Detection of Water Vapor in Halley's Comet

Abstract

Gaseous, neutral H₂O was detected in the coma of comet Halley on 22.1 and 24.1 December 1985 Universal Time. Nine spectral lines of thev₃ band (2.65 micrometers) were found by means of a Fourier transform spectrometer (λ/▵λ ∼ 10⁵) on the NASA-Kuiper Airborne Observatory. The water production rate was ∼6 x 10²⁸ molecules per second on 22.1 December and 1.7 x 10²⁹ molecules per second on 24.1 December UT. The numbers of spectral lines and their intensities are in accord with nonthermal-equilibrium cometary models. Rotational populations are derived from the observed spectral line intensities and excitation conditions are discussed. The ortho-para ratio was found to be 2.66±0.13, corresponding to a nuclear-spin temperature of 32 K (+5 K, -2 K), possibly indicating that the observed water vapor originated from a low-temperature ice.