Is There Life on Saturn’s Moons?

Huygens was a pinnacle among many high peaks for the Cassini mission, marking the first time humans had landed a spacecraft on any body in the outer Solar System. It saw a terrain sculpted by wind and liquid, with vast hydrocarbon dunes and sinuous chan­nels carved into the shoreline. Volcanoes in the distance are likely to emit water instead of lava. Meanwhile, a tepid ocean sits under the icy crust of little Enceladus. These two moons are worlds at once alien and familiar, and if they host life, it will be unlike any form of life known on Earth. In 2011, the sum of all the evidence so far caused scientists at a major meeting to elevate Enceladus to the status of the “most habitable spot in the Solar System” beyond Earth, above Titan, and even above Mars.58

One of the conundrums of the search for life beyond Earth is the difficulty of moving away from anthropocentric thinking. All life on this planet is one thing, derived from a single common ancestor and a single implementation of information storage in genetic ma­terial. There’s no way to inductively formulate a “general theory” of biology or know how life might have evolved if conditions had been slightly different, or if it would have evolved at all. Since the hypotheticals cannot be answered, we don’t know whether the origin of life on Earth was a historical accident or the nearly inevitable outcome of physical and chemical conditions plus the passage of time. These two scenarios project very different roles for life elsewhere in the universe: sparseness or abundance. We do know that life has adapted to almost every conceivable ecological niche on the Earth, including environments below water’s freezing point and places where the source of energy is not sunlight or pho­tosynthesis. Life grips the planet like a fever, thriving in almost all places we can imagine and in some that are nearly unimaginable.

If Titan and Enceladus host life, we’ll not just have to expand the envelope of our traditional thinking about life processes, we’ll have to throw away the box completely. If the moon of a giant planet is hospitable to biology, then by focusing only on finding Earth-l ike terrestrial planets we’ll be missing a large part of the story. If a tiny moon far from the Sun can be habitable, the num­ber of potential living worlds throughout the Milky Way galaxy rises dramatically to several billion. Biochemistry based on ethane and methane, augmented by water and ammonia, would be unlike anything we’ve ever seen on Earth. Computer and lab simulations might help, but we still know too little about physical and chemi­cal conditions on these moons for them to be reliable. The next time window to get a gravity assist and revisit the Saturn system is 2015 to 2017, which adds a sense of urgency since the oppor­tunity following that one is not until 2030. Cassini and Huygens have given us just a taste of the potential for life beyond Earth; if we want to address this profound question we’ll have to go back.