No. 64: Jul-Aug 1989
Can this be the comet Halley of the textbooks? Comets are supposed to be as old as the solar system itself (4.6 billion years), born of solar-system stuff when a gaseous cloud condensed. Above all, comets do not "burn" or combust!
The vision of a "burning" comet was advanced by recent observations that the velocity and temperature of the gases escaping from Halley are higher than one would expect from the sublimation of ices under solar radiation. Also, the concentration of expelled material in large, hypersonic jets carrying large quantities of fine dust further undermine the sublimation model.
E.M. Drobyshevski has concluded
"The new observations, together with some earlier data still poorly understood (e.g., the appearance in the coma of large amounts of C3 ) can be accounted for by assuming the cometary ices to contain, apart from the hydrocarbons, nitrogen-containing compounds, etc., also of free oxygen (about 15 wt. %). Under these conditions, burning should occur in the products of sublimation under deficiency of oxidizer accompanied by the production of 'soot,' 'smoke,' etc. The burning should propagate under the surface crust and localize at a few sites.
"The presence of oxygen in cometary ices follows from a new eruption theory assuming the minor bodies of the Solar System to have formed in explosions of the massive ice envelopes saturated with electrolysis products on distant moon-like bodies of the type of Ganymede and Callisto."
(Drobyshevski, E.M.; "Combustion as the Cause of Comet P/Halley's Activity," Earth, Moon, and Planets, 43:87, 1988. Cr. L. Ellenberger.)
Drobyshevski's combustion theory assumes a "local" origin (within the solar system) for Halley. But measurements of the ratio of carbon-12 to carbon-13, made during the 1986 flyby, produced a ratio of 65:1. This compares to 89:1 for solar system material. The 65:1 ratio, it turns out, is more typical of interstellar material. This datum seems to place Halley's birthplace somewhere outside the solar system.
(Weiss, R.; "Carbon Ratio Shows Halley May be Alien," Science News, 135:214, 1989.)
As if all this were not bad enough, calculations of the amount of matter expelled from Halley and incorporated in known meteor streams allow an estimate of Halley's residence time in the inner solar system. (One has, of course, Halley's present mass, but must estimate its original mass!) The conclusion is that Halley has spent only 23,000 years in its present orbit!
(Maddox, John; "Halley's Comet Is Quite Young," Nature, 339:95, 1989.)
Comments. Admittedly, this is a pretty shaky calculation, but it accords with the calculation that Halley had a close encounter with Jupiter about 20,000 years ago.
The manifest contradictions in the inferences made above from recent observations of Halley mean that we still have a lot to learn about comets, Halley in particular. One should also recall that the solar system has other features that may be youthful, such as Saturn's rings. (See ARL16 in our catalog: The Moon and the Planets)
For other cometary anomalies, see Chapter AC in our catalog volume: The Sun and Solar System Debris. Both catalog volumes are described here.