MID-INFRARED SPECTROPHOTOMETRIC OBSERVATIONS OF FRAGMENTS B AND C OF COMET 73P/SCHWASSMANN-WACHMANN 3

We present mid-infrared spectra and images from the Gemini-N (+ Michelle) observations of fragments SW3-[B] and SW3-[C] of the ecliptic (Jupiter family) comet 73P/Schwassmann-Wachmann 3 pre-perihelion. We observed fragment B soon after an outburst event (between 2006 April 16–26 UT) and detected crystalline silicates. The mineralogy of both fragments was dominated by amorphous carbon and amorphous pyroxene. The grain size distribution (assuming a Hanner-modified power law) for fragment SW3-[B] has a peak grain radius of ap ∼ 0.5 μm, and for fragment SW3-[C], ap ∼ 0.3 μm; both values are larger than the peak grain radius of the size distribution for the dust ejected from ecliptic comet 9P/Tempel 1 during the Deep Impact event (ap = 0.2 μm). The silicate-to-carbon ratio and the silicate crystalline mass fraction for the submicron to micron-sized portion of the grain size distribution on the nucleus of fragment SW3-[B] were 1.341+0.250−0.253 and 0.335+0.089−0.112, respectively, while on the nucleus of fragment SW3-[C] they were 0.671+0.076−0.076 and 0.257+0.039−0.043, respectively. The similarity in mineralogy and grain properties between the two fragments implies that 73P/Schwassmann-Wachmann 3 is homogeneous in composition. The slight differences in grain size distribution and silicate-to-carbon ratio between the two fragments likely arise because SW3-[B] was actively fragmenting throughout its passage while the activity in SW3-[C] was primarily driven by jets. The lack of diverse mineralogy in the fragments SW3-[B] and SW3-[C] of 73P/Schwassmann-Wachmann 3 along with the relatively larger peak in the coma grain size distribution suggests that the parent body of this comet may have formed in a region of the solar nebula with different environmental properties than the natal sites where comet C/1995 O1 (Hale–Bopp) and 9P/Tempel 1 nuclei aggregated.