The other day, I posted on the creation of bacteria that use arsenic instead of phosphorus as one of the essential building blocks of life. In the post, I wrote, “I think the true eureka moment will come when we find an exoplanet (one that orbits a star other than the sun) that has a large percentage of molecular oxygen in its atmosphere.” I noted that, so far, only Earth was known to have an atmosphere rich in oxygen, thanks to the abundance of life here.
The very next day, I learned that another heavenly body has been found to have an oxygen-rich atmosphere, and it’s right here in the dear old solar system: Saturn’s second-largest moon, Rhea. The Cassini Solstice Mission to Saturn, which has been one of the most spectacularly successful space missions ever and is still pouring forth data, has just discovered it. It recently detected an atmosphere on Rhea with densities of about 1 billion oxygen molecules and 600 million molecules of carbon dioxide per cubic foot.
This doesn’t mean there are little green Rheans running around. For one thing, it’s far too cold there for anything resembling Earth-like biochemistry to be taking place. With sunlight only about 1 percent as strong as on earth, the temperature on Rhea is about -179° Celsius (-290° Fahrenheit). For another, the atmosphere is extremely tenuous, about five trillion times less dense than Earth’s atmosphere. The wonder is that a body as small as Rhea, only about 900 miles in diameter, has any atmosphere at all. Our moon, which has more than twice the diameter of Rhea (and is much more dense as well; Rhea is essentially a large ice ball), has none. As one of the scientists involved said, “Rhea’s oxygen appears to come from water ice on Rhea’s surface when Saturn’s magnetic field rotates over the moon and showers it with energetic particles trapped in the magnetic field.”
The significance here, of course, is that life is obviously not the only way to produce an oxygen-rich atmosphere and that “The new results suggest that active, complex chemistry involving oxygen may be quite common throughout the solar system and even our universe.” Still, this wispy-thin atmosphere was detectable only by a probe in very close proximity to Rhea. At interstellar distances, only dense, warm, oxygen-rich atmospheres are going to be discovered anytime soon. And should one be, it would still be a powerful indicator of life.
What a wondrous golden age of astronomy we live in. An astronomer friend of mine, Doug Finkbeiner, leads a team that recently uncovered two vast energy bubbles erupting from the center of the Milky Way. Now oxygen on Rhea. There’s been nothing like it since Galileo first pointed the newly invented telescope at the sky and said, in a rough translation from the Italian, “Holy cow! Look at that!”
