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J.P. Emery (NASA Ames Research Center), R.H. Brown (Univ. Arizona/Lunar and Planetary Lab)
We have continued our program to measure near-IR (0.8 - 4.0 micron) spectra of Trojan asteroids. To date, we have obtained K-band (1.95 - 2.5 micron) spectra of 24 objects, I, J, H-band (0.8 - 1.75 micron) spectra of 17 objects, and L-band (2.85 - 4.0 micron) spectra of 8 objects.
Trojan asteroids orbit the sun at 5.2 AU, trapped in Jupiter's stable lagrange points. Their unique location beyond the outer edge of the main belt yet interior to Kuiper belt objects and Centaurs falls amidst the transition region between the rocky inner and icy outer solar system. Dynamical and physical considerations have led to the suggestion of possible links between Trojan asteroids and other groups of minor bodies (e.g. short period comets, irregular satellites, KBOs/Centaurs) [1,2]. These asteroids are generally very dark and very red (in both visible and near-IR), placing them in the P and D taxonomic classes [3]. The low albedo and red spectral slope have generally been attributed to organics on the surfaces of these objects, but a case has recently been made that silicates may be the reddening agent [4].
These recently measured data are combined with previously published data to construct spectra covering the visible and near-IR (0.3 - 4.0 micron), and the composite spectra are analyzed quantitatively using the formulation for scattering in a particulate medium developed by Hapke. Under this rigorous examination, it is found to be unlikely that the red spectral slope is a result of organics on the surfaces, due mainly to the lack of absorptions in the L-band. These surfaces are compatible with mixtures of anhydrous silicates and carbonaceous material. Upper limits are placed on the amount of water ice and hydrated silicates present on the surfaces. Comparison is also made to other groups of dark solar system objects.
[1] W. K. Hartmann, D. J. Tholen (1990) Icarus, 448. [2] E. M. Shoemaker et al. (1989) In Asteroids II, 487. [3] D. J. Tholen et al. (1989) In Asteroids II, 1139. [4] D. P. Cruikshank et al. (2001) Icarus, 348.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.