minerals. Sulfuric acid is used as a defense by certain marine species, for example, the phaeophyte alga Desmarestia munda (order Desmarestiales) concentrates sulfuric acid in cell vacuoles. Stratospheric aerosol In the stratosphere, the atmosphere's second layer that is generally between 10 and 50 km above Earth's surface, sulfuric acid is formed by the oxidation of volcanic sulfur dioxide by the hydroxyl radical: Because sulfuric acid reaches supersaturation in the stratosphere, it can nucleate aerosol particles and provide a surface for aerosol growth via condensation and coagulation with other water-sulfuric acid aerosols. This results in the stratospheric aerosol layer. Extraterrestrial sulfuric acid The permanent Venusian clouds produce a concentrated acid rain, as the clouds in the atmosphere of Earth produce water rain. Jupiter's moon Europa is also thought to have an atmosphere containing sulfuric acid hydrates. Manufacture Sulfuric acid is produced from sulfur, oxygen and water via the conventional contact process (DCDA) or the wet sulfuric acid process (WSA). Contact process In the first step, sulfur is burned to produce sulfur dioxide. The sulfur dioxide is oxidized to sulfur trioxide by oxygen in the presence of a vanadium(V) oxide catalyst. This reaction is reversible and the formation of the sulfur trioxide is exothermic. The sulfur trioxide is absorbed into 97–98% to form oleum (), also known as fuming sulfuric acid or pyrosulphuric acid. The oleum is then diluted with water to form concentrated sulfuric acid. Directly dissolving in water, called the "wet sulfuric acid process", is rarely practiced because the reaction is extremely exothermic, resulting in a hot aerosol of sulfuric acid that requires condensation and separation. Wet sulfuric acid process In the first step, sulfur is burned to produce sulfur dioxide: (−297 kJ/mol) or, alternatively, hydrogen sulfide () gas is incinerated to gas: (−1036 kJ/mol) The sulfur dioxide then oxidized to sulfur trioxide using oxygen with vanadium(V) oxide as catalyst. (−198 kJ/mol) (reaction is reversible) The sulfur trioxide is hydrated into sulfuric acid : (−101 kJ/mol) The last step is the condensation of the sulfuric acid to liquid 97–98% : (−69 kJ/mol) Other methods A method that is the less well-known is the metabisulfite method, in which metabisulfite is placed at the bottom of a beaker and 12.6 molar concentration hydrochloric acid is added. The resulting gas is bubbled through nitric acid, which will release brown/red vapors of nitrogen dioxide as the reaction proceeds. The completion of the reaction is