This is a drawing of what the surface of a comet might look like.
Click on image for full size
JPL
The Comet Nucleus
A comet nucleus is
made of a special sort of dust. This theorized dust is called "fluffy" because it could be as light weight and full of holes as a sponge. The holes are filled with frozen gases made of carbon dioxide, carbon monoxide and other gases. Observations of the nuclei of comet
Hale-Bopp and
Hyakutake have given scientists fresh ideas about comet composition and evolution.
Scientists do not know whether the nucleus is very hard (like solid ground) or very soft and breakable (like a snowball). The Rosetta mission hopes to land a probe on the surface of a comet to find out just how hard it is.
As a comet approaches the Sun, it begins to sublimate, forming the coma and a spectacular tail.
This picture shows that sublimation may happen only in specific places on the nucleus. These spots of evaporation are called "jets". The jets can sometimes help to turn the comet and make it tumble in space. Halley's comet was photographed with three distinct jets on its surface as it approached the Sun in 1986.
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