Ammonia release experiments in the high atmosphere and dissociation processes in comets

Abstract

The mechanism of formation of radicals and ions observed in comets (CN, C 2, NH 2, CO +, N 2 +, CO 2 +) is one of the major problems of cometaryphysics. Photodissociation and photoionization have been proposed as sources of these species. Two releases of ammonia at an altitude of 180 km have been carried through in July 1964 by ESRO in order to test the relevance of such experiments for the photodecomposition hypothesis. NH 3 absorption in the range λ2250 to λ1500 Å yields the radical NH 2, which has resonance bands in the yellow-red. The released masses of about 42 kg appeared in the twilight sky as diffuse rings expanding with a velocity of 0.5 to 0.8 km/sec, and decresed rapidly in brightness. The configuration remained visible for only 20 seconds and a spectrum of the expanding mass of material obtained between 12 and 20 seconds after the beginning of the release showed only the twilight continuum with no trace of any NH 2 bands. An upper limit on the concentration of NH 2 within the cloud was determined from the brightness of the sky background. The low upper limit so derived points to a space lifetme of the NH 3 molecules of at least 10 3.5 to 10 4 seconds. Stimulated by the release experiments, a study has been made of the space lifetimes of the precursor particles of the radicals and ions in cometary atmospheres. It seems highly probable that the lifetimes of these precursors at solar distance of 1 a.u. are <10 3 seconds, a result incompatible with the photodissociation hypothesis. It shows further that the production rate of CO + ions is orders of magnitude too fast to be explained by a photoionization mechanism. The radicals and ions have the same local source close to the nucleus, a volume with a radius of only a few hundred kilometers. Outbursts of radicals and ions happen simultaneously. Bredohl observed the same releases and those repeated in 1966. Although the results are perhaps not absolutely convincing, he may have detected a weak NH 2 emission on an intense continuous background. Such an observation does not contradict the conclusions drawn above.