Hitching a ride on a comet may be like latching onto the tailpipe of a city bus. For years, astronomers have thought that the tails trailing for millions of miles behind a comet were composed mostly of water. The gas is similar to the non-ionized form that hovers over the freeways of New York, Los Angeles, or Phoenix. The Astrophysical Journal published the study aresults in the February 10th issue. At the heart of a comet is a dirty snowball a few kilometers in size that sheds gas and dust when heated by the sun.
Comets generally have two tails.
One is made of dust, the other gas. For this reason, scientists previously assumed that the tails of comets would also be made mostly of water. Surprisingly, they are not, according to the ASU findings. Susan Wyckoff, ASU astronomy professor and lead author of the paper. For nearly 50 years we've been using the dirty snowball model for comets.