INFRARED SPECTROSCOPY OF COMET 73P/SCHWASSMANN-WACHMANN 3 USING THESPITZER SPACE TELESCOPE

Abstract

We have used the Spitzer Space Telescope Infrared Spectrograph (IRS) to observe the 5-37 µm thermal emission of comet 73P/Schwassmann-Wachmann 3 (SW3), components B and C. We obtained low spectral resolution (R∼100) data over the entire wavelength interval, along with images at 16 and 22 µm. These observations provided an unprecedented opportunity to study nearly pristine material from the surface and what was until recently the interior of an ecliptic comet -cometary surface having experienced only two prior perihelion passages, and including material that was totally fresh. The spectra were modeled using a variety of mineral types including both amorphous and crystalline components. We find that the degree of silicate crystallinity, ∼35%, is somewhat lower than most other comets with strong emission features, while its abundance of amorphous carbon is higher. Both suggest that SW3 is among the most chemically primitive solar system objects yet studied in detail, and that it formed earlier or farther from the sun than the bulk of the comets studied so far. The similar dust compositions of the two fragments suggests that these are not mineralogically heterogeneous, but rather uniform throughout their volumes. The best-fit particle size distribution for SW3B has a form dn/da∼a −3.5 , close to that expected for dust in collisional equilibrium, while that for SW3C has dn/da∼a −4.0 , as seen mostly in active comets with strong directed jets such as C/1995 O1 Hale-Bopp. The total mass of dust in the comae plus nearby tail, extrapolated from to the field of view of the IRS peakup image arrays, is 3-5 x 10 8 kg for B and 7-9 x 10 8 1 Guest observer, Spitzer Space Telescope -3kg for C. Atomic abundances derived from the spectral models indicates a depletion of O compared to solar photospheric values, despite the inclusion of water ice and gas in the models. Atomic C may be solar or slightly sub-solar, but its abundance is complicated by the potential contribution of spectrally featureless mineral species to the portion of the spectra most sensitive to the derication of the C abundance. We find a relatively high bolometric albedo, ∼0.13 for the dust, considering the large amount of dark carbonaceous material, but consistent with the presence of abundant small particles and strong emission features.

Understanding measured water rotational temperatures and column densities in the very innermost coma of Comet 73P/Schwassmann–Wachmann 3 B

“…Some recent IR measurements of 73P/Schwassmann-Wachmann 3 using the Spitzer Space Telescope report a more precise dust content of the fragments B and C of the comet (Sitko et al, 2011). We arbitrarily chose the nature of the dirt to be magnetite but using another species would not have critically affected the results.…”

Mid-infrared spectra of comet nuclei

“…In addition to Hale-Bopp and Schwassmann-Wachmann 1, we selected comets from the literature that were observed with Spitzer /IRS at high signal-to-noise ratios: Tempel 1 (pre-Deep Impact experiment, nucleus removed; Lisse et al 2006, Kelley andWooden 2009), 17P/Holmes (post-megaoutburst; Reach et al 2010), 73P/Schwassmann-Wachmann 3 fragments B and C (Sitko et al 2011), and Oort cloud comet C/2007 N3 (Lulin) (Woodward et al 2011). The Schwassmann-Wachmann 1 spectrum is based on our own processing and spectral extraction, following the methods of Kelley et al (2006).…”

“…We examined the mineral decompositions of the spectra of comets Holmes, Hale-Bopp, Schwassmann-Wachmann 1, Schwassmann-Wachmann 3, and Lulin (Harker et al 2002, Reach et al 2010, Sitko et al 2011, Woodward et al 2011, Schambeau et al 2015 Wooden et al (1999) indicates the shape of the short-wavelength shoulder is controlled by the relative amounts of amorphous and crystalline pyroxene (see their Figs. 5, 6, and 7), but in other comets the coma grain size distribution must also play a role.…”

Infrared Spectroscopy of HR 4796A's Bright Outer Cometary Ring + Tenuous Inner Hot Dust Cloud

“…The cometary nuclei sources have been actively outgassing, driven by either water ice crystallization or CO 2 sublimation, for ~ 1 Myr. What is left over is similar to dust emitted from comets in our Solar System, i.e., darkish, red, and made up of devolatilized, large, porous, fluffy dust aggregates composed of amorphous carbon, ferromagnesian silicates, and ferromagnesian sulfides (Lisse et al 2006(Lisse et al , 2007Sitko et al 2011, Egrand et al 2016). This composition is consistent with the findings of Li & Lunine (2003), Koheler et al (2008), Nilsson et al (2010, Rodigas et al (2015), and Milli et al (2017).…”