In Whipple's standard model of the cometary nucleus, the basic features of cometary activity are controlled by the thermodynamics of water ice. Water ice sublimates appreciably at heliocentric distances less than 5 or 6 AU. At larger distances, water is effectively frozen as ice, leading to the expectation that distant comets should be inactive. This indeed appears to be the case for a great many distant comets.

However, a subset of the comets support actively regenerated comae even at heliocentric distances larger than 5 or 6 AU. Over the past 4 decades, a number of hypotheses have been proposed to account for the presence of activity in distant comets. For example, some researchers supposed that the solid state phase change from amorphous (disordered) water ice to crystalline water ice could drive the observed activity, since this phase change is known to be exothermic. Others argued that slow internal propagation of conducted heat in periodic comets might account for sporadic activity observed in a number of distant comets. And still others noted that, while water ice is solid at large heliocentric distances, other so-called "super-volatiles" would not be. Candidate super-volatiles include carbon dioxide, carbon monoxide and diatomic nitrogen. Even hydrogen and helium have, at times, been suggested as potential sources of activity in distant, cold comets.

The advent of sensitive, ground-based telescopes operating in the submillimeter and millimeter wavelength regimes allows us to directly search for the source of activity in distant comets. Starting with the archetypal distant, active comet P/Schwassmann-Wachmann 1 we detected strong rotational transitions unambiguously associated with carbon monoxide. The strengths of the CO lines indicate outgassing at rates in the range 1000 kg/s to 2000 kg/s (1 tonne/s to 2 tonne/s). Following our discovery of CO in P/Schwassmann-Wachmann 1 (Senay and Jewitt 1994), we detected comparably prodigious CO outgassing in distant comet C/Hale-Bopp (Jewitt, Senay and Matthews 1996). Recently, we have identified CO in comet C/Hyakutake (1996 B2), where the outgassing rate is again close to 1 tonne/s.

As a result of these measurements, we feel that the source of the activity observed in distant comets has been identified. The observed CO outgassing is so strong that it is easily capable of supporting the diverse phenomena (dust coma, jets) long associated with activity in distant comets. Other suggested processes (amorphous-crystalline phase change, propagating thermal waves) may also operate, but we do not need to invoke them to explain what we see. By Occam's Razor, we accept that CO is responsible.

The presence of near-surface CO in vast supply tells us immediately that the comets formed and have been stored in a cold place. It would be difficult to account for the CO if the comets formed at a temperature much in excess of 50K, and lower formation temperatures are possible. These temperatures are characteristic of heliocentric distances of order 30 to 50 AU, and place the origin of the nuclei in the Neptune region or the Kuiper Belt beyond.

Many interesting molecules have transitions in the submillimeter and millimeter wavelength regimes. Sensitive observations at these wavelengths have become possible only in the last few years. Currently, we are using JCMT to study the bright near-Earth comet Hyakutake (1996 B2), and the promising incoming Hale-Bopp (1995 O1).

This page presents some of the spectra obtained at our favorite submillimeter telescope, the James Clerk Maxwell Telescope, located on our favorite shield volcano, Mauna Kea, Hawaii.

David Jewitt

Submillimeter Jewitt

Last Updated February 1997