The oxygen emission at 557.7 nm is a ubiquitous component of the spectrum of the terrestrial polar aurora and the reason for its usual green colour(1). It is also observed as a thin layer of glow surrounding the Earth near 90 km altitude in the dayside atmosphere(2,3)but it has so far eluded detection in other planets. Here we report dayglow observations of the green line outside the Earth. They have been performed with the Nadir and Occultation for Mars Discovery ultraviolet and visible spectrometer instrument on board the European Space Agency's ExoMars Trace Gas Orbiter. Using a special observation mode, scans of the dayside limb provide the altitude distribution of the intensity of the 557.7 nm line and its variability. Two intensity peaks are observed near 80 and 120 km altitude, corresponding to photodissociation of CO(2)by solar Lyman alpha and extreme ultraviolet radiation, respectively. A weaker emission, originating from the same upper level of the oxygen atom, is observed in the near ultraviolet at 297.2 nm. These simultaneous measurements of both oxygen lines make it possible to directly derive a ratio of 16.5 between the visible and ultraviolet emissions, and thereby clarify a controversy between discordant ab initio calculations and atmospheric measurements that has persisted despite multiple efforts. This ratio is considered a standard for measurements connecting the ultraviolet and visible spectral regions. This result has consequences for the study of auroral and airglow processes and for spectral calibration. The oxygen emission at 557.7 nm, responsible for the green colour of auroras on Earth, is present in two layers at 80 and 120 km altitude on the dayside atmosphere of Mars. Simultaneous observations of the oxygen both in this visible line and in the ultraviolet could also constrain the elusive visible/ultraviolet intensity ratio of the auroral emission to a value of 16.5.