A Tale of Three Moons: Is There Life in the Outer Solar System? (Are There Two Habitable Zones in Planetary Systems?)

The search for life in our Solar System outside Earth, 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 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, the largest moon of Saturn. Europa has a thin atmosphere of oxygen and water, 10^-12 bars, and Enceladus has a tenous variable atmosphere of water vapor.

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 of Jupiter and Saturn that were thought to be cold and frozen for eons are now known to be geologically active. Io is the most volcanically active body known in the Solar System. At least three other Galilean moons, Europa, Ganymede, and Callisto, and the Saturnian moon, Europa, appear to have extensive subsurface oceans. On the surface, they are all ice worlds with no atmospheres to speak of, but below, they are ocean worlds.

This brings up an interesting point in that Europa and Enceladus are currently considered to be natural places to look for LAWKI despite not having atmospheres and are not in the region that we called the Habitable zone of our Solar System!

And, finally, there's Titan, the largest moon of Saturn with its rain, rivers, lakes and seas made of liquid methane rather than water. Who would have thought that we would find so many places for LAWKI in the outer Solar System? We look at the three moons, Europa, Enceladus, and Titan in a bit more detail.

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I. Ocean Worlds

Ocean Worlds of the Solar System Click on the picture of Europa to the right to see a video on the ocean worlds of our Solar System. Start around 1:18 for a discussion of the Jovian moons.

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II. Europa and Enceladus

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 as an inhabited world beneath its ice crust was a foreshadowing of the upcoming Voyager and Galileo results. Voyager and Galileo gave us our real close-up looks of this intriguing place. Europa has an ice shell covered by long cracks and fissures giving it an appearance much like the ice floes at the poles on Earth. The cracking is thought to be due to tidal stresses placed on Europa by Jupiter and the other Galilean moons. More surprising though was the discovery that, like on Earth, this ice cover was floating on top of a deep layer of liquid water. The ice shell is 10 to 15 miles (15 to 25 kilometers) thick floating on an ocean 40 to 100 miles (60 to 150 kilometers) deep. This was found through measurements of Europa's magnetic field by Galileo; observations which suggested some sort of conducting liquid (most likely a salty water ocean) must exist in the interior of Europa beneath the outer ice shell. Europa is one-fourth the size of Earth but its ocean may contain twice as much water as Earth. On Europa, the water layer appears to cover the entire moon, a global subsurface ocean. How is this possible? If there are liquid oceans, there must be a source of heat 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 and its other Galilean moons provides the heat needed to haves oceans rather solid layers of ice on Europa, much as the gravitational forcing on Io leads to its warm interior and extensive vulcanism. As a consequence of the tidally heated interior of Europa the seafloor of the subsurface ocean on Europa is thought to be similar to that found near our ocean bottoms. No sunlight, but volcanic vents supplying heat (energy) and minerals (fuel) to power simple forms of life. Those hydrothermal are ideal for some 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. Although not a settled question, it is also plausible that life on Earth originated near such hydrothermal vents (see right, black smoker) at the bottoms of our oceans, regions out of contact with our atmosphere and sunlight.

left image is true color, right is enhanced to show features on surface

Are There Surface Features and Other Evidence that Point to the Existence of Hydrothermal Vents and Perhaps LAWKI? Europa is criss-crossed by linear fissures (cracks) many of which overlap. The fissures may result from the tidal forcing causing stresses and cracking in Europa's icy surface. The whitish and bluish regions are pure water ice while the reddish and brownish regions carry non-ice components such as salts and sulfur compounds dredged up from the ocean floor.. The large dark bands are regions of surface spreading (plate tectonics). If the spreading is caused by convective flows, the convection may take materials produced on Europa's surface to the bottom of the ocean floor where it may be used with materials coming from the interior of Europa at hydrothermal vents as energy source for life. The active geology suggested by these features is supported by the young age of the surface, 40 to 90 million years old. Jumbled, reddish chaotic regions where the ice has been broken up and refrozen into new patterns ( Conomara region). The chaos regions although interesting in their own right, may offer clues as to how life could arise on Europa (start around 1:30). Near the dark bands, an interesting discovery was made using the Hubble Space telescope; plumes similar to those of geysers on the Earth, may exist on Europa. These properties are so tanatalizing that we will go back to Europa in the 2020s (the Europa Clipper) to study further this interesting Jovian moon.

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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 geologial activity. Enceladus, as is Europa, is encased in ice; its ice shell may be as thin as half a mile to 6 miles (0.8 to 10 kilometers) at its south pole with average thickness about 12 to 16 miles (20 to 25 kilometers). The evidence that led to the discovery of Enceladus's sub-surface ocean came from gravity and magnetic field measurements; astronomers found evidence of a slight wobble in the motion of Enceladus as it orbited Saturn. and a weak magnetic field around Enceladus. The Cassini mission made the crucial observation of geysers, plumes of material erupting from the south polar region through large, warmer cracks nicknamed, tiger stripes. Cassini flew directly through the geysers analyzing their composition finding that they were composed of water vapor, 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? Cassini made several more discoveries during its Grand Finale where Cassini dove deep through Enceladus's geyser plumes. Cassini's deepest dive through the plume reaching within 30 miles of Enceladus, yielded very 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. Why this is so interesting will be answered shortly. Further missions are likely needed to answer the question of life but the possibilities are exciting given our current understanding of Enceladus. We know that Enceladus seems to possess two of the prime needs for life, We know that Enceladus has liquid water, a key ingredient for life as we know it. This result was surprising for a moon but was easily understood. Enceladus has a liquid ocean because of tidal heating due to Saturn similar to the tidal heating of Europa by Jupiter. Current studies also suggest another key ingredient: Enceladus has an energy source. based on what we know about hydrothermal vents on the Earth. This is further supported by a 2016 study which concluded that tiny silica grains detected by Cassini can only have been produced in hot water at high pressure. Methanogenesis Deep-sea chemical reactions and Hydrothermal Vents. It has been speculated that life on Earth originated around hydrothermal vents 3.8 billion years ago from simple metabolic reactions. Today, 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), 4H2+CO2--->CH4+2H2O The detection of H2 and CO2 in the plumes suggests methanogenesis may be happening deep beneath 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 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."

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Europa Clipper To launch in 2020s and perform a detailed investigation of Europa Europa Clipper will go into orbit around Jupiter and make 45 flybys of Europa at closest approach distances ranging from 16 miles ot 1,700 miles above Europa's surface.. Nine instruments have been selected for the Europa Clipper; Plasma Instrument for Magnetic Sounding (PIMS) Interior Characterization of Europa using MAGnetometry (ICEMAG) Mapping Imaging Spectrometer for Europa (MISE) Europa Imaging System (EIS) Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) Europa THermal Emission Imaging System (E-THEMIS) MAss SPectrometer for Planetary EXploration/Europa (MASPEX) Ultraviolet Spectrograph/Europa (UVS) SUrface Dust Mass Analyzer (SUDA)

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III. Titan

Titan, the largest moon of Saturn is even more interesting in some ways. It is perpetually shrouded in a thick smoggy atmosphere of 98.4 % nitrogen, 1.4 % methane, and (0.1-0.2) % hydrogen. Consequently, its 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.

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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.

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