Planetary science. The nightside of Venus.

On 20 November 1999, Slanger et al. [HN1] (1) pointed the giant, 10-m-diameter, Keck I telescope [HN2] at the night side of Venus, our closest planetary neighbor. Venus [HN3] has been scrutinized for 35 years by ground-based telescopes and an armada of spacecraft [HN4], but it took just 8 min of exposure time for the high-resolution Keck spectra [HN5] to yield an important discovery, namely the first evidence for atomic oxygen airglow (2) at visible wavelengths on the nightside of Venus. The telltale sign of this airglow [HN6] was a diffusive emission from the upper atmosphere in the 557.7-nm atomic oxygen green line. This type of emission is prominent in Earth's aurora [HN7] and diffuse background nighttime airglow, but its appearance in the spectrum of the Venus night sky is surprising for several reasons.