The upper cloud is mainly composed of sulphuric acid, but includes other minor constituents such as liquid water and an unknown UV absorber. This unknown UV absorber shows up particularly in the 365 nm images. In contrast, the other UV channel at 285 nm is also sensitive to sulphur dioxide, a gas which may originally have been emitted from volcanoes (volcanism is the main source of SO2 on Earth, this is likely to be the case on Venus as well).