Gas-Dust Coma Dynamics of Comet 67P/Churyumov-Gerasimenko during its 2021 Perihelion via VLT/MUSE

Abstract:

Comet 67P/Churyumov-Gerasimenko, hereafter referred to as 67P, reached perihelion at 1.21 au on 12 November 2021. We observed  it over 15 observational epochs pre- and post-perihelion, using the Multi-Unit Spectroscopic Explorer (MUSE) integral field unit spectrograph, mounted at the Very Large Telescope (VLT). These observations captured the evolving gas-dust coma in sub-arcsecond detail, across a wide field of view, roughly 55''x55'', at 0.2''/pixel (~60-360km/pixel) sampling. We probed the inner, middle, and outer coma (ρ=10³-10⁵ km) from 2.25 au on the first observation in May 2021, to 1.28 au near-perihelion in late September 2021, and to 1.9 au post-perihelion by March 2022. Here, we present a full morphological and spectroscopic analysis of the gas-dust coma, including dust colour maps, molecular coma species maps, and gas production pathways, with clear detections of strong emissions from C₂, NH₂, and CN gas species. 

Introduction:

Visited by the ROSETTA orbiter, comet 67P is among the most well-characterized small-bodies in the Solar System and is a prime target for long-term evolutionary monitoring. The nucleus is bilobate in structure, joined by a thick neck about which the comet rotates at a 58° obliquity, which causes strong seasonal variability (Motolla et al. 2014). The two lobes, and neck, host a diverse range of morphological features and units, likely created by prolonged activity. As shown by Sierks et al. (2015), the smooth northern region of the neck, designated Hapi, is responsible for the majority of activity on the comet, however, smaller active regions are also seen throughout the northern hemisphere. Seasonal and diurnal effects further influence this activity. As 67P approaches perihelion, the subsolar point migrates to the southern sky after equinox. It illuminates previously shaded reservoirs of icy grains and material, which instigates more energetic activity and new volatile sublimation trends, not unlike like the twin outbursts recorded on 29 October and 17 November 2021 (Sharma et al. 2021) and enhanced CN coma (Opitom et al. 2019). Diurnal changes in insolation primarily influence volatile sublimation due to thermal lag into the subsurface reservoirs. Water ice is most strongly affected by thermal lag, as deposits can be as shallow as a few mm (Marboeuf et al. 2014), while more volatile species, like CO₂ and CO, are more heavily insulated by their greater depth (Hässig et al. 2015). With this context, we aim to analyse our MUSE dataset by isolating the dust, C₂, NH₂, [OI], and CN spectral features in our datacubes, from which we can probe the majority of the coma, its trends, and relations to the nucleus around 67P's 2021 perihelion.

Observing:

We observed 67P with the MUSE integral field unit (IFU) spectrograph, which collects three-dimensional datacubes, comprising two spatial and one spectral dimension (x,y,λ). These ground-based MUSE observations were collected over almost a year, from 15 May 2021 to 09 March 2022, as a part of ESO programmes 105.2086.001 and 108.223B.001. We utilised MUSE in the wide field mode (WFM) without adaptive optics, covering 4800 to 9300Å with an average resolving power of 3000 (Bacon et al. 2010). We exposed MUSE for 600 seconds using non-sidereal tracking and ensured comprehensive coverage through dithering and rotating observations by 90° from North to East between exposures, to minimize detector artefacts. The sky observations were positioned 5 to 10 arcminutes away from the system to ensure no contamination from the diffuse coma. We also conducted standard star observations for flux calibrations. The complete dataset (67P, sky, standard star) was reduced with the ESO MUSE Pipeline (Wielbacher et al. 2020) and ESO Molecfit Package (Smette et al. 2015).

Analysis:

To understand the dynamics of the gas and dust in the coma, we first reduced all datacubes and corrected for telluric absorption on a nightly basis, which significantly improved our detections of the C₂ (0,0) Swan band (~5100Å), NH₂ bands (~6000-7400Å), and the CN (1,0) Red band (~9100-9300Å). We ensured a satisfactory fit, and proceeded with the isolation of gas species via a dust and solar continuum removal technique described in Opitom et al. (2019). We utilized a reference 67P MUSE spectrum in which no gas species were detected to fit and subtract the dust continuum from each spectrum in our datacubes, iteratively. We used these dust-free cubes to isolate C₂, NH₂, and CN emissions and create maps of the molecular coma for all dates, sampled in Figure 1. We employ azimuthal median enhancement to our maps to enhance the substructure of each gas component. 

Fig. 1: Top panels: continuum-subtracted spectrum from 40'' circular aperture around 67P, on 30 September 2021. Dashed-red line is ideal continuum subtraction, solid black line is gas emission, solid red region is flux used to create maps. Lower panels: species maps of 67P coma. Red X is comet optocenter, red arrow is comet rotation axis and North pole direction, yellow arrow is sun angle, green arrow is velocity angle. 

Throughout the campaign, we saw strong C₂ coma emissions, primarily pre-perihelion, and sometimes filling the entire FoV (ρ>5x10⁴km). NH₂ coma emissions were equally prominent and persisted after perihelion, and only began to weaken by our last observations in March 2022. Additionally, NH₂ emissions did not always share the same morphology and orientation as the C₂ and dust coma, perhaps hinting at the existence of extended sources via ammoniated icy grains - to be tested by Haser models. CN was not robustly detected before August 2021, which is congruent with the expected sublimation after the southern vernal equinox passage the month prior. Post-equinox, we saw the expansion of the CN coma, which lasted until a few months after perihelion. Finally, dust colour largely followed expected sorting trends, becoming bluer at larger cometocentric distances. Further modelling to derive species scale lengths, production rates, and relations to the nucleus are underway.