By Josh Roberts
As Dr. Anthony Aguirre said in a recent Dean Lecture here at Morrison Planetarium, “The Universe doesn’t seem to make single copies of things.” To this point, recent discoveries on Mars show us how alike Earth and Mars were ages ago. And now we have learned that Europa, a major moon of Jupiter, may have a kindred world in Enceladus, a minor moon of Saturn.
Europa has long been held as a prototype of a possible harbor for life. Planetologists have envisioned the icy surface as warmed by gravitational interactions with Jupiter and becoming molten, which would both lubricate further gravitational squeezing and cause the denser water to sink below the ice, protecting it from harsh surface conditions. Some estimates have placed the volume of Europa’s oceans as twice that of Earth, and although the chemical conditions might not be an ideal starting point for terrestrial life, evidence suggests that Europa’s sister moon, the ever-volcanic Io, is contributing mineral salts to Europa’s surface, allowing for a great diversity of chemicals within its dark oceans and raising its potential for harboring life.
While the grand count of extra-terrestrial life forms is still resting at ZERO, many astrobiologists agree that if worlds like Europa are common, then the likelihood of life existing somewhere out there in the Universe is high.
The question becomes: just how common are they?
Evidence for oceans under the surface of Enceladus comes from variations in both the gravity and the shape of the moon. The Cassini spacecraft has measured both with some precision, and the results suggest an increase in density under the south pole. What’s denser than ice? Water, of course! (The Washington Post has a great article that describes the discovery and asks whether NASA should send a spacecraft to go check it out.) Although the oceans of Enceladus might only be associated with its southern hemisphere and might not be as extensive as those on Europa, they do have one major trait that could make them more life-likely: their proximity to the surface.
Some estimates put the icy crust of Europa at over fifty miles thick and while huge basins of shattered ice (called “catastrophic lakes”) might allow for chemical interchange in places, most of Europa’s oceans would be isolated from the surface. Because Enceladus is much smaller, the crust is thinner and the ocean is closer to the surface. Perhaps one day an explorer from Earth could access this alien ocean and see if anything lives there.
My bet? Sea monkeys!
These findings from the gravity measurements of Enceladus appear in the recent edition of the journal Science.
Josh Roberts is a Senior Presenter and astronomer at the California Academy of Sciences. He also contributes to Morrison Planetarium productions and is involved in Bay Area astronomy outreach.