sulfur dioxide - an invisible but very noxiously odorous gas which can be formed by burning sulfur in Earth air. Its chemical formula is SO_2. It is poisonous to humans and has a choking, pungent odor. At very low temperatures it condenses into an ice, and is a liquid over a narrow range of temperature and pressure. It was probably formed on Io by chemistry between oxygen- and sulfur-containing compounds, possibly water and hydrogen sulfide, which are thought to be the dominant compounds of oxygen and sulfur in the hydrogen-rich material which accumulated into the solar system. The reaction producing sulfur dioxide would have taken place as Io heated up inside and slowly lost its hydrogen.