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

TL;DR

This paper reanalyzes Spitzer observations of comet 29P/Schwassmann-Wachmann 1 using updated data processing, revealing new nucleus measurements and detailed dust composition analysis, improving understanding of this comet's properties.

Contribution

It provides a new analysis with advanced image processing, incorporating previously unobserved data, and offers refined measurements of the comet's nucleus and dust composition.

Findings

Nucleus radius measured at 30.2 km

Infrared beaming parameter of 0.99

Dust mainly composed of amorphous ferromagnesian silicates

Abstract

We present a new analysis of ${\it Spitzer}$ 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 ${\it Spitzer}$ 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 $^{+3.7}_{-2.9}$ km and an infrared beaming parameter value of $\eta = 0.99$ $^{+0.26}_{-0.19}$. 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 $S' = 14.94 \pm 1.09$ [% (1000 \r{A})$^{-1}$] (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.