Is There Life On Earth? Would Aliens Know? Proving It From A Distance

Sunlight reflected from Earth illuminates the night side of the Moon with earthshine in this image taken at moonrise. Giant Jupiter has just emerged from behind the Moon, along with its moons Europa on the left and Io, Ganymede, and Callisto on the right. Europa may have habitats for life beneath its ice surface. Discovering life there will be challenging. Imagine the difficulty of studying an Earth-like planet orbiting a distant star. This is “the new moon in the old moon’s arms,” a reversal of the usual saying that describes an evening crescent.

Photo by S. J. Edberg (c)2011

Sometimes I come across ideas so clever that I wish I had thought of them. Like asking the question, “Is there life on Earth?”

Of course, most people would probably answer “Well, yeah!” But the perspective of an astrobiologist looking 20 light-years away at a star that has an Earth-similar planet in its habitable zone is different. He or she would ask, “What signatures demonstrate there’s life on Earth?” Our observer could spend a few evenings or mornings of work spread over a few months looking at … the earthlit moon.

When the moon is a crescent we see a fraction of its dayside hemisphere. A careful look shows the rest of the moon, shining faintly. This is the moon’s night side, illuminated by “earthshine”: sunlight reflected by Earth to the Moon, which reflects it back to our eyes. Therein is the key to figuring out whether or not there is life on Earth!

From firsthand experience you know that deserts, forests, and farmland reflect different amounts of different colors. Ocean doesn’t reflect as much light but in shallow areas any reflection will have appear more blue-green than yellow-orange-red. So the overall color of the earthshine on the moon will vary depending on what part of Earth is reflecting sunlight to the moon. Depending on the portion of Earth reflecting sunlight to the moon, the color of the earthshine will change (very subtly).

Our astrobiologist can use instruments to make careful measurements of the color of the earthlit moon. Combining measurements of earthshine made from around the world and over a few months or a year, the astrobiologist can see changes in the earthshine’s color, reflecting the part of Earth illuminating the Moon: Eurasia and Africa will show some combination of colors strong in green (plants) and earth tones (deserts). Earthshine reflecting from the Pacific Ocean will not be as bright. And reflections from clouds and snow will change the earthshine too, indicating weather and seasonal changes over time.

If our astrobiologist really cranks up the sensitivity, the presence of gases like methane, ozone, and water and the characteristic fingerprint of chlorophyll can be found in the light reflected back to Earth by the moon.

Comparing earthshine measurements with exoplanet measurements lets our astrobiologist determine if there is strong case for life on an exoplanet.

Isn’t it cool to think that the crescent moon is illuminating the search for life on exoplanets?