3-13 Micron Spectroscopy of Comet 73P/Schwassmann- Wachmann 3 M.L. Sitko (Space Science Inst. & Univ. Of Cincinnati) D.K. Lynch, R.W. Russell (The Aerospace Corp.) W.J. Carpenter, R.L. Kimes (Univ. of Cincinnati) E.F. Polomski (Univ. of Minnesota)

Observing recently-fragmented comets allows us to observe material once hidden beneath their surfaces The two brightest components of the fragmenting Comet Schwassmann-Wachmann 3 were observed using the IRTF with The Aerospace Corporation’s Broad-band Array Spectrograph System (BASS) on May 18 & 19, 2006 (UT)

The 2 brightest components, B & C, exhibited significant variability over time scales of hours Component C underwent an 80% increase in 2 hours on May 19, presumably due to either more fresh surface being exposed to the Sun by rotation of the nucleus, or the occurrence of a fragmentation event

Both components exhibited a strong 10 micron silicate band of nearly identical structure. The strength of the band in SW3 indicates (as did the Deep Impact of Tempel 1) that Jupiter family comets are formed with small grains throughout their interiors, but that long-term loss of these grains occurs from their surfaces. In Oort cloud comets, this band can be very strong (twice the flux level of the continuum) or nearly absent. In Jupiter family comets, it is almost always weak.

The large wavelength coverage of BASS, and its ability to obtain precise photometric imaging at visible wavelengths, allows greater constraints to be placed on models than is possible with the 8-13 micron coverage of most mid-IR spectrographs. Above: unfiltered (false color) image of SW3-C obtained with the BASS guide camera. Right: BASS spectra and photometry obtained with the guide camera using narrow- amorphous carbonamorphous olivineamorphous pyroxenecrystalline olivine band filters (blue points), allow the spectral energy distribution to be determined from microns. The grain components include amorphous carbon, amorphous olivine, amorphous pyroxene, and crystalline olivine. The sum of these is the upper black curve. To this is added the scattered solar spectrum, normalized to the narrow-band photometry. The extension into the visible indicates a mean reflectivity of 13%, and decreasing toward shorter wavelengths.