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

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 from the surface we do not believe that these numbers are diagnostic of the comet's 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 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 × 10²⁶ molecules/s while for April 27 at 1.76 AU it is 4.65 × 10²⁶. We find that about 83% of the H₂O 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 H₂O 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 H₂O derives from the neck area. A rough estimate of the water surface evaporation rate of the illuminated nucleus for April 27 yields about 5 × 10¹⁹ molecules/s/m².

Spatial radial profiles of H₂O 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 CO₂ profiles do not exhibit any acceleration into the coma but are closely matched by a 1/ρ profile.