The extended formaldehyde source in comet P/Halley

Details The extended formaldehyde source in comet P/Halley The extended formaldehyde source in comet P/Halley

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...277..677m&link_type=abstract

Astronomy and Astrophysics, Vol. 277, p. 677 (1993)

Astronomy and Astrophysics

Astrophysics

83

Molecular Processes - Comets: General - Comets: Individual: P/Halley - Carbon Monoxide - Pom - Ion Spectra

Scientific paper

The Neutral Mass Spectrometer on the Giotto spacecraft measured the neutral and ion composition in the coma of comet P/Halley. In the ion mass spectra, the 31 amu/e peak is dominated by the protonated formaldehyde ion H3CO+ and can be used to calculate the H2CO density. The radial profile of the amplitude of the 31 amu/e peak can not be reconciled with H2CO being only a parent molecule evaporating from the nucleus. From the radial profile of the H3CO+ density, we derive the total production of formaldehyde and the strength of its extended source within the contact surface located at 4660 km. The total H2CO production inside the contact surface is Q(H2CO)/Q(H2O) = 0.029, essentially independent of model assumptions. The extrapolation of our data to the nucleus is fully compatible with no H2CO being released directly from the nucleus. As upper limit for the nuclear production rate we estimate Q(H2CO)/Q(H2O) 0.004. Extrapolating the measured extended source strength to the region outside the contact surface leads to a total formaldehyde production in P/Halley of Q(H2CO)/Q(H2O) = 0.038. This probably represents a lower limit as the extrapolation may underestimate the H2CO production. The absolute error in the H2CO production rate due to the uncertainties in the measured ion densities and in the rate constants used in modelling the ion chemistry is estimated to be less than 35%.
The H2CO abundance in P/Halley is about one to two orders of magnitude larger than in four other comets recently investigated with the IRAM 30 m radio telescope. Most of this difference may be due to the presence of the extended H2CO source in P/Halley. In addition, the H2CO abundance seems to show temporal variations within one comet.
The photodissociation of H2CO provides an extended source for CO. Within the contact surface the CO production from formaldehyde accounts for two thirds of the measured density increase of CO. Formaldehyde polymers, coating dust grains and evaporating as monomers provide the most likely explanation for the extended H2CO source. We see no evidence for high concentrations of polyoxymethylenes (POMs) in the gas phase. Our results are in agreement with the icy mantels of interstellar grains being a major source of all volatiles including H2O and acting also as the extended source for H2CO.

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