A New Analysis of Spitzer Observations of Comet 29P/Schwassmann-Wachmann 1

Abstract

We present a new analysis of $Spitzer$ observations of comet 29P/Schwassmann-Wachmann 1 taken on UT 2003 November 21, 23, and 24, similar to a previous investigation of the observations (Stansberry et al. 2004), but using the most recent $Spitzer$ data pipeline products and intensive image processing techniques. Analysis of images from the IRAC 5.8 & 8.0 $\mu$m bands and the MIPS 24.0 & 70.0 $\mu$m bands resulted in photometry measurements of the nucleus after a suite of coma modeling and removal processes were implemented. SW1 was not identified in the 5.8 $\mu$m image from the previous work so its incorporation into this analysis is entirely new. Using the Near Earth Asteroid Thermal Model (Harris 1998) resulted in a nucleus radius measurement of $R$ = 30.2 ${}_{-2.9}^{+3.7}$ km and an infrared beaming parameter value of $\eta =0.99$ ${}_{-0.19}^{+0.26}$. We also measured an infrared geometric albedo, $p_{5.8}$ = 0.5 $\pm$ 0.5. Extrapolating a 0.04 V-band albedo and using a normalized reflectivity gradient $S^{\prime }=14.94\pm 1.09$ [% (1000 Å)${}^{-1}$] (Duffard et al. 2014) we recover an infrared albedo of $p_{5.8}$ = 0.31 in the near infrared consistent with the value recovered from thermal modeling. The dust composition extracted from IRS spectra are very comet-like, containing mainly amorphous ferromagnesian silicates (but with a minority of crystalline silicates as well), water ice, and metal sulfides.

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