The chemical fingerprints of vegetation and gases associated with life can be picked out of the Earthshine.
- Scientists use light reflected back to Earth from the moon to get a planet-wide picture.
- It is the angle, or polarization, of the light that is key.
- Scientists would like to get a look at Earthshine without having it bounce off the moon.
Using the moon as a giant mirror, scientists looked at reflected light from Earth and figured out a way to pick out telltale signs of life, a practice run for looking for life on planets beyond the solar system.
The research builds on earlier work to collect all the light reflected off Earth and analyze it for the chemical fingerprints of vegetation, oxygen, water and other signs of life. Rather than looking at the brightness, however, the scientists study how the light is polarized, a measurement that can more easily be made from very distant exoplanets as well.
"In the past, there have been very accurate polarization measurements of the Earth's blue sky from the ground and these measurements showed the absorption bands (of gases) and the vegetation very clearly. However, those were local measurements -- a small path of the sky overhead -- so they didn't prove you can also see it in the whole Earth observations," Daphne Stam, with the Netherlands Institute for Space Research, wrote in an email to Discovery News.
In exoplanet research, polarization is a very useful tool for two reasons, adds Christoph Keller, a professor of experimental physics at The Netherlands' Leiden Observatory.
First is that starlight reflected by an exoplanet is polarized, like sunlight bouncing off Earth, but the light coming directly from the star is not.
"Polarization measurements are like filters that block the much brighter starlight and let only the much dimmer exoplanet light through," Keller told Discovery News.
Second, is that these measurements of polarized light contain much more information about the surface and atmosphere of planets than regular measurements of light's intensity.
"Polarization, for instance, can say something about the size and composition of liquid droplets and small particles in a planetary atmosphere," Keller said.
Ideally, scientists would like to get a look at Earthshine without having it bounce off the moon, but instruments to make that measurement from space currently don't exist.
"This is the only way to see the Earth (as) it looks from space, but actually observing from the ground," lead researcher Michael Sterzik, deputy director of the European Space Agency's La Silla Paranal Observatory in Chile, wrote in an email to Discovery News.
"The final motivation is to establish a viable astronomical techniques to study and analyze the atmopsheres and surfaces of exoplanets, and in particular biosignatures," Sterzik added.
For now, since Earth is the only example of a life-hosting planet, it is the best stand-in for Earth-like planets beyond.
Sterzik's research appears in this week's Nature.