Astronomers and science fiction writers once dreamed of life sprinkled throughout the solar system, led by Martian canal-builders and oceans-dwellers on Venus. As spacecraft revealed the bleak and boiling state of those planets hopes faded, but are now staging a tentative, if much less dramatic, revival. However, debate rages as to where scarce resources should be deployed if we are to find life elsewhere in the solar system.

The outer solar system used to be thought too cold to support life, except perhaps under Titan's thick atmosphere. However, the discovery of tidal flexing has opened up thoughts of life on several moons of Jupiter and Saturn.

^{Professor Kenneth R. Lang, Tufts University} 

As illustrated in the sketch above, when a planet has multiple moons their gravity affects each other. An outer satellite will pull an inner one into a non-circular orbit. At perihelion the inner moon will be pulled out of shape by planet's stronger gravitational effect on the near side, just as our own moon raises a tide on Earth. When the moon is further from the planet it can relax towards perfect roundness. The moon is squeezed and relaxed like a giant stress ball, generating heat in the process.

Back To Mars
While tidal flexing is opening up a lot of new possibilities for life locations Professor Malcolm Walter, Director of the Australian Centre for Astrobiology argues, “Mars is clearly the best opportunity for both scientific and pragmatic reasons. Early in its history Mars was very similar to Earth environmentally, so we can make 1:1 comparisons, can compare places on Mars to places in [Australia's] Pilbara.”

Mars remains the focus for life-seeking exploration, notably the Curiosity and Opportunity rovers. The ExoMars mission, designed explicitly to search for Martian exobiology (life beyond the Earth) is a joint project of the European and Russian Space Agencies due for launch in 2016, with a possible follow up two years later. Even private companies are working on missions. Meanwhile three spacecraft sit in Martian orbit with two on their way.

Mars has plenty of features to keep us interested, but the quest for life drives these efforts. Yet oddly, we have approached this task indirectly, “There have been no specific searches for life on Mars since the Viking missions,” says Walter. “Everything since has looked at the atmosphere or geology. Curiosity might have the capacity to detect organic compounds but was not specifically designed for the purpose.”

Over time hopes for finding life have risen and crashed several times. Most recently, in September 2013 it was reported that Curiosity had found no sign of methane, considered a likely marker of microbes.

The methane absence was particularly disappointing because it contradicted previous results. Nevertheless, we have been lower still before. In the 1990s Mars was thought not only to be dead rock, but to probably lack water. Walter remains hopeful. “We can't conclude that life is rare – the methane study is controversial, there is some evidence currently being produced for detection, although they source could be microbes or volcanic.”

Definitive evidence of water is yet to be found, but the signs keep getting more encouraging. Gale and Endeavour Craters, Curiosity and Opportunity’s respective landing sites, both host sedimentary rocks that appear to have precipitated in water. There are also increasing signs that liquid water might still exist, albeit temporarily. 

Mission planners are focused on the question “If I was a Martian where would I live", hoping to find the perfect mix of rocks from wetter times, soil soft enough for deep investigations and safe landing sites. Debate rages between promising but already semi-explored places like Gusev Crater and newer horizons.

Europa

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_NASA

Europa was the first place where tidal flexing excited thoughts of life. . It's slightly smaller than our Moon, but beneath its smooth icy surface an ocean of liquid water larger than those on Earth is thought to lie, an unexpected discovery by Voyager. 

As Europa shifts slightly towards and away from Jupiter enough heat is generated to keep the ocean liquid, assisted by the insulating blanket of a layer of ice perhaps 20km thick. The existence of an ocean is thought to explain why Europa has very few craters, with evidence of past collisions erased by geysers or oceanic tides buckling the ice above. The cracks that crisscross Europa can also be explained by giant tides.

Internal heat within Europa's core could give rise to hotspots on the ocean floor rather like the black smokers, proposed as the places where Earth's life first appeared. Black smokers provide energy and nutrients, but advanced life forms still probably need oxygen. Presumably Europa lacks photosynthesizing plants or algae, but it has been calculated that cosmic rays striking Europa could free up oxygen, a process limited on Earth by our atmosphere.

Unfortunately, if life is restricted to the bottom of Europa's ocean it is going to be very hard for us to find. First we would have to get through all that ice, and then to the bottom of an ocean that could be 100km deep. Just getting more than a few meters down will be a challenge.

