Water and carbon dioxide distribution in the 67P/Churyumov-Gerasimenko coma from VIRTIS-M infrared observations

Abstract

Context. Studying the coma environment of comet 67P/Churyumov-Gerasimenko (67P) is one of the primary scientific goals of the VIRTIS experiment on the ESA Rosetta mission. Aims. The distribution and variability of water vapour and carbon dioxide in the comet's coma are needed to estimate their production rate, abundances in the nucleus, and the spatial distribution of the active regions. Methods. Infrared emission lines from vibrational bands of water and carbon dioxide at 2.67 and 4.27 µm, respectively, were observed by the VIRTIS-M imaging channel and mapped from close to the nucleus up to ∼10 km altitude with a resolution of ∼40 m/px. A dataset consisting of 74 observations in the 1−5 µm spectral range acquired from 8 to 14 April 2015 when 67P was at a heliocentric distance of 1.9 AU is analysed in this work. A statistical correlation between the gas distribution and the surface's active regions was performed. Results. The maximum H 2 O emission is observed within 3 km from the nucleus and is mainly concentrated above two active regions, Aten-Babi and Seth-Hapi, while the CO 2 distribution appears more uniform with significant emissions coming from both the "head" and southern latitude regions. In the equatorial region, the column densities of both species decrease with altitude, although CO 2 decreases more rapidly than H 2 O. The calculated CO 2 /H 2 O column density ratios above Aten-Babi and Seth-Hapi are 2.4 ± 0.6% and 3.0 ± 0.7%, respectively. A value equal to 3.9 ± 1.0% is observed at equatorial latitudes in the region encompassing Imothep. Conclusions. VIRTIS-M has mapped the distribution of water vapour and carbon dioxide around the nucleus of 67P with unprecedented spatial resolution. The different water and carbon dioxide outgassing above the surface, seen in the VIRTIS-M data, might be indicative of a different thermal history of the northern and southern hemispheres of 67P.

Seasonal erosion and restoration of the dust cover on comet 67P/Churyumov-Gerasimenko as observed by OSIRIS onboard Rosetta

“…While it is difficult to exclude definitively any possibility, we consider it unlikely for super-volatiles to be the main driver of the observed changes. The observational evidence thus far indicates higher abundance of CO 2 from the unilluminated southern heminucleus (Hässig et al 2015) and, in particular, its depletion in Ma'at, Ash, Seth(-Hapi), and (Aten-)Babi regions in the northern hemi-nucleus where surface changes were detected (Bockelée-Morvan et al 2015;Fink et al 2016;Migliorini et al 2016). Podolak et al (2016) suggested that the measured CO 2 production rate, a few percent of water production, is insufficient to account for the typical speed of dust grains at meters per second.…”

“…Heat propagation to x > 0.5 m would take longer than the time frame of surface changes, in which case, the observed changes would imply a (spatial) distribution of super-volatile ices in deeper subsurface. This scenario is contested by observations, if not excluded (Bockelée-Morvan et al 2015;Fink et al 2016;Migliorini et al 2016). …”

Production of O2 through dismutation of H2O2 during water ice desorption: a key to understanding comet O2 abundances

“…On the surface and in the depth of cometary nuclei, there are huge gradients of temperature, and the desorption of water or other molecules is spatially inhomogeneous and subject to diurnal variation (e.g. Bockelée-Morvan et al 2015;Lee et al 2015;Le Roy et al 2015;Migliorini et al 2016). Topological and geological diversity makes a comet a very rich but inhomogeneous medium.…”

Properties of the dust in the coma of 67P/Churyumov-Gerasimenko observed with VIRTIS- M

“…A detailed analysis of the CO 2 outgassing has been done in Fink et al (2016) and will not be reported here. As shown in that paper and in the publications of Migliorini et al (2016) and Fougere et al (2016), CO 2 does not come from the illuminated northern hemisphere but rather from the dark but more primitive southern hemisphere. While the maximum of CO 2 in all this data set appears above the head region, we note that when we used the comet viewing program by Bieler to look down on the comet from the direction of the Sun, the CO 2 emissions along the LOS appear to originate from the terminator region separating the northern and southern hemispheres.…”

“…In their work, the CO 2 -to-H 2 O column density ratio varies from 2 to 30 per cent, depending on the region observed. Migliorini et al (2016) reported that the H 2 O emission is mainly concentrated above two active regions, while the CO 2 distribution appears more uniform with significant emission coming both from the head and from the south latitude regions. In a comprehensive paper, Fink et al (2016) presented an investigation of the emission intensity of CO 2 and H 2 O and their distribution in the coma of 67P/CG.…”