Discovery of Crystalline Silicates in Comet C/2001 Q4 (NEAT)

Abstract

We present the 10 μm silicate feature of the dynamically new Oort Cloud comet C/2001 Q4 (NEAT) 5 days prior to perihelion (rh=0.97 AU, Δ=0.35 AU, 2004 May 11.25 and 11.30 UT) observed with the NASA Ames HIFOGS spectrophotometer. The silicate feature of comet Q4 contains strong crystalline peaks at 10.0 and 11.2 μm, along with weaker peaks at 9.3, 10.5, and 11.8 μm, which are characteristic of crystalline olivine and crystalline orthopyroxene. The relative heights of the resonant peaks as well as the shape of the silicate feature in comet Q4 is the same as in comet C/1995 O1 (Hale-Bopp) preperihelion (rh=1.21 AU). Thermal emission modeling shows Q4 and Hale-Bopp have similar relative abundances of the silicate minerals and high silicate crystalline-to-amorphous ratios. The silicate-to-amorphous carbon ratio derived for comet Q4, however, is lower than in Hale-Bopp and varies by a factor of ~2 in 2 hr, potentially sampling material from different jets in the coma. Owing to the similarity in the silicate mineralogy between Q4 and Hale-Bopp, either these two icy planetesimals formed in the same regime or crystalline silicates were widely distributed within the comet-forming zone.