addition to 3.2 volume % AOAs. Although our paperincluded references for the abundance of <10% CAIs for CVchondrites (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 derivedfrom spectral modeling of 10%, an abundance we reported as anapproximate value with appropriately large uncertainties in(2).
Recognizing that the modal mineralogy of carbonaceous chondritesin general and Allende in particular remains a subject of debate,a potentially lower abundance of CAIs in Allende does not changethe underlying foundation of our work. Although the same techniqueswere 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 valueof 10% for Allende in order to infer 30 ± 10% for theasteroids). A simple comparison of the spectra of bulk Allendeand the asteroids reveals a higher albedo and more pronouncedspectral features in the asteroids, which supports the relativeenrichment of spinel-rich CAIs in the asteroids. Thus, our inferencethat the CAI-rich asteroids contain two to three times moreCAIs than even the most CAI-rich meteorites is particularlyrobust.
We find it puzzling that Hezel and Russell stand by estimatesof CAI abundances on these asteroids derived by Burbine et al.(8), which seem to yield a result (5 to 10% CAIs) that theyfind more agreeable. The difference between our work and thatof Burbine et al. (5), however, has nothing to do with initialassumptions of CAI abundances in CV chondrites but rather reflectsa greatly improved understanding of the nature of modern asteroidregoliths (i.e., they are well mixed) and the modeling thereof,as well as higher-quality spectra of both the asteroids andthe CAIs. These advances were acknowledged by Burbine in a commentary(9) that accompanied our study. In (2), we argued that the asteroidswe studied likely formed earlier in the history of the solarsystem, when CAIs were forming before the injection of 26Alinto the solar nebula, thus preventing melting. If substantiated,a lower absolute abundance of CAIs might mitigate the need forearly formation, as would heterogeneous distribution of 26Al,incorporation of ice, or an anomalous thermal history, all ofwhich were possibilities we mentioned. Nonetheless, we standby our conclusion that these asteroids are greatly enrichedin the earliest solar system materials compared with the suiteof meteorites available in the world's collections and thatthey therefore remain enticing targets for future spacecraftexploration.
7. A. J. Brearley, R. H. Jones, in Reviews in Mineralogy, Vol 36, Planetary Materials, J. J. Papike, Ed. (Mineralogical Society of America, Washington, DC, 1998).
8. T. H. Burbine, M. J. Gaffey, J. F. Bell, Meteorit. Planet. Sci.27, 424 (1992).
3 volume %, far below the 
