A Tale of Three Moons: Is There Life in the Outer Solar System?

(Are There Two Habitable Zones in Planetary Systems?)

Until fairly recently, the search for life elsewhere in the solar system has focused primarily on Mars, as it is the most Earth-like of all the other planets in the solar system. The possibility of finding any kind of life farther out in the outer solar system was considered very unlikely at best; too cold, too little sunlight, no solid surfaces on the gas giants and no atmospheres to speak of on any of the moons apart from Titan.

But now, some of the places that were previously considered the least likely to hold life have turned out to be perhaps some of the most likely to provide habitable environments. Moons that were thought be cold and frozen for eons are now known to be geologically active, in surprising ways. One of them is the most volcanically active place known in the solar system. At least two others appear to have oceans of liquid water beneath their surfaces. That's right, oceans. And geysers. On the surface, they are ice worlds, but below, they are water worlds. Then there's the one with rain, rivers, lakes and seas, but made of liquid methane instead of water. Billions of kilometres farther out from the Sun than the Earth. Who would have thought? Let's look at those last three in a bit more detail

Ever since the film 2001: A Space Odyssey first came out, Europa has been the subject of fascination. A small, icy moon orbiting Jupiter, its depiction in that movie, as an inhabited world beneath its ice crust was like a sort of foreshadowing, before the Voyager and Galileo spacecraft gave us our first real close-up looks of this intriguing place. Its surface shell of ice is covered with long cracks and fissures, giving it an appearance much like ice floes at the poles on Earth. More surprising though, was the discovery that, also like on Earth, this ice cover most likely is floating on top of a deep layer of liquid water below. In Europa's case though, the water layer appears to cover the entire moon, a global subsurface ocean. How is this possible? If there is liquid water, there must be heat (or high concentrations of salts or ammonia), and if you have water and heat, could there be something living in those waters? Gravitational tugging from Jupiter indeed appears to provide enough heat to keep the water liquid instead of frozen. The environment is now thought to be similar to ocean bottoms on Earth. No sunlight, but if there are volcanic vents generating heat and minerals, as on Earth, such a spot could be ideal for at least simple forms of life. On Earth, places like these deep in the oceans are brimming with organisms which don't require sunlight to survive.

Then there's Enceladus. Another very small icy moon, orbiting Saturn Geological activity was considered very unlikely on such a tiny world, only a few hundred kilometres in diameter. But then Cassini saw the geysers, plumes of material erupting from the south polar region through large, warmer cracks nicknamed tiger stripes. Cassini has now flow directly through the geysers, analyzing their composition, which is mostly water vapour, ice particles, salts and organics. The latest analysis based on the Cassini data indicates that they almost certainly originate from a sea or ocean of liquid water below the surface. Warm, salty water loaded with organics; could Enceladus be another possible niche for extraterrestrial life? As with Europa, only further missions will be able to answer these questions, but the possibilities are exciting.

Titan, the largest moon of Saturn is even more interesting in some ways. It is perpetually shrouded in a thick smoggy atmosphere of nitrogen and methane, so the surface had never been visible until Cassini and its small lander probe Huygens , first looked below the smog and clouds. Titan is like an eerily alien version of Earth, surface pressure 1.5 atm with rain, rivers, lakes and seas, but being far too cold for liquid water, T ~ -180 C (not much heat here), its water cycle may be composed of liquid methane/ethane.

At left, we show a Phase Diagram for water. The diagram shows the phase (solid, liquid, vapor) expected for water for a given temperature and pressure. Typical of conditions near those found at the surface of the Earth, water is near its Triple Point where it can exist in all 3 phases and we find polar caps, clouds, and oceans on the Earth--we have the Water Cycle. On Titan, a similar situation exists for Methane (see Phase Diagram at right) and methane polar caps, methane oceans, and methane clouds can form--there may be a Methane Cycle on Titan. Note that 100 Kelvin is -173 Celsius (Centigrade), around 200 degrees below the temperatures at the surface of the Earth, but about the surface temperature on Titan.

Appearance-wise, the surface and geology look amazingly Earth-like, but the conditions are uniquely Titan. For that reason, it has long been considered that the chances of any kind of life existing here are remote at best.

In the last ten years, scientists have started to consider the possibility of life forming in Titan-like environments using liquids other than water, such as methane. Could life occur in a liquid methane lake or sea? How would it differ from water-based life? A discovery was made by Cassini/Huygens which could be interpreted as evidence of methane-based life on Titan. There was a seeming disappearance of hydrogen from Titan's atmosphere near its surface and a lack of acetylene on the surface of Titan. Previous theoretical studies suggested that those two circumstances, if ever found, could be evidence for methane-based lifeforms; lifeforms that consumed hydrogen and acetylene rather than oxygen. This is highly speculative; a chemical explanation is probably more likely according to the scientists involved, however, biology cannot be ruled out. Future proposed missions for Titan include a floating probe to land in one of the lakes and a balloon to soar over the landscape, pursuing such mysteries as never before. How cool is that?

Comments by Chris McKay on the Cassini/Huygens results.

Is Life Forming on Titan?

A team of investigators led by University of Arizona graduate student Sarah Horst has approximated, in a French lab, atmospheric conditions on Saturn's moon Titan. Through a series of experiments, they bombarded the gases with radiation, producing a number of compounds, including amino acids.

Could these molecules be the basis for the development of life on Titan?

Imagine a puree of plant matter in a blender. Then imagine an army of very tiny tweezers selecting and throwing out most of the important chemicals and elements, like the lipids, vitamins, metal ions, phosphates, sugars, and most of the amino acids. Then add a broad mixture of thousands of other random chemicals to the few remaining but now pulverized and randomized plant-derived chemicals. Finally, imagine tossing the brew into the atmosphere and claiming that it is now ready to serve as a springboard to life. Imagine no more.

Horst and the other researchers supplied radiation powerful enough to break the triple bond between the two nitrogen atoms that comprise nitrogen gas a known constituent of Titan's atmosphere. which freed the nitrogen atoms to bond with other nearby atoms, including carbons.

What was in the resulting concoction? Assuming there are at least three or four structural variations of each, we are talking up to 20,000 molecules that could be in there, including a handful of some of the smallest chemical units found in cells, according to University of Arizona information.

Oh, and the moon that is the most volcanically active place in the solar system? Io, although with the only known forms of liquid there being extremely hot lavas on that sulfuric hothouse, the chances of life are still thought to be unbelievably slim. But that's ok when you start to find out that worlds with oceans and lakes, etc. may be much more common than previously imagined.