Wyckoff says survivability is the key factor to explain her team's observations. The gas tails of comets extend tens of millions of kilometers from the nuclei. 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. Water molecules break apart into hydrogen and oxygen atoms after only about one day of exposure to the sun.
Carbon monoxide is much heartier.
It can survive the sun's ultraviolet light for about 10 days. As a result, mostly carbon monoxide gas, not water, reaches the comet's tail. The sun's magnetized solar wind propels it tailward. The discovery is based on observations of two of the brightest comets of this century. The ASU team was prompted to look at carbon monoxide levels after finding almost no water in Hale-Bopp's tail.