Some sources ([wikipedia Italy](https://it.wikipedia.org/wiki/Cometa)) say that comets are mostly made of gas and “ice”. Other sources ([INAF](https://www.media.inaf.it/2017/08/22/comete/)) say that Rosetta has shown evidence that “ice” is no more than 20% of the mass of a comet. So which one is true?
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> So which one is true?
Both! Comets are usually made up of a combination of rock, dust, ice, and frozen gasses. But their ratio of those substances varies. One comet might be mostly gas and ice, while another is mostly rock with smaller proportions of gas and ice.
Comets are balls of rock, dust, water ice, and frozen gases. In other words, exactly what Wikipedia and that article told you. I’m not sure where that article you linked got that figure from because it didn’t cite, but I can’t find anything to back up that number. Regardless, remember that each time a comet passes close to the sun, some of that water ice and frozen gas sublimates away, so over time, comets become more rocky and less ice-y.
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