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

Europa
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.
Europa is a largish moon, 3100 km is diameter nearly 90 % the size of our Moon. Europa, unlike our Moon, is covered by a surface shell of ice 15 to 25 km in thickness, covered by long cracks and fissures giving it the appearance much like that of ice floes at the poles on Earth. The surface is very young, but more surprising was the discovery that, also like on Earth, this ice cover most likely is floating on top of a deep layer of liquid water of depth 60 to 150 kilometers. The average depth of the Earth's oceans is around 3.7 kilometers. In Europa's case though, the water layer appears to cover the entire moon, a global subsurface ocean. Additionally, the Hubble telescope detected what appear to be plumes rising from the surface of Europa.

How are these things possible?
Relatively speaking, Europa has much more water than does the Earth. This is understandable because Europa, like Jupiter formed outside of the snowline of the protosolar nebula, the large gas cloud from which the Sun and planets formed. Outside of the snowline, water can become ice and so is easily incorporated into the forming planets and moons.
Because there are liquid oceans on Europa, there must be a source of heat (or high concentrations of salts or ammonia) to keep Europa's water liquid. Europa is too far from the Sun for Solar heating to work, it is too small for it to retain heat from its formation for 4.6 billion years, and too small to contain enough radioactive elements to keep its interior warm. Gravitational tugging from Jupiter appears to provide enough heat to keep the water liquid instead of frozen (see next panel). The subsurface environment of Europa is thought to be similar to ocean bottoms on Earth. No sunlight, but volcanic vents generating heat and minerals. Such spots could be ideal for at least simple forms of life. Places such as these deep in the oceans on Earth are brimming with organisms which don't require sunlight to survive.

Gravitational tugging from Jupiter provides enough heat to Europa's interior. As Europa orbits Jupiter it experiences tidal forces, like the tides caused by the Moon in our oceans. Tidal forces flex and stretch Europa because its orbit is elliptical in shape. The tide is higher when Europa is close to Jupiter than when it is farther. The flexing heats Europa making Europa's interior warmer than it would be from the Sun's heat alone. Flexing might also produce volcanic activity from the rocky interior as on the neighboring moon Io. The tidal forces also cause Europa's icy outer shell to flex likely causing the cracking seen in Europa's surface.
In the movie Europa is seen in a cutaway view through two cycles of its 3.5 day orbit about the giant planet Jupiter. Like Earth, Europa is thought to have an iron core, a rocky mantle and a surface ocean of salty water. Unlike on Earth, however, this ocean is deep enough to cover the whole moon, and being far from the sun, the ocean surface is globally frozen over. Europa's orbit is eccentric, which means as it travels around Jupiter, large tides, raised by Jupiter, rise and fall. Jupiter's position relative to Europa is also seen to librate, or wobble, with the same period. This tidal kneading causes frictional heating within Europa, much in the same way a paper clip bent back and forth can get hot to the touch, as illustrated by the red glow in the interior of Europa's rocky mantle and in the lower, warmer part of its ice shell. This tidal heating is what keeps Europa's ocean liquid and could prove critical to the survival of simple organisms within the ocean, if they exist. The giant planet Jupiter is now shown to be rotating from west to east, though more slowly than its actual rate.

In the fall of this year, October 2024, the Europa Clipper

will be launched for Jupiter.

Enceladus
Then there's Enceladus. Enceladus is one of the major inner moons of Saturn along with Dione, Tethys, and Mimas. It orbits Saturn at a distance of 148,000 miles (238,000 km), falling between the orbits of Mimas and Tethys. It is tidally locked with Saturn, keeping the same face toward the planet. It completes one orbit every 32.9 hours within the densest part of Saturn's E Ring, the outermost of its major rings, and is its main source. Enceladus is trapped in an orbital resonance; its resonance with Dione excites its orbital eccentricity, which is damped by tidal forces with Saturn tidally heating its interior, and possibly driving the geological activity.
Several years ago, Cassini made the startling observation of geysers, plumes of material erupting from the south polar region through large, warmer cracks nicknamed tiger stripes. Subsequently, Cassini has flown directly through the geysers, analyzing their composition finding they are water vapour, ice particles, salts, and organics. Analysis of 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.
Later Cassini observations taken during its October 2015 dive through Enceladus' geyser plume, Cassini's deepest dive through the plume itself getting to within 30 miles of Enceladus' surface yielded much interesting information (Waite et al. 2015). Using Cassini's Ion and Neutral Mass Spectrometer (INMS) instrument, Waite and colleagues calculated that molecular hydrogen H₂ made up between 0.4 % and 1.4 % of the volume of Enceladus' geyser plume. Further calculations revealed that carbon dioxide CO₂ made up an additional 0.3 % to 0.8 % of the plume's volume. The H₂ is likely produced by reactions between hot water and rock in and around Enceladus' core, Waite and colleagues concluded; neither Enceladus' ocean nor its ice shell are viable long-term reservoirs for volatile H₂, and processes that disassociate H₂ from water ice in the shell don't seem capable of generating the volume measured in the plume.
An explanation for the data is that there may be hydrotheraml vents near the botton of the ocean on Enceladus (see next section). This is consistent with a 2016 study by another group which had concluded that tiny silica grains detected by Cassini could have been produced only in hot water at significant depths.
These results about Enceladus and its plumes are exciting given what they say about the possibility of extra-Terrestrial life. We next follow the logic chain that suggests these results suggest that Enceladus should be a prime target for searches for extra-Terrestrial life.

Firstly, we know that Enceladus has water, a key ingredient for life as we know it (LAWKI). This is understand because Saturn (and thus Enceladus) formed beyond the snowline of the protosolar nebula.
Enceladus not only has water, it has a sub-surface liquid ocean because of tidal heating due to Saturn (similar to the way in which Jupiter tidally heats Europa).
Current studies suggest Enceladus has another key ingredient for life: Enceladus has an energy source.

Deep-sea hydrothermal vents and chemical reactions. Earth's deep-sea hydrothermal vents support rich communities of life, ecosystems powered by chemical energy rather than sunlight.

"Some of the most primitive metabolic pathways utilized by microbes in these environments involve the reduction of carbon dioxide CO₂ with H₂ to form methane CH₄ by a process known as methanogenesis (Seewald 2016).
The inferred presence of H₂ and CO₂ in Enceladus' ocean coming from the observations of the plumes by Cassini, suggests methanogenesis takes place deep below Enceladus's icy shell.
"Indeed, the observed H₂ levels indicate that a lot of chemical energy is potentially available in the ocean," Glein said. "It's quite a bit larger than the minimum energy required to support methanogenesis," he said. Glein stressed, however, that nobody knows whether such reactions are actually occurring on Enceladus. "This is not a detection of life," Glein said. "It increases the habitability, but I would never suggest that this makes Enceladus more or less likely to have life itself. I think the only way to answer that question is, we need data."
Seewald also cautioned on an astrobiological interpretation noting that H₂ is rare in Earth's seawater, because hungry microbes quickly gobble it all up. quickly gobble it up. "Is the presence of H₂ an indicator for the absence of life, or is it a reflection of the very different geochemical environment and associated ecosystems on Enceladus?" Seewald wrote. "We still have a long way to go in our understanding of processes regulating the exchange of mass and heat across geological interfaces that define the internal structure of Enceladus and other ice-covered planetary bodies."

Titan
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.

Is There Evidence for Life on Titan?
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.