The Nature and Frequency of the Gas Outbursts in Comet 67p/Churyumov–gerasimenko Observed by the Alice Far-Ultraviolet Spectrograph on Rosetta

Abstract

Alice is a far-ultraviolet imaging spectrograph onboard Rosetta that, amongst multiple objectives, is designed to observe emissions from various atomic and molecular species from within the coma of comet 67P/Churyumov-Gerasimenko. The initial observations, made following orbit insertion in August 2014, showed emissions of atomic hydrogen and oxygen spatially localized close to the nucleus and attributed to photoelectron impact dissociation of H 2 O vapor. Weaker emissions from atomic carbon were subsequently detected and also attributed to electron impact dissociation, of CO 2 , the relative H I and C I line intensities reflecting the variation of CO 2 to H 2 O column abundance along the line-of-sight through the coma. Beginning in mid-April 2015, Alice sporadically observed a number of outbursts above the sunward limb characterized by sudden increases in the atomic emissions, particularly the semi-forbidden O I λ1356 multiplet, over a period of 10-30 minutes, without a corresponding enhancement in long wavelength solar reflected light characteristic of dust production. A large increase in the brightness ratio O I λ1356/O I λ1304 suggests O 2 as the principal source of the additional gas. These outbursts do not correlate with any of the visible images of outbursts taken with either OSIRIS or the navigation camera. Beginning in June 2015 the nature of the Alice spectrum changed considerably with CO Fourth Positive band emission observed continuously, varying with pointing but otherwise fairly constant in time. However, CO does not appear to be a major driver of any of the observed outbursts.

Comet 67P outbursts and quiescent coma at 1.3 au from the Sun: dust properties from Rosetta/VIRTIS-H observations

“…Grün et al (2016) review observations by nine Rosetta instruments of a dusty outburst which occurred on 19 February 2016, when the comet was at 2.4 AU from the Sun. Gaseous outbursts with no dust counterparts were also observed (Feldman et al 2016).…”

Ingredients for solar-like systems: protostar IRAS 16293-2422 B versus comet 67P/Churyumov–Gerasimenko

“…Thanks to the continuous monitoring of the target by the orbiter, it was realized that there is much variability in the outgassing of ices hidden underneath the surface, which has to do with seasonal and diurnal variations as probed with Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), Microwave Instrument for the Rosetta Orbiter (MIRO) and Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) instruments aboard Rosetta (e.g., De Sanctis et al 2015;Lee et al 2015;Bockelée-Morvan et al 2015b;Biver et al 2015;Luspay-Kuti et al 2015;Hässig et al 2015;Filacchione et al 2016a,b;Fornasier et al 2016;Hansen et al 2016;Bockelée-Morvan et al 2016;Barucci et al 2016;Migliorini et al 2016;Gasc et al 2017;Marshall et al 2017;Filacchione et al 2019). The bi-lobate geometry of the nucleus and the associated self-shielding, its changing rotational period, backfall of granular material, short-lived outbursts, active sinkhole pits and orbital trajectory change the irradiance of its surface during a single apparition (Keller et al 2015;Vincent et al 2015Vincent et al , 2016Feldman et al 2016;Keller et al 2017;Kramer et al 2018), but also in the long term upon repeated approaches to the Sun. Nevertheless, it is possible to extract bulk abundances of the interior ices upon careful data analysis (e.g., Calmonte et al 2016) and to peek at them on special occasions such as cliff collapses (Pajola et al 2017).…”

H2O and O2 absorption in the coma of comet 67P/Churyumov–Gerasimenko measured by the Alice far-ultraviolet spectrograph on Rosetta

“…These stellar sight lines allow the coma of 67P/C-G to be studied in far-UV absorption, where column densities can be measured directly. Alice's previous characterizations of the coma of 67P/C-G have primarily used emission lines from CO and atomic hydrogen, oxygen, carbon, and sulphur (e.g., Feldman et al 2015Feldman et al , 2016. While the strengths of these emission lines can only be used to derive molecular column densities under specific assumptions (i.e., pure resonance fluorescence), the ratios of strong, commonly observed lines can be diagnostic of physical conditions in the coma.…”