Detection of Water Vapor in Halley's Comet.

Gaseous, neutral H(2)O was detected in the coma of comet Halley on 22.1 and 24.1 December 1985 Universal Time. Nine spectral lines of thev(3) band (2.65 micrometers) were found by means of a Fourier transform spectrometer (lambda/triangle uplambda approximately 10(5)) on the NASA-Kuiper Airborne Observatory. The water production rate was approximately 6 x 10(28) molecules per second on 22.1 December and 1.7 x 10(29) 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.