Ask A Scientist© Astronomy Archive

Question:  What are comets made of?
derek1

Answer:  Comets may be composed of many different things, depending on their
origins, but many contain large quantities of ice
Of the other contents, there may be metals such as iron, iridium, and other
minerals.  Trapped in the ice, moreover, there may be samples of gasses, if the
comet came from some planetary form or if it passed through a gaseous cloud.
The tail of the comet is simply a stream of evaporating water and gasses from
its surface.
Wordsworth