Solar radiation constrains the atmospheric habitable zone to between 51 km (65 °C) and 62 km (−20 °C) altitude, within the acidic clouds.[3] It has been speculated that clouds in the atmosphere of Venus could contain chemicals that can initiate forms of biological activity.[11][12] It has been speculated that any hypothetical microorganisms inhabiting the atmosphere, if present, could employ ultraviolet light (UV) emitted by the Sun as an energy source, which could be an explanation for the dark lines observed in the UV photographs of Venus.[13][14]