Goldilocks’ Outdoor Hammock

‘The Goldilocks Zone’. The name given to the distance from the sun where liquid water can exist. Earth is thought incredibly special and lucky because it orbits the sun in this zone, and as a result is covered with liquid water. In the search for extraterrestrial life, people have often thought of the Goldilocks Zone around other stars as the most important place to look, perhaps the only place worth looking. But this could easily be false. There are other places in our solar system where life forms could conceivably exist, and they probably aren’t where you expect.

Look away from Earth, towards Mars. And then keep going because we’re not talking about Mars. Stop at Jupiter. The big one. Jupiter’s a gas planet, in that while there is a rocky core at the center, The bulk of the planet is swirling gas. We’re not going in there. We’re talking about Europa.

Out of Jupiter’s 60-and-counting moons, The four most ‘important’ are Ganymede, Callisto, Europa, and Io. With these four being so far from the sun, they would almost be completely overlooked in any search for life.

But while Europa’s surface is ice, there is evidence that there could be a liquid ocean of water beneath the surface. For one thing, the ice appears fairly smooth, not heavily eroded, as if it froze recently (relatively speaking), which would mean the water below may have a greater chance of remaining liquid under the insulating ice surface. Europa’s ice also has characteristic cracks that you might see if there was a great body of liquid beneath, rising and falling with tides and straining the surface. Furthermore, since Europa is orbiting Jupiter, it gets gravitationally “stretched” and “flexed” during its orbit, which provides heat energy to the moon, the same way pulling a rubber-band repeatedly warms it up. This process would be important in keeping Europa’s under-ocean from freezing.

Europa’s surface of seemingly young ice, with tell-tale stress-induced cracks. This could be the roof of a living ocean. Credit: NASA/JPL/University of Arizona. Photo from European Space Agency, taken from Flickr.

So if deductions about Europa’s sub-surface ocean are true, that means there’s a huge body of water in our very solar system, and a prime candidate for the search for life. And plans have been suggested to search for it there. In the coming years, NASA has plans to send spacecraft to the moons of Jupiter, observing them more closely. Eventually even landing a craft on Europa to analyse the chemistry of the ice. As always, they will have to be careful to keep the spacecraft completely sterile, to avoid releasing Earth-life into Europa’s ocean!

There’s another moon we need to talk about though, and it’s even further out. Around Saturn is the moon Titan, which has a lot in common with Earth. A thick, nitrogen-heavy atmosphere, liquid oceans on the surface, and even rain. The difference being that the liquid here isn’t water. It’s made of hydrocarbons like methane and ethane, what we’d call natural gas on Earth. The life we’re used to is dependent on water, but organic carbon molecules like these could be the basis for a completely different kind of life. Our biological molecules are based around carbon because of its chemical versatility in building large complex molecules. In an ocean full of carbon, there is no telling what kind of complex molecules may arise.

Titan’s hazy surface. Credit: ESA, NASA, JPL, University of Arizona. Image taken from Flickr (John Batchelor)

We can’t afford to be narrow minded about the possibilities of where and how life may exist, or what life looks like and what it even means. Perhaps the Goldilocks zone is an outlier, and the really reliable places for life in the universe are outer moons, sheltered by ice and parent planets from the heat of stars and from asteroid impacts. Earth life has always been at risk to these forces, while Europan and Titanian life would have a different set of challenges. It’s a chilling thing to think of something swimming quietly in an underground ocean far away, unaware that there’s even a sky.


Further reading: