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Comets

Putting the CO in coma

Nature volume 399, pages 640–641 (1999)Cite this article

When comet Hale-Bopp was at its closest to the Sun (roughly 0.9AU, where 1AU = 1 astronomical unit, the Earth-to-Sun distance) at the beginning of April 1997, it released about 300 tons of water and 100 tons of carbon monoxide (CO) per second. Whereas cometary water is known to arise from the sublimation of ices in the nucleus, the origin of cometary CO is more controversial. On page 662of this issue, DiSantiet al .1 report infrared observations of CO emission from comet Hale-Bopp over a range of distances from the Sun. They reveal that cometary CO has a dual origin — 50% from the nucleus and 50% from a distributed source in the cometary atmosphere. Moreover, they show that the distributed source switches off at a certain distance from the Sun, suggesting a thermal production process and giving some clues to its identity.

Within the ‘dirty snowball’ model, first proposed by Fred Whipple in 1950, comet nuclei are thought to consist of ices — mainly frozen water, with some carbon dioxide, methane and ammonia — mixed up with dust particles. When these icy cores approach the Sun, radiation causes the ices to vaporize, releasing a diffuse envelope of dust and gas called the coma, and several tails. The comet nuclei are too small to be observed directly, but the coma can be millions of kilometres across, making comets such as Hale-Bopp visible to the unaided eye. Until recently, the chemical composition of the nucleus was inferred from dissociation products in the visible spectra of comets. The advent of detectors working in new spectral domains — ultraviolet, radio and infrared — has directly confirmed the composition of cometary gases and extended the list to include more than 20 volatile species2. One of the most important is CO, which in some comets is more than 10% relative to water.

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Figure 1: Cometary activity is governed by heat from the Sun, which vaporizes ices from the nucleus.

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  1. Observatoire de Paris, F-92195, Meudon, France

    Jacques Crovisier

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  1. Jacques Crovisier
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Crovisier, J. Putting the CO in coma. Nature 399, 640–641 (1999). https://doi.org/10.1038/21326

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