Detection of Water Vapor in Halley's Comet

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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.