Thinking about what type of life could exist on Europa is “Very speculative because we know so little,” says Walter. “Europa could have 100km deep ocean, we can model a moon of that sort, can suggest probably hydrothermal systems at the base of the ocean, but that is as far as we can go, beyond that it is all speculation. It is going to be very hard to conceive of an experiment that would lead to a positive result – cracks could have water come to the surface and there might be biomarkers, but as yet dealing with speculation.”

Titan

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^{Titan's atmosphere in true color - methane molecules are being broken apart by sunlight and producing ethane and acetylene}

We've covered this one before. Titan's thick atmosphere, oceans and chemistry resembling the early Earth draw plenty of attention. Alas these oceans are of hydrocarbons, probably methane and ethane, rather than water. Nevertheless, simulations by NASA indicate that complex organic compounds can form in Titan's atmosphere. 

Titan is too far from Saturn for tidal flexing to matter. Titan's greenhouse effect raises the surface temperature compared to Saturn's other moons to a still chilly -179.2°C, although things get 60°C warmer in the upper atmosphere. Titan's lower atmosphere is depleted of hydrogen to an extent consistent with some prior modeling of the effects of widespread methanogenic life-forms, but other explanations also exist.

“We've learned a lot about Titan recently,” says Walter. ”There is a lot of interest for using it as a model for the very earliest Earth, when there may have been no life, but there might have been organic compounds in abundance. Most people thinking of it as a model for prelife, rather than expecting to find something living.” Others disagree arguing Titan should be the top priority for limited life-seeking resources.

Enceladus

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^NASA
Perhaps the most exotic prospect for life within our solar system is Enceladus ( En – sel – a das). Like Europa tidal flexing melts water internally, but with some extra quirks.

At just 504km wide Enceladus didn't attract much attention prior to the Voyager missions. However, these revealed both heavily cratered areas and regions that showed signs of being recently remade. Cassini revealed south polar volcanoes, most active when the moon is furthest from Saturn, are spitting jets of water into space (above) where they slowly fall back as a sort of snow, or become part of Saturn's E Ring. Spectral analysis reveals the presence of salt and organic compounds. “It sounds crazy but it could be snowing microbes on the surface of this little world,” said NASA's Dr Carolyn Porco.

Enceladus' current orbit is only elongated enough to produce a quarter of the moon's thermal emissions. The favored explanation is that Enceledus once had a much more eccentric orbit and built up plenty of heat, which is still being dissipated. We don't know how much temperatures may have varied, but large fluctuations are not life's friend. There are also theories that other orbital conditions may have seen Enceledus freeze solid at times.

Nevertheless, in 2011 Enceladus was described as “emerging as the most habitable spot beyond the Earth in the Solar System for life was we know it,” although the statement did come from a NASA Enceladus Focus Group conference. Enceladus “has liquid water, organic carbon, nitrogen [in the form of ammonia], and an energy source," The Focus Group's Chris McKay told Nature. "There is no other environment in the Solar System where we can make all those claims". Finding life on Enceladus may be relatively easy. Even if it only survives within the ocean, chemical traces should be found in the volcanic emissions.

Enceladian life is thought most likely to be in the form of methanogenic microbes binding hydrogen to carbon dioxide to produce methane and water (CO2 + 4 H2→ CH4+ 2 H2O). The substantial presence of methane in the volcanic plumes interests McKay, but geological sources are also possible. 

Even 1.5 billion km from the sun changed particles in the solar wind could react with molecules spewed out by Enceladus and form potential energy sources such as methanol and hydrogen peroxide. Material may slowly circulate between the bottom of Enceladus' ocean and space, with microscopic life using each stage of the process to grow. Comparisons with Antarctic Lakes suggest such processes are possible.

An alternative fuel source is acetylene, known to exist on comets, with hints of a presence of Enceladus as well. The conversion from acetylene to ethanol and acetate powers some life forms on Earth whose simple biochemistry suggests they might be among the first life to appear.

Jupiter's Other Moons

Ganymede and Calisto have less powerful sources of heat than Europa, and their surfaces are cratered. Nevertheless, evidence has emerged that they may also contain gigantic underground resevoirs – in Ganymede's case possibly several liquid layers with different types of ice in between. Ganymede also has the advantage of a magnetic field, which Europa lacks, which on Earth has been important for life. The obstacles to actually finding any life are likely to be even greater although Walter argues that for any of the moons if life is abundