Post-perihelion observations of water in comet Halley

Abstract

Our previous pre-perihelion observations¹ of comet Halley from the NASA Kuiper Airborne Observatory (KAO) resulted in the first definite detection of water vapour in a comet. This success allowed us to carry out an ambitious post-perihelion KAO observing programme which significantly extended the scope of our Halley water investigation. The spectral bandwidth of the post-perihelion data included essentially all of the ν3 band. Fifteen spectral lines of water were detected, including three previously undetected lines in the ν3 band (A ≃ 2.65 μm) and three lines in the (011–010) band. The observed relative intensities illustrate that water in the cometary coma is rotationally relaxed, as predicted by recent nonthermal-equilibrium models^2,3. An ortho/para ratio of ∼3 is consistent with both pre- and post-perihelion data. The water spatial brightness profile can be fit by that of a parent molecule, and a significant sunward-tailward brightness asymmetry was observed which suggests ejection primarily into the sunlit hemisphere. Comet Halley exhibited marked temporal activity on timescales as short as 2 h. The largest water production rate measured was 2.3 × 10³⁰ mol s⁻¹ on 24:18ut March 1986, which is about an order of magnitude (or more) larger than the production rates we measured pre-perihelion at the same heliocentrie distance (∼1 AU), indicating a large pre- to post-perihelion asymmetry in gas production.