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 colour¹. 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 CO₂ by solar Lyman α 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.

557.7 nm处的氧气发射是地球极光极光光谱的普遍组成部分，也是其通常绿色的原因¹。在白天的大气层中，还观察到它在90公里高空周围环绕着地球的薄薄一层辉光^2,3但是到目前为止，它还没有被其他行星发现。在这里，我们报告了对地球外部绿线的日光观测。它们是由欧洲航天局ExoMars微量气体轨道飞行器上的天底和火星探测掩星术用紫外可见光谱仪完成的。使用特殊的观察模式，对日间肢的扫描可提供557.7 nm线的强度及其变化的高度分布。在80和120 km高度附近观察到两个强度峰，对应于CO _2的光解离分别由太阳莱曼α和极端紫外线辐射引起。在297.2 nm处的近紫外光中观察到的发射光较弱，该发射光来自氧原子的相同含量。两条氧气线的这些同时测量值使得可以直接得出可见光发射量与紫外线发射量之间的比率为16.5，从而澄清了尽管有许多努力，但从头算计算和大气测量之间仍然存在争议。该比率被认为是连接紫外和可见光谱区域的测量标准。该结果对极光和气辉过程的研究以及光谱校准具有重要意义。