FUV Spectral Signatures of Molecules and the Evolution of the Gaseous Coma of Comet 67P/Churyumov–Gerasimenko

Abstract

The Alice far-ultraviolet imaging spectrograph onboard Rosetta observed emissions from atomic and molecular species from within the coma of comet 67P/Churyumov-Gerasimenko during the entire escort phase of the mission from 2014 August to 2016 September. The initial observations showed that emissions of atomic hydrogen and oxygen close to the surface were produced by energetic electron impact dissociation of H 2 O. Following delivery of the lander, Philae, on 2014 November 12, the trajectory of Rosetta shifted to near-terminator orbits that allowed for these emissions to be observed against the shadowed nucleus that, together with the compositional heterogeneity, enabled us to identify unique spectral signatures of dissociative electron impact excitation of H 2 O, CO 2 , and O 2 . CO emissions were found to be due to both electron and photoexcitation processes. Thus we are able, from far-ultraviolet spectroscopy, to qualitatively study the evolution of the primary molecular constituents of the gaseous coma from start to finish of the escort phase. Our results show asymmetric outgassing of H 2 O and CO 2 about perihelion, H 2 O dominant before and CO 2 dominant after, consistent with the results from both the in situ and other remote sensing instruments on Rosetta.

Long-term monitoring of the outgassing and composition of comet 67P/Churyumov-Gerasimenko with the Rosetta/MIRO instrument

“…23, the CO/H 2 O ratios measured by MIRO (r h < 2.4 AU) are all below 1%. Measurements with the Alice instrument shortly after perihelion lead to CO/H 2 O ∼ 0.5% (Feldman et al 2018), in agreement with MIRO determinations. A low (<1%) CO/H 2 O abundance near perihelion is also consistent with the upper limit measured from infrared spectra obtained by VIRTIS (Bockelée-Morvan et al 2016).…”

A photochemical model of ultraviolet atomic line emissions in the inner coma of comet 67P/Churyumov-Gerasimenko

“…Our calculated volume emission rates clearly show that photodissociation of H 2 O contributes to the total H I Lyman-β emission by an order of magnitude higher compared to solar resonance fluorescence close to the cometary surface (see Figure 3). Feldman et al (2018) also found that the contribution from resonance scattering for H I Lyman-β emission is negligible in the Alice observations. It should be noted that the Rosetta was orbiting at radial distances around 90 km on 2015 March 29 while Alice was making nadir observations, hence the contribution from solar resonance fluorescence in the nadir observed HI Lyman-β intensity can be neglected.…”

“…This calculation show that resonant scattering of atomic oxygen can also determine the oxygen emission ratio when substantial atomic oxygen is present in the inner coma. Feldman et al (2018) also noticed in the limb observation around the perihelion made on 2015 Aug 19 that resonance fluorescence is important excitation source in producing OI 1304 Å emission. Global kinetic models such as Tenishev et al (2008), which accounts for strong momentum exchange between hot oxygen atoms to surrounding water molecules, can determine accurate atomic oxygen densities close to the nucleus which is not possible by our one-dimensional approach.…”

“…In all these cases, electron impact dissociative excitation processes are negligible (<1%) in determining O I emission intensities. Table 2 of Feldman et al, 2018) and Cal. is model calculated emission intensities;…”

“…Similarly, as the comet was moving towards the Sun, the wide Angle camera of Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) and Alice onboard Rosetta instruments also observed the changes in the intensities of various spectral emissions of cometary species (Bodewits et al, 2016;Feldman et al, 2018). By analysing the Alice observed far-ultraviolet spectra, Feldman et al (2018) have studied the evolution of cometary coma and suggested that various emission features of 67P/C-G are majorly driven by electron impact dissociative excitation processes. When the comet was moving towards the Sun from ∼2 to 1.5 AU, Bodewits et al (2016) have noticed that the OSIRIS observed emission intensities of various species are decreased though the gas production rate was increasing in this period.…”