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.
According to Wyckoff, survivability is the key factor that can explain the observations. CO in Comets Page 2 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.