 The Venusian atmosphere has been found to be sufficiently out of equilibrium as to require further investigation. Analysis of data from the Venera, Pioneer, and Magellan missions has found the chemicals hydrogen sulfide (H2S) and sulfur dioxide (SO2) together in the upper atmosphere, as well as carbonyl sulfide (OCS). The first two gases react with each other, implying that something must produce them. Although carbonyl sulfide is difficult to produce inorganically, it is present in the Venusian atmosphere. It should be noted, however, that the presence of carbonyl sulfide can be accounted for by the planet's volcanism. In addition, one of the early Venera probes detected large amounts of toxic chlorine just below the Venusian cloud deck.
