Venus’ escaping atmosphere

The plasma package called the Analyser of Space Plasmas and Energetic Atoms (ASPERA-4) on board ESA’s Venus Express has established, for the
first time, the composition of escaping plasma on Venus.

Venus loses mainly oxygen, hydrogen, and helium via its atmosphere’s interaction with the solar wind. The hydrogen that escapes, originates from
the small amounts of water present in the atmosphere. Water molecules in the atmosphere, or water vapour, break down under the action of the Sun’s
ultraviolet radiation to produce one oxygen atom and two hydrogen atoms. Solar ultraviolet light ionises the atoms that are then picked up by the
solar wind and accelerated. They eventually escape the planet. Oxygen can also originate from breaking carbon dioxide molecules.