The heart of a comet is its nucleus, a dirty snowball a few kilometers in size that sheds gas and dust when heated by the Sun. Comets generally have two tails, a dust and a gas tail. Because the icy nucleus is made primarily of water ice, not dry ice (frozen carbon dioxide) or other ices, scientists previously assumed that the tails of comets would likewise be comprised mostly of water. Surprisingly, according to the ASU study, they're not.

According to Susan Wyckoff, ASU astronomy professor and lead author of the paper, "these results are exciting because for nearly fifty years we've been using the dirty snowball model for comets and gotten a lot of evidence that water is the predominant molecule in the nuclei. When we measured ionized water and carbon monoxide levels in the tails, 10 to 20 million kilometers from the nuclei, we found that the ratio actually reverses."

The ASU team, including Wyckoff, computer specialist Rodney Heyd and undergraduate student Rebecca Fox, explain that the lack of water ions in comet tails is due largely to the relatively fragile nature of water molecules compared with carbon monoxide molecules when exposed to sunlight.

According to Wyckoff, survivability is the key factor that can explain the observations. "Because the gas tails of comets extend tens of millions of kilometers from the nuclei," said Wyckoff, "the molecules need to be able to survive the trip that takes several days while exposed to ultraviolet sunlight."

Ultraviolet sunlight is very energetic and can break chemical bonds. Because the bond strength of carbon monoxide is much greater than that of water, carbon monoxide can survive this trip much better than water can. While a water molecule will break apart into hydrogen and oxygen atoms after only about one day of exposure to the sun, carbon monoxide is much heartier in the Sun's ultraviolet light, and can survive for about ten days. For this reason, it is mostly carbon monoxide gas, not water, which finally reaches the tail. The gas that makes this trip is ionized (missing one electron) and is propelled tailward by the Sun's magnetized solar wind.

Observations that led to this discovery included the two recent comets-two of the brightest of this century-Hale-Bopp (of California cult fame) and Hyakutake, and were made using telescopes at the Kitt Peak National Observatory and the University of Arizona's Steward Observatory. The team was prompted to look at carbon monoxide levels after finding almost no water in the tail of Hale-Bopp.

In addition to finding high levels of carbon monoxide, the ASU team also discovered an unidentified molecule in the comets' gas tails, one first seen first in Halley's Comet 13 years ago. According to Wyckoff, in the 13 years since that discovery the molecule has still not been identified. Wyckoff hopes that NASA's recently launched Stardust mission, set to rendezvous with comet Wild2 and bring back materials from the tail, will allow scientists to finally analyze and identify the unknown molecule.

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To see the ASU team's paper, visit http://www.journals.uchicago.edu/ApJ/journal/ and click on Rapid Release ApJ Letters. Photography showing gas and dust tails on Comet Hale-Bopp can be found at http://pulsar.la.asu.edu/~chris/comarch2/031597c-f.jpg and at http://pulsar.la.asu.edu/~chris/comarch/comarch.html.

Note: If no author is given, the source is cited instead.

• Asteroids, Comets and Meteors
• Extrasolar Planets
• Astronomy
• Stars
• Sun
• Astrophysics

• Comet Hale-Bopp
• Comet
• Comet Shoemaker-Levy 9
• Interstellar medium
• Near-Earth asteroid
• Solar wind