Investigation into the disparate origin of CO2 and H2O outgassing for Comet 67/P

Abstract We present an investigation of the emission intensity of CO 2 and H 2 O and their distribution in the coma of 67P/ Churyumov–Gerasimenko obtained by the VIRTIS-M imaging spectrometer on the Rosetta mission. We analyze 4 data cubes from Feb. 28, and 7 data cubes from April 27, 2015. For both data sets the spacecraft was at a sufficiently large distance from the comet to allow images of the whole nucleus and the surrounding coma. We find that unlike water which has a reasonably predictable behavior and correlates well with the solar illumination, CO 2 outgasses mostly in local regions or spots. Furthermore for the data on April 27, the CO 2 evolves almost exclusively from the southern hemisphere, a region of the comet that has not received solar illumination since the comet's last perihelion passage. Because CO 2 and H 2 O have such disparate origins, deriving mixing ratios from local column density measurements cannot provide a meaningful measurement of the CO 2 /H 2 O ratio in the coma of the comet. We obtain total production rates of H 2 O and CO 2 by integrating the band intensity in an annulus surrounding the nucleus and obtain pro-forma production rate CO 2 /H 2 O mixing ratios of ∼5.0% and ∼2.5% for Feb. 28 and April 27, respectively. Because of the highly variable nature of the CO 2 evolution from the surface we do not believe that these numbers are diagnostic of the comet's bulk CO 2 /H 2 O composition. We believe that our investigation provides an explanation for the large observed variations reported in the literature for the CO 2 /H 2 O production rate ratios. Our mixing ratio maps indicate that, besides the difference in vapor pressure of the two gases, this ratio depends on the comet's rotational orientation combined with its complex geometric shape which can result in quite variable rates of erosion for different surface areas such as the northern and southern hemisphere. Our annulus measurement for the total water production for Feb. 28 at 2.21AU from the Sun is 2.5 × 10 26 molecules/s while for April 27 at 1.76 AU it is 4.65 × 10 26 . We find that about 83% of the H 2 O resides in the illuminated portion of our annulus and about 17% on the night side. We also make an attempt to obtain the fraction of the H 2 O production coming from the highly active neck of the comet versus the rest of the illuminated surface from the pole-on view of Feb. 28 and estimate that about 60% of the H 2 O derives from the neck area. A rough estimate of the water surface evaporation rate of the illuminated nucleus for April 27 yields about 5 × 10 19 molecules/s/m 2 . Spatial radial profiles of H 2 O on April 27 on the illuminated side of the comet, extending from 1.78 to 6.47 km from the nucleus center, show that water follows model predictions quite well, with the gas accelerating as it expands into the coma. Our dayside radial profile allows us to make an empirical determination of the expansion velocity of water. On the night side the spatial profile of water follows 1/ ρ . The CO 2 profiles do not exhibit any acceleration into the coma but are closely matched by a 1/ ρ profile.