The heterogeneous coma of comet 67P/Churyumov-Gerasimenko as seen by ROSINA: H₂O, CO₂, and CO from September 2014 to February 2016

Abstract

Context. The ESA Rosetta mission has been investigating the environment of comet 67P/Churyumov-Gerasimenko (67P) since August 2014. Among the experiments on board the spacecraft, the ROSINA experiment (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) includes two mass spectrometers to analyse the composition of neutrals and ions and a COmet Pressure Sensor (COPS) to monitor the density and velocity of neutrals in the coma. Aims. We study heterogeneities in the coma during three periods starting in October 2014 (summer in the northern hemisphere) and ending in February 2016 (end of winter in the northern hemisphere). We provide a detailed description of the main volatiles dynamics (H 2 O, CO 2 , CO) and their abundance ratios. Methods. We analysed and compared the data of the Reflectron-type Time-Of-Flight (RTOF) mass spectrometer with data from both the Double Focusing Mass Spectrometer (DFMS) and COPS during the comet escort phase. This comparison has demonstrated that the observations performed with each ROSINA sensor are indeed consistent. Furthermore, we used a Direct Simulation Monte Carlo (DSMC) model to compare modelled densitites with in situ detections. Results. Our analysis shows how the active regions of the main volatiles evolve with the seasons with a variability mostly driven by the illumination conditions; this is the case except for an unexpected dichotomy suggesting the presence of a dust layer containing water deposited in the northern hemisphere during previous perihelions hiding the presence of CO 2 . The influence of various parameters is investigated in detail: distance to the comet, heliocentric distance, longitude and latitude of sub-satellite point, local time, and phase angle.

Surface localization of gas sources on comet 67P/Churyumov-Gerasimenko based on DFMS/COPS data

“…The localization of the most active emitting areas in Sect. 5 is in good agreement to Hoang et al (2017) and Kramer et al (2017). This activity pattern shows a high correlation (0.7) to active gas emitters with short living dust locations derived from OSIRIS and NAVCAM images by Vincent et al (2016).…”

“…4 in Kramer et al (2017), which shows our inverse model data on a 100 km surface. As suggested by VIRTIS-H observations in Bockelée-Morvan et al (2016), by modeling results in Fougere et al (2016b) and Hoang et al (2017), CO2, CO are decoupled from H2O at the time before inbound equinox. This matches our observation in interval A, that CO2 and CO are mainly located in the southern hemisphere, while H2O originates from the northern hemisphere.…”

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

“…Gasc et al (2017) reported a NH 3 /H 2 O value as low as 0.04% for 2.0-2.7 AU postperihelion, whereas the MIRO average value for this period is 0.1%. The CO/H 2 O ratio derived from ROSINA instruments ranges from ∼1 to ∼50%, with strong local and temporal variations (Le Roy et al 2015;Fougere et al 2016;Hoang et al 2017;Gasc et al 2017). A steep decrease from 3.6 to 2.6 AU inbound was observed before it stabilized at a value of about 5% (Fougere et al 2016).…”

Observations of a mix of cold and warm electrons by RPC-MIP at 67P/Churyumov-Gerasimenko

“…As the total electron plasma frequency resonance in anti-phased SDL sub-mode can not always be identified, the resonance located close to the cold plasma frequency could sometimes be identified as the main resonance which thus leads in some cases to an overestimation of the total plasma density (around 20%). Moreover, the total electron density increased close to the perihelion due to the increase of outgassing activity (Hoang et al 2017;Héritier et al 2018). Assuming that the temperature of the electrons is constant (Myllys et al 2019), the Debye length decreases close to the perihelion thus enabling RPC-MIP to detect more easily the total electron plasma frequency.…”

“…As a comet nucleus approaches the Sun, the solar thermal forcing increases and the cometary volatiles sublimate so that the outgassing activity of neutral particles increases (e.g., Hansen et al 2016). A cometary atmosphere, mainly composed of water, carbon monoxide and carbon dioxide (Gasc et al 2017;Hoang et al 2017), expands because of the low cometary gravity. This atmosphere gets ionized through different mechanisms (Cravens et al 1987;Galand et al 2016;Héritier et al 2017Héritier et al , 2018: (i) by photoionisation by extreme ultra violet solar flux, (ii) by electron-impact ionisation by energetic electrons or, (iii) by solar wind impact ionisation and charge exchange, to form a cometary ionosphere that eventually interacts with the surrounding solar wind plasma.…”