Mid-infrared spectroscopy of 11 main-belt asteroids

Abstract

We present mid-infrared (8-13 μm) spectra of 11 main-belt asteroids: 1 Ceres, 3 Juno, 7 Iris, 11 Parthenope, 20 Massalia, 24 Themis, 41 Daphne, 42 Isis, 44 Nysa, 67 Asia, and 88 Thisbe. This paper makes the first report on the mid-infrared spectrum for 5 asteroids. Our observation was conducted with Michelle on UKIRT. The modified Standard Thermal Model (STM) has provided a slightly better fit to the observed spectra than the model without any modification. For 1 Ceres, we detected an emission feature that surpasses the thermal continuum by 6.2±1.1%. For the other 10 asteroids, no feature has been detected above their observational errors. However, their S/N ratios are sufficient to only detect 6% emission excess. As the causes of the observed spectral distinction, we examine possible (1) chemical and (2) physical differences on the surfaces of the asteroids: (1) 1 Ceres has silicates with a lower degree of polymerization than do the other asteroids; (2a) the dominant grain size on 1 Ceres is nearer to 200 μm, and probably smaller than that on the other asteroids; (2b) In addition, 1 Ceres has very small (< 5 μm) grains on its surface, while the other asteroids have grains that are moderately small, but larger than on 1 Ceres. In either case, the observed spectral distinctiveness of 1 Ceres, the largest asteroid, suggests that the properties of an asteroid surface may be correlated with the asteroid size. © 2011. Astronomical Society of Japan.