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Water ice and organics on the surface of the asteroid 24 Themis


It has been suggested1,2,3 that Earth’s current supply of water was delivered by asteroids, some time after the collision that produced the Moon (which would have vaporized any of the pre-existing water). So far, no measurements of water ice on asteroids4,5 have been made, but its presence has been inferred from the comet-like activity of several small asteroids, including two members of the Themis dynamical family6. Here we report infrared spectra of the asteroid 24 Themis which show that ice and organic compounds are not only present on its surface but also prevalent. Infrared spectral differences between it and other asteroids make 24 Themis unique so far, and our identification of ice and organics agrees with independent results7 that rule out other compounds as possible sources of the observed spectral structure. The widespread presence of surface ice on 24 Themis is somewhat unexpected because of the relatively short lifetime of exposed ice at this distance (3.2 au) from the Sun. Nevertheless, there are several plausible sources, such as a subsurface reservoir that brings water to the surface through ‘impact gardening’ and/or sublimation.

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Figure 1: Rotationally resolved near-infrared reflectance spectra of 24 Themis.

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H.C. and K.H. are visiting astronomers at the Infrared Telescope Facility (IRTF), which is operated by the University of Hawaii under cooperative agreement no. NCC 5-538 with NASA. All the observations used in this publication were obtained at the IRTF. H.C. acknowledges support from NASA’s Planetary Astronomy programme and from the US National Science Foundation. H.C. was a visiting Fulbright Scholar at the “Instituto de Astrofísica de Canarias” in Tenerife, Spain, and a visiting astronomer at Observatoire de la Côte d’Azur, Nice, France. J.L. acknowledges support from the Spanish ‘Ministerio de Ciencia e Innovación’. T.M.-D. was supported by the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico and by the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro. We benefited from discussions with D. Cruikshank, M. Delbó, P. Michel, A. Morbidelli and T. Roush.

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Authors and Affiliations



H.C. and K.H. carried out the observations, the data reduction and the interpretation of the results. N.P.-A. carried out the spectral modelling and interpretation. E.S.H. provided software and participated extensively in the data reduction. M.S.K. and Y.F. provided software and participated extensively in the thermal modelling of the asteroid. T.M.-D. carried out the spectral comparison with meteorites and mineral samples. J.L. and J.Z. contributed to the interpretation of the spectroscopic results. All authors discussed the results and commented on the manuscript.

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Correspondence to Humberto Campins.

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The authors declare no competing financial interests.

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Supplementary Information

This file contains Supplementary Information comprising: Observations and Data Reduction; Spectral Modelling; Supplementary Table 1; Supplementary References and Supplementary Figures S1-S2 with legends. (PDF 219 kb)

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Campins, H., Hargrove, K., Pinilla-Alonso, N. et al. Water ice and organics on the surface of the asteroid 24 Themis. Nature 464, 1320–1321 (2010).

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