Glimpse of Earth as seen from afar

Glimpse of Earth as seen from afar
Lunar eclipse paints portrait of Earth that could aid hunt for distant habitable planets.
Eric Hand Published online 10 June 2009 | Nature
The rosy glow of a lunar eclipse helped astronomers capture the Earth's transmission spectrum.Daniel Lopez
Astronomers have seen what the Earth's atmosphere might look like from outer space by using the Moon as a giant mirror. Sunlight that bounced back from the Moon carried a fingerprint of the Earth's atmosphere that could help astronomers determine if the extrasolar planets they're finding harbour life.
The astronomers, at Spain's Institute of Astrophysics of the Canary Islands, made their observations on 16 August 2008 during a lunar eclipse — in which the Moon moves into Earth's shadow. Even when the Moon is totally eclipsed by Earth, it is still bathed in a dim red light — from sunlight that has been bent as it passes through the edge of Earth's atmosphere. Using Earth-based telescopes, the astronomers detected some of this light after it bounced back from the Moon, and captured a 'transmission spectrum' of the light that had passed through Earth's atmospheric halo.
Because gases in Earth's atmosphere absorb certain wavelengths of light, the astronomers were able to pick out key biosignatures in the spectrum — gases such as methane and oxygen that are associated with life on Earth. But they were surprised to find signatures that they expected to be too weak to detect: evidence of Earth's protective ionosphere, which absorbs some of the Sun's highest energy photons, and evidence of nitrogen, which makes up the bulk of the atmosphere but is difficult to detect.
"We find that these signatures are actually much stronger than the models predicted," says institute astronomer Enric Pallé, lead author of a study published today in Nature1. "They will be easier to detect on an exosolar planet."