The Cometary Nucleus

As previously noted, the traditional picture of a comet with a hazy head and a spectacular tail applies only to a transient phenomenon produced by the decay in the solar heat of a tiny object known as the cometary nucleus. In the largest telescopes, the nucleus is never more than a bright point of light at the centre of the cometary head. At substantial distances from the Sun, the comet seems to be reduced to its starlike nucleus. The nucleus is the essential part of a comet because it is the only permanent feature that survives during the entire lifetime of the comet. In particular, it is the source of the gases and dust that are released to build up the coma and tail when a comet approaches the Sun. The coma and tail are enormous: typically the coma measures a million kilometres (621,371 miles) or more in diameter, and the tail may extend about 100 million kilometres in length. They scatter and continuously dissipate into space but are steadily rebuilt by the decay of the nucleus, whose size is usually in the range of 10 kilometres (6 miles).

The evidence on the nature of the cometary nucleus remained completely circumstantial until March 1986, when the first close-up photographs of the nucleus of Comet Halley were taken during a flyby by the Giotto spacecraft of the

Composite image of the nucleus of Comet Halley produced from 68 original photographs taken by the Halley Multicolour Camera on board the Giotto spacecraft on March 13 and 14,1986. Courtesy of H.U. Keller; copyright Max-Planck-Institut für Aeronomie, Lindau, Ger., 1986

European Space Agency. Whipple's basic idea that the cometary nucleus was a monolithic piece of icy conglomerate had been already well supported by indirect deductions in the 1960s and '70s and had become the dominant though not universal view. The final proof of the existence of such a "dirty snowball," however, was provided by the photographs of Comet Halley's nucleus.

If there was any surprise, it was not over its irregular shape (variously described as a potato or a peanut), which had been expected for a body with such small gravity (10-4g, where g is the gravity of Earth). Rather, it was over the very black colour of the nucleus, which suggests that the snows or ices are indeed mixed together with a large amount of sootlike materials (i.e., carbon and tar in fine dust form). The very low geometric albedo (2 to 4 percent) of the cometary nucleus puts it among the darkest objects of the solar system. Its size is thus somewhat larger than anticipated: the roughly elongated body measures 15 by 8 kilometres (9 by 5 miles) and has a total volume of some 500 cubic kilometres (120 cubic miles). Its mass is rather uncertain, estimated in the vicinity of 1017 grams, and its bulk density is very small, ranging anywhere from 0.1 to 0.8 g/cm3 (0.06 to 0.46 oz/in3). The infrared spectrometer on board the Soviet Vega 2 spacecraft estimated a surface temperature of 300 to 400 K for the inactive "crust" that seems to cover 90 percent of the nucleus. Whether this crust is only a warmer layer of outgassed dust or whether the dust particles are really fused together by vacuum welding under contact is still open to speculation.

The 10 percent of the surface of Halley's nucleus that shows signs of activity seems to correspond to two large and a few smaller circular features resembling volcanic vents. Large sunward jets of dust originate from the vents; they are clearly dragged away by the gases vaporizing from the nucleus. This vaporization has to be a sublimation of the ices that cools them down to no more than 200 K in the open vents. The chemical composition of the vaporizing gases, as expected, is dominated by water vapour (about 80 percent of the total production rate). The next most abundant volatile (close to 10 percent) appears to be carbon monoxide (CO), though it could come from the dissociation of another parent molecule (e.g., carbon dioxide [CO2] or formaldehyde [CH2O]). Following CO in abundance is CO2 (close to 4 percent). Methane (CH4) and ammonia (NH3), on the other hand, seem to be close to the 0.5 to 1 percent level, and the percentage of carbon disulfide (CS2) is even lower; at that level, there also must be unsaturated hydrocarbons and amino compounds responsible for the molecular fragments observed in the coma.

This is not identical to—though definitely reminiscent of—the composition of the volcanic gases on Earth, which also are dominated by water vapour. However, their CO :CO, CO :CH , and SO2:S2 ratios are all larger than in Comet Halley, meaning that the volcanic gases are more oxidized. The major difference may stem from the different temperature involved—often near 1,300 K in terrestrial volcanoes, as opposed to 200 K for cometary vaporizations. This may make the terrestrial gases closer to thermody-namic equilibrium. The dust-to-gas mass ratio is uncertain but is possibly in the vicinity of 0.4 to 1.1.

The dust grains are predominantly silicates. Mass spectrometric analysis by the Giotto spacecraft revealed that they contain as much as 20-30 percent carbon, which explains why they are so black. There also are grains composed almost entirely of organic material (molecules made of atoms of hydrogen, carbon, nitrogen, and oxygen).

There is some uncertainty concerning the rotation of Halley's nucleus. Two different rotation rates of 2.2 days and 7.3 days have been deduced by different methods. Both may exist, one of them involving a tumbling motion, or nutation, that results from the irregular shape of the nucleus, which has two quite different moments of inertia along perpendicular axes.

Scientific knowledge of the internal structure of the cometary nucleus was not enhanced by the flyby of Comet Halley, and so it rests on weak circumstantial evidence from the study of other comets. Earlier investigations had established that the outer layers of old comets were processed by solar heat. These layers must have lost most of their volatiles and developed a kind of outgassed crust, which probably measures a few metres in thickness. Inside the crust there is thought to exist an internal structure that is radially the same at any depth. Arguments supporting this view are based on the fact that cometary comas and tails do not become essentially different when comets decay. Since they lose more and more of their outer layers, however, the observed phenomena come from material from increasingly greater depths. These arguments are specifically concerned with the dust-to-gas mass ratio, the atomic and molecular spectra, the splitting rate, and the vaporization pattern during fragmentation.

Before the Giotto flyby of Comet Halley, other cometary nuclei had never been resolved optically. For this reason, their albedos had to be assumed first in order to compute their sizes. Techniques proposed to deduce the albedo yielded only that of the dusty nuclear region made artificially brighter by light scattering in the dust. In 1986 the albedo of Comet Halley's nucleus was found to be very low (2 to 4 percent). If this value is typical for other comets, then 11 of 18 short-period comets studied would be between 6 and 10 kilometres (3 and 6 miles) in diameter; only 7 of them would be somewhat outside these limits. Comet Schwassmann-Wachmann 1 would be a giant with a diameter of 96 kilometres; 10 long-period comets would all have diameters close to 16 kilometres (59 miles; within 10 percent).

Since short-period comets have remained much longer in the solar system than comets having very long periods, the smaller size of the short-period comets might result from the steady fragmentation of the nucleus by splitting. Yet, the albedo may also diminish with aging. At the beginning, if the albedo were close to that of slightly less dirty snow (10 percent), the nuclear diameter of long-period comets would come very close to that of the largest of the short-period comets. The diameters of new comets also have been shown to be rather constant and most likely measure close to 10 kilometres (6 miles). Of course, these are mean "effective" diameters of unseen bodies that are all likely to be very irregular.

The region around the nucleus, up to 10 or 20 times its diameter, contains an amount of dust large enough to be partially and irregularly opaque or at least optically thick. It scatters substantially more solar light than is reflected by the black nucleus. Dust jets develop mainly sunward, activated by the solar heat on the sunlit side of the nucleus. They act as a fountain that displaces somewhat the centre of light from the centre of mass of the nucleus. This region also is likely to contain large clusters of grains that have not yet completely decayed into finer dust; the grains are cemented together by ice.

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