During the day, solar radiation excites atoms and molecules in the atmospheres of Earth and Venus to higher energy levels. As darkness falls, the atoms and molecules drop to lower energy states and emit radiation - this is 'nightglow'. Slanger's team used the Keck I telescope in Hawaii to study two distinct emission lines in the nightglow of Venus: green light emitted by atomic oxygen as it drops from a higher to a lower excited state, and red light subsequently emitted as it falls further to its ground state. In theory, every green emission could be followed by a red emission, but this only happens above an altitude of 200 km. Below this level the lower excited states are 'quenched' by collisions with other atoms and molecules.

Astronomers expect to see much greater levels of red emission because they believe that daytime solar energy breaks up carbon dioxide molecules high in the Venusian atmosphere to produce atomic oxygen that is mainly in its lower excited state. However, they found that the green emission is eight times stronger than the red emission. This could mean that the green light comes from a different process deeper in the Venusian atmosphere. Alternatively, scientists may need to modify their theories of the how excited states of oxygen are created and transported through the atmosphere.

The green line intensity of Venus is comparable with that from aurora on Earth - a phenomenon produced as the terrestrial magnetic field interacts with the solar wind. But the atmospheres of the two planets are enormously different in composition, temperature and pressure. "A green line does not mean that a planet has an oxygen atmosphere because Venus has extremely low levels of oxygen", Slanger told PhysicsWeb. This result has implications for researchers studying the atmospheres of extra-solar planets.

The team also hopes its findings may shed light on the apparent variability of the emission lines: the Russian Venera orbiters visited Venus in 1975 and found no sign of the green signal. "We do not understand how the variability can be this large", said Slanger, although the team speculates that the fluctuations could be connected with the solar cycle.