Spitzer-IRS spectral maps of 67P/Churyumov-Gerasimenko

Comets formed from the material surrounding the young Sun, about 4.6 billion years ago. After their formation, the nuclei were scattered and stored in their current reservoirs [1,2], where they remained mainly unaltered, preserving most of the chemical and physical properties linked to their formation site.

Cometary nuclei contain a mix of various silicates (mainly olivine and pyroxene that can be found in the amorphous and/or crystalline form), organic refractory material and other minor elements [3]. Comet dust appears different from comet-to-comet, and partially different from the one observed in the interstellar medium [4]. These differences may correlate in principle with some of the processes that were in act in the protoplanetary disc at the time and in the region where the nuclei formed. A detailed study of the cometary dust can thus improve our understanding of the formation and evolution of matter in the early Solar System, and test the most recent theoretical models of cometary nuclei [5].

Mid-infrared spectroscopy in the wavelength range between 7 to 40 μm can give information on the temperature and chemical compostition of the dust, and help inferring dust grains size and structure [3,4,6]. Spectral maps of active comets in this spectral range allow measurements of the chemical and temperature variations within a large portion of the coma. If observed, these fluctuations can be in principle associated to different processes (e.g., fragmentation processes), and can help to put some constraints on the physical properties of the observed dust, more stringent than the ones retrieved just observing in proximity of the nucleus.

The coma of 67P was mapped with the InfraRed Spectrometer (IRS) on board of the Spitzer telescope between the 28^thand 30^thof November, 2008. At the observing epochs, the comet was at about 1.65 au from the Sun and about 1.00 au from the spacecraft. The maps were taken using both the short-wavelength (7-14 μm) and the long-wavelength (14-38 μm) IRS spectrometers.

We reduced the data using the official pipeline CUBISM [7] that performs all the basic reduction and calibration steps (i.e., background subtraction, bad pixels removal, flux calibration for extended sources comprising aperture correction and slit loss correction). The spectral cubes produced by CUBISM were re-sampled to eliminate some residual aliasing effects resulting from the reduction process, and spectral cubes from different settings (short and long wavelengths) were combined together. In this way, we were able to extract the full 7-38 μm spectrum for each pixel in a FOV of about 27”x38”.

In general, we observe smoothed spectra and no signs of crystalline silicates; we identify clearly an amorphous silicate band around 11 μm. Smoothed spectra could be indicative of a low abundance of crystalline silicates or related to the presence of larger and/or less porous dust grains in the coma. The thermal analysis of the extracted spectra, performed with the model developed by [8], do no show any particular temperature variation, nor visible signs of fragmentation. Finally, we investigated the coma morphology, searching for the presence of jets and/or anisotropies.

Acknowledgements: This work has received funding from the European Research Council (ERC) - program grant CAstRA, agreement ID 757390. Data were downloaded from the Spitzer Heritage archive, and collected under the program ID 50022, ’Creating Synergy Between Spitzer and Rosetta: The Coma of Comet 67P’, PI - M. Kelley.

References: [1]. Gomes, R., et al., 2005, Nature, 435, 446 – [2]. Morbidelli, A., et al., 2007, AJ, 134, 1790 – [3]. Hanner, M. S., 1999, Space Science Reviews, 90, 99 – [4]. Wooden, D. H., et al., 2017, Phil. Trans. R. Soc. A 375 – [5]. Ciarniello, M. et al., 2022, Nat. Astron., 262 – [6]. Kelley, M. S. & Wooden, D. H., 2009, Planetary and Space Science, 57, 1133, and ref. therein – [7]. Smith, J. D. T., et al., 2007, PASP, 119.1133S – [8]. Markkanen, J. & Agarwal, J., 2020, A&A, 643, A16.