Putting the CO in coma | Nature

Access through your institution Buy or subscribe 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, DiSanti_et 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. This

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during checkout ADDITIONAL ACCESS OPTIONS: * Log in * Learn about institutional subscriptions * Read our FAQs * Contact customer support REFERENCES * DiSanti, M. A. _ et al._ _Nature_ 399,

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* http://stardust.jpl.nasa.gov * http://sci.esa.int/missions/rosetta * http://nmp.jpl.nasa.gov/st4 Download references AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Observatoire de Paris,

F-92195, Meudon, France Jacques Crovisier Authors * Jacques Crovisier View author publications You can also search for this author inPubMed Google Scholar RIGHTS AND PERMISSIONS Reprints and

permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Crovisier, J. Putting the CO in coma. _Nature_ 399, 640–641 (1999). https://doi.org/10.1038/21326 Download citation * Issue Date: 17 June

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