Detection of ice and organics on an asteroidal surface

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Recent observations, including the discovery1 in typical asteroidal orbits of objects with cometary characteristics (main-belt comets, or MBCs), have blurred the line between comets and asteroids, although so far neither ice nor organic material has been detected on the surface of an asteroid or directly proven to be an asteroidal constituent. Here we report the spectroscopic detection of water ice and organic material on the asteroid 24 Themis, a detection that has been independently confirmed2. 24 Themis belongs to the same dynamical family as three of the five known MBCs, and the presence of ice on 24 Themis is strong evidence that it also is present in the MBCs. We conclude that water ice is more common on asteroids than was previously thought and may be widespread in asteroidal interiors at much smaller heliocentric distances than was previously expected.

At a glance


  1. The reflectance spectra of 24 Themis are well fitted by a mixture of ice-coated pyroxene grains and amorphous carbon.
    Figure 1: The reflectance spectra of 24 Themis are well fitted by a mixture of ice-coated pyroxene grains and amorphous carbon.

    The spectrum of 24 Themis from 2008 (filled symbols) is plotted (binned ×6) along with an example spectral model that includes a thin coating of water ice on surface grains (grey line). The 24 Themis data are binned to lower spectral resolution and plotted as geometric albedo. The model is an intimate mixture containing 29% pyroxene coated with a 0.045-μm-thick layer of water ice and 71% amorphous carbon6, 14. All grains have a diameter of 30μm. Error bars, 1sample s.d.

  2. Organic material is present on the surface of 24 Themis.
    Figure 2: Organic material is present on the surface of 24 Themis.

    The spectrum of 24 Themis from 2008 has been divided by the model shown in Fig. 1 to identify residual absorptions (open symbols). A residual absorption band centred near 3.4μm and with a width of ~0.2μm is well matched by complex organic materials. The thick black line is ice tholin (the residual of an irradiated mixture of water ice and ethane; calculated from optical constants; ref. 9). Offset from the ice tholin and the spectrum/model data by 0.1 units, the dashed line is asphaltite15, the heavy grey line is the carbonaceous meteorite Cold Bokkeveld (a slope from hydrated silicates has been removed; ref. 11) and the thin black line is an average of six polycyclic aromatic hydrocarbons (ref. 10). Absorption features in organic materials near 3.4 and 3.5μm are typically –CH2 and –CH3 stretch bands, whereas those near 3.3μm in the polycyclic aromatic hydrocarbons are aromatic stretch bands. Error bars, 1sample s.d.


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Author information


  1. Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA

    • Andrew S. Rivkin
  2. Earth and Planetary Science Department, University of Tennessee, Knoxville, Tennessee 37996, USA

    • Joshua P. Emery


A.S.R. performed all of the telescopic observations and reduced all of the data, including the thermal flux removal. J.P.E. performed the spectral modelling of the ice and organics and performed spectral library searches. The authors contributed equally to interpretation and analysis.

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The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (2.4M)

    This file contains Supplementary Information and Data comprising: Consideration of alternate explanations for 3.1-µm absorption, Supplementary References, Supplementary Table S1 and Supplementary Figures S1-S4 with legends.

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