comet to comet. One exception does exist, and that is CO + , whose fluorescence 

 in the comet-tail band system in the blue gives the ion tail its visibility. This 

 species is often absent from the coma, and no ion tail is seen, particularly in 

 old, short-period comets. On the basis that spectacular ion tails are most often 

 associated with bright, new comets, and that both CO and C0 2 are considerably 

 more volatile than H 2 0, it has been suggested that new comets contain a signifi- 

 cant fraction of either of these species and that, for at least a part of their orbit, 

 the sublimation of gases from the nucleus is controlled by this more volatile 

 fraction. 



Spectroscopy in the far-ultraviolet provides a convenient means of studying 

 compositional variations of the dominant volatile species. H 2 0, C0 2 , and CO 

 all have signatures in this spectral region, as do the dissociation products of 

 H 2 (H, O, and OH), as well as C, C , CO , and C0 2 . Several sulfur com- 

 pounds also fluoresce in the far-ultraviolet, including S, CS, and the recently 

 discovered diatomic sulfur, S 2 . Since the terrestrial atmosphere is opaque to 

 wavelengths below 3000 A, observations in the far-ultraviolet have to be made 

 from above the absorbing 2 and 3 . 



In May 1983, comet IRAS-Araki-Alcock (1983d) passed to within 0.032 AU 

 of the Earth, providing a unique opportunity to study the structure of a come- 

 tary coma on a scale of tens of kilometers from the ground and from Earth 

 orbit. A major contribution of the IUE satellite observatory was the discovery of 

 S 2 in a region ~500 km around this comet. The spatial distribution and the 

 expected photochemical lifetime of S 2 imply that it is produced directly from 

 the nucleus at ~10~ 3 the rate of water molecule production. Furthermore, 

 the short lifetime makes this species an ideal tracer of short-term cometary activ- 

 ity. IUE data on S 2 obtained over a 32-hour period have shown a marked tem- 

 poral variation that can be associated with the visual Sunward "fan" of this 

 comet. The presence of S 2 in cometary ice is also potentially an indicator of the 

 physical conditions prevalent at the time of the formation of the comets (or 

 "cometesimals") in the solar or pre-solar nebula. S 2 is generally much reduced 

 in abundance relative to other sulfur polymers in the gas phase at low tempera- 

 tures. The significance of S 2 will not be fully appreciated until its presence in 

 other comets is confirmed in future ultraviolet observations. 



The successful encounter missions with comet Halley in March 1986 (Giotto, 

 Vega, and Suisei) and the plethora of Earth-based observations carried out 

 during this apparition have served to largely confirm the aforementioned models 

 of the cometary coma. It is still too early to draw any significant conclusions 

 from the images of the nuclear region of comet Halley regarding the mechanism 

 of dust and gas ejection (or sublimation) from the dark nucleus, but the in situ 

 measured chemical composition of both the gaseous and solid components of 

 the coma was found to be consistent with what was expected on the basis of 

 remote observations of comets performed over the past decades. Water, the 

 dominant constituent, was also detected remotely using very-high-resolution 



39 



