One of the Earth's most distinctive features is its color, a beautiful light blue. Iconic photo after iconic photo has been taken highlighting the planet's place of the color spectrum, from NASA's various "Earthrise" photos to Voyager One's "Pale Blue Dot." So it was worth asking, Joshua Krissansen-Totton and David Catling figured, to ask if that color was in fact a sign of life. The two scientists, funded by NASA Astrobiology Institute's Virtual Planetary Laboratory, have just released a paper revealing that the answer is both yes and no.
"One important takeaway is that color should be used with caution because we found it's relatively easy to make lifeless planets that are pale blue in color," says Krissansen-Totten. But, he "was very excited to find that Earth's spectrum has an intriguing signature that is biogenic, unique and potentially quite useful." In other words, while the general visual blue can be replicated by all sorts of planets, Earth still has unique spectral signatures.
The bad news is that, as far as the eye can see, it's easy to imitate blue. Gas giants can be blue. The methane clouds of Neptune and Uranus are blue. All that's needed for these planets to present as blue is to bounce back absorb all the red in the color spectrum.
However, once you move into the realm of ultraviolet light, Earth starts to become unique. The widespread appearance of blue in both sea and sky comes from an effect known as Rayleigh scattering, which scatters particles and is more effective at handling shorter wavelengths. The blue end of the visual spectrum happens to be made up of short wavelengths. Our " O2-rich, oxidizing air is transparent to sunlight, allowing prominent Rayleigh scattering," Krissansen-Totten's paper notes. This scattering takes place across a U-shaped spectrum on Earth, and no other planet we've ever found has a spectrum quite like it.
It's an intriguing place from which to start a search for life, but the search for a U-shaped specturm can't begin quite yet—current telescopes can only obtain the colors of young, gas giant-sized planets. But, as Krissansen-Totten says, "nature is weird and full of surprises." Hopefully telescopes will be able to catch up with these findings soon.