How to rediscover life on Earth by looking at the Moon

Feb 29, 2012
This view shows the thin crescent moon setting over ESO's Paranal Observatory in Chile. As well as the bright crescent the rest of the disc of the Moon can be faintly seen. This phenomenon is called earthshine. It is due to sunlight reflecting off the Earth and illuminating the lunar surface. By observing earthshine astronomers can study the properties of light reflected from Earth as if it were an exoplanet and search for signs of life. This picture was taken on Oct. 27, 2011, and also records the planets Mercury and Venus. Credit: ESO/B. Tafreshi/TWAN (

( -- By observing the Moon using ESO's Very Large Telescope, astronomers have found evidence of life in the universe -- on Earth. Finding life on our home planet may sound like a trivial observation, but the novel approach of an international team may lead to future discoveries of life elsewhere in the Universe. The work is described in a paper to appear in the March 1, 2012, issue of the journal Nature.

"We used a trick called earthshine observation to look at the as if it were an exoplanet," says Michael Sterzik (ESO), lead author of the paper. "The shines on the Earth and this light is reflected back to the surface of the Moon. The acts as a giant mirror and reflects the Earth's light back to us — and this is what we have observed with the VLT."

The analyse the faint earthshine light to look for indicators, such as certain combinations of gases in the Earth's atmosphere, that are the telltale signs of organic . This method establishes the Earth as a benchmark for the future search for life on planets beyond our Solar System.

When the Moon appears as a thin crescent in the twilight skies of Earth it is often possible to see that the rest of the disc is also faintly glowing. This phenomenon is called earthshine. It is due to sunlight reflecting off the Earth and illuminating the lunar surface. After reflection from Earth the colours in the light, shown as a rainbow in this picture, are significantly changed. By observing earthshine astronomers can study the properties of light reflected from Earth as if it were an exoplanet and search for signs of life. The reflected light is also strongly polarised and studying the polarisation as well as the intensity at different colours allows for much more sensitive tests for the presence of life. Credit: ESO/L. Calçada

The fingerprints of life, or biosignatures, are hard to find with conventional methods, but the team has pioneered a new approach that is more sensitive. Rather than just looking at how bright the reflected light is in different colours, they also look at the polarisation of the light, an approach called spectropolarimetry. By applying this technique to earthshine observed with the VLT, the biosignatures in the reflected light from Earth show up very strongly.

Co-author of the study Stefano Bagnulo (Armagh Observatory, Northern Ireland, United Kingdom) explains the advantages: "The light from a distant exoplanet is overwhelmed by the glare of the host star, so it's very difficult to analyse — a bit like trying to study a grain of dust beside a powerful light bulb. But the light reflected by a planet is polarised, while the light from the host star is not. So polarimetric techniques help us to pick out the faint reflected light of an exoplanet from the dazzling starlight."

This video is not supported by your browser at this time.
This video takes us around the Moon and shows how its surface is illuminated not just by the brilliant light of the Sun, but also by light reflected from the Earth. We start on the side facing away from Earth. Part of the surface is brightly illuminated by the Sun, but the rest is totally dark. As we move around the Moon the Earth rises and its reflected bluish light illuminates the Moon’s surface. This dull glow is the earthshine. It can be clearly seen from Earth when the Moon appears as a crescent in the evening or morning sky. When the Sun emerges from behind the Moon the brilliant crescent is seen, with earthshine still faintly visible. Credit: ESO/M. Kornmesser

The team studied both the colour and the degree of polarisation of light from the Earth after reflection from the Moon, as if the light was coming from an . They managed to deduce that the Earth's atmosphere is partly cloudy, that part of its surface is covered by oceans and — crucially — that there is vegetation present. They could even detect changes in the cloud cover and amount of vegetation at different times as different parts of the Earth reflected light towards the Moon.

"Finding life outside the Solar System depends on two things: whether this life exists in the first place, and having the technical capability to detect it," adds co-author Enric Palle (Instituto de Astrofisica de Canarias, Tenerife, Spain). "This work is an important step towards reaching that capability."

"Spectropolarimetry may ultimately tell us if simple plant life — based on photosynthetic processes — has emerged elsewhere in the Universe," concludes Sterzik. "But we are certainly not looking for little green men or evidence of intelligent life."

The next generation of telescopes, such as the E-ELT (the European Extremely Large ), may well be able to bring us the extraordinary news that the Earth is not alone as a bearer of life in the vastness of space.

Explore further: How baryon acoustic oscillation reveals the expansion of the universe

More information: This research was presented in a paper, "Biosignatures as revealed by spectropolarimetry of Earthshine", by M. Sterzik et al. to appear in the journal Nature on 1st March 2012.

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