FOR IMMEDIATE RELEASE: 18 FEBRUARY 1999
Contact: James Hathaway
Arizona State University College of Liberal Arts & Sciences
Comets, Like Cars, Leave Carbon Monoxide In Their Wake
Hitching a ride on a comet may be like latching onto a bus's
tailpipe, a recent Arizona State University study found. The
study, done by ASU astronomers and published in the February 10
issue of The Astrophysical Journal, found that comet gas tails,
previously thought to be composed mostly of water, actually
contain high concentrations of ionized carbon monoxide (CO) gas
similar to the non-ionized form that hovers over the freeways of
New York, Los Angeles and Phoenix.
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
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
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.
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