Investigation into the disparate origin of CO 2 and H 2O outgassing for comet 67P

We present an investigation of the emission intensity of CO₂ and H₂O 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, CO₂ outgasses mostly in local regions or spots. Furthermore for the data on April 27, the CO₂ 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 CO₂ and H₂O have such disparate origins, deriving mixing ratios from local column density measurements cannot provide a meaningful measurement of the CO₂/H₂O ratio in the coma of the comet. We obtain total production rates of H₂O and CO₂ by integrating the band intensity in an annulus surrounding the nucleus and obtain pro-forma production rate CO₂/H₂O mixing ratios of ~5.0% and ~2.5% for Feb. 28 and April 27 respectively. Because of the highly variable nature of the CO₂ evolution we do not believe that these numbers are diagnostic of the comets bulk CO₂/H₂O composition. We believe that our investigation provides an explanation for the large observed variations reported in the literature for the CO₂/H₂O 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 geometric shape, illumination and past orbital history.Our annulus measurement for the total water production for Feb. 28 at 2.21AU from the sun is 2.5x10²⁶ molecules/s while for April 27 at 1.76 AU it is 4.65x10²⁶. We find that about 83% of the H₂O resides in the illuminated portion of our annulus and about 17% on the night side. A rough estimate of the water surface evaporation rate of the illuminated nucleus for April 27 yields about 5x10¹⁹ molecules/s m².