Spectroscopy of asteroids formed beyond the snowline

Minerals containing bound water (OH) or water molecules (H20) adsorbed onto mineral surfaces give rise to prominent spectral absorptions near 3 pm that are observed in some asteroid classes (Rivkin et al., 2002). These absorptions have different shapes (Fig. 12.14) -the hydroxl feature is sharp and the H20 feature is more subdued. Infrared spectra for asteroids with high albedos are generally characterized by the water feature, whereas spectra of low-albedo asteroids tend to have the sharp OH feature. Although the hydroxl band is partly obscured when viewed through the Earth s atmosphere (the dashed lines in Fig. 12.14), it is still an easily recognizable feature in the spectra of some asteroid classes. 

The low-albedo C-, B-, G-, and F-class asteroids dominate the middle part of the main belt. The sharp 3 pm feature in the spectra of these objects indicates that hydrous 

Different shapes of the -3 pm absorption feature in asteroid spectra. Low-albedo asteroid spectra show a sharp feature attributed to hydroxl-bearing minerals, whereas high-albedo asteroid spectra have a more subdued feature arising from adsorbed water molecules. Dashed lines in the middle of the spectra are regions obscured by the Earth s atmosphere. 

The D- and P-class asteroids dominate the outer main belt and Trojan asteroids located in Jupiter s orbit. With only a few exceptions, the spectra of these asteroids show no 3 pm absorption bands (Jones et al., 1990). The D and P asteroids are thought to contain ice that has never been melted. However, it is also possible that D and P asteroids could contain hydrated silicates, and that the 3 pm feature is masked by an increasing abundance of elemental carbon with heliocentric distance. The unique carbonaceous chondrite Tagish Lake has a reflectance spectrum quite similar to D-class asteroids, and it has been hypothesized to be a sample of this class. However, Tagish Lake shows a significant 3 pm absorption. 

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