Earth’s transmission spectrum from lunar eclipse observations

Part of the little we know about the atmospheres of extrasolar planets is gleaned from spectra obtained from starlight that has passed through the planetary atmosphere as it transits the stellar disk. As planet-finding techniques improve, the point where analogues of Earth are likely to be found is approaching. So what are we looking for? A good starting point for comparisons would be a transmission spectrum of our own planet. Now we have one, based on the characterization of sunlight reflected from the Moon towards the Earth during a lunar eclipse, a scenario that resembles the observing geometry of a planetary transit. This indirect method produces an optical and near-infrared transmission spectrum of the Earth. Some biologically relevant atmospheric features that are weak in reflected spectra are much stronger in the transmission spectrum, and stronger than predicted by modelling.