Main belt comets are a type of comet that is found in the main asteroid belt between Mars and Jupiter. They are thought to be made up of the same material as asteroids, but they also contain water ice. When main belt comets get close to the sun, the heat from the sun causes the water ice to sublimate, or turn directly from a solid to a gas. This releases gas and dust into the comet's atmosphere, creating a coma, or a cloud of gas and dust. The coma can be millions of kilometers wide, and it is what makes main belt comets visible from Earth.