In the analysis of mission data from the Venera, Pioneer Venus and Magellan missions, it was discovered that carbonyl sulfide, hydrogen sulfide and sulfur dioxide were present together in the upper atmosphere. Venera also detected large amounts of toxic chlorine just below the Venusian cloud cover.[7] Carbonyl sulfide is difficult to produce inorganically,[8] but it can be produced by volcanism.[9] Sulfuric acid is produced in the upper atmosphere by the Sun's photochemical action on carbon dioxide, sulfur dioxide, and water vapour.[10]