Those elements--mainly, water and oxygen--show up distinctly when looking at "earthshine", or Earth's reflection from the moon. "As a result," says Nick Woolf of Arizona, "it is possible to use the moon to integrate light from the Earth and to determine what the spectrum of the Earth would be like if it were seen from far away as a planet. We need this information to prepare to observe Earth-like planets around other stars."
A simulated image, using the Steward Observatory 90-inch telescope at Kitt Peak, Ariz, shows how the moon "saw" the Earth at the time.
The Earth, as seen from a distant vantage point, has long captivated the imagination of planet finders. And in 1993, a team of researchers inspired by Carl Sagan, used an Earth fly-by of the Galileo spacecraft on its way to Jupiter to catch a glimpse of how the Earth might appear from afar. For astrobiologissts, Sagan's results were surprising.
Pale Blue Dot?
Rather than seeing the Earth as an obvious candidate for life, the Galileo pictures gave surprisingly few clues of the biological potential of our own planet. What was learned however, was first to look close to home before concluding about the biological potential of far-off candidate planets. How did Galileo miss the obvious signs of life we would have expected to see? One answer may lie in the fact that the spacecraft made its observations while still quite close to the Earth. "The spectrograph was designed to look at small areas of Jupiter, so the field of view of the spectrograph was quite small," says Woolf.
"Also, since the surface brightness of Jupiter is far less than the Earth, the spectrograph detectors saturated except when the spectrograph was pointed at the darkest area of Earth - a cloud-free section of sea." The cloud-free sea is considered very dark relative to the dominance of bright clouds in a global picture of Earth. Thus it should come as no surprise that Galileo was successful in only imaging a relatively dark and lifeless planet, mainly because its design was not intended to look at Earth, but to probe Jupiter instead.
A Complex Matter
The Earth as seen by the Moon shows just such a signature. "The visible ozone band is a weak band, which is also rather hard to measure," says Woolf. "The oxygen A band is the strong band which best indicates the large amount of oxygen that has been produced by photosynthesis. Any planet with an oxygen band like this would have so much oxygen on it that no reasonable alternative explanation would exist to it having been formed by living processes."
Of course even to the human eye, an Earth-like planet shows up as not just a blue dot. Central to its astrobiological potential are spectral bands in green light -- those showing photosynthesis and vegetation. "The vegetation feature in the infrared is caused by the scattering of light at plant cell walls and organelles," says Woolf. "This scattering is eliminated at wavelengths shortward of ~7200A by the absorption of chlorophyll A. So the effect of vegetation being present shows as an escarpment in the spectrum. There is a minor return of scattering in the mid green, where the dye absorption is reduced, and this is what produces the vegetation green color. If our eyes had a slightly greater red sensitivity, we would talk about vegetation as being red rather than green!"
These results indicate that the Earth's spectral pattern is complex, particularly when the short wavelength (ultraviolet) and long wavelength (infrared) ranges have to include the dynamic effects of how our own atmosphere distorts such lunar reflections. "The Kitt Peak observations were made over the wavelength range 5000-9400A," says Woolf, with the higher wavelengths in red and the lower wavelengths in blue-green.
"The main concerns," says Woolf, "are both for correcting for regular moonlight scattered over the image, and correcting for the slightly different color that the moon has when light is reflected straight back and when it is reflected at a substantial angle. For the first of these we made observations of the sky near the dark limb of the moon, and observed earthshine just inside the limb. For the second we used observations made in 1973 which were not fully adequate for our needs, but were the best available measures."
"Peaking" Their Interest
Despite these challenging corrections to what a land-based telescope might image of Earth, the team has offered three tell-tale signatures for further planet finders to look for in future. Water and oxygen bands are strong and easier to detect, while sub-peaks for chlorophyll and ozone (charged oxygen, or O3-) are likely byproducts of vegetation that also have piqued their interest for further study.
"There is an interesting question whether a peak near 7500A is a universal feature of complex photosynthetic life forms on land, or whether it is unique to Earth's biochemistry. There are some reasons for suspecting that it is universal, and relates to the absorption spectrum of seawater, and processes when vegetation migrated onto land."
Thus, it came as a surprise to see such a sharp rise in the far-red picture of the Earth, because that portion of the spectrum is particularly tuned to land vegetation and not the roughly 83% of the sea reflection that made up the earthshine photos.
"An extraterrestrial observing Earth would have noticed that about 400 - 500 million years ago, vegetation took root on land," says Woolf. "And since land vegetation requires different parts -- roots and leaves, for example -- it would indicate that life had taken hold strongly on our planet. Spectral features of oxygen and ozone would indicate photosynthesis by living organisms, further confirming the evidence. Any advanced intelligence that cared to inquire would know that life has been present on Earth for a very long time," Woolf concluded.
"The giant telescopes planned for land would be somewhat like the giant single dish radio telescopes," says Woolf, "but working at optical wavelengths, and employing adaptive optics to make very sharp images. There are a number of organizations around the world currently investigating the ways of making these telescopes, but the number of telescopes is likely to be small."
Both NASA and the European Space Agency (ESA) propose space missions to look for Earth-like planets in the infrared. NASA is developing the Terrestrial Planet Finder (TPF) project, part of the Jet Propulsion Laboratory Navigator Program, and ESA is developing its DARWIN project. The European Southern Observatory is exploring the possibility of ground-based searches using the future Overwhelmingly Large Telescope (OWL) project.
Collaborations with Nick Woolf of the University of Arizona Steward Observatory included Wes Traub of the Harvard-Smithsonian Center for Astrophysics, Paul Smith, also at Arizona, and Ken Jucks, a colleague assisting Wes Traub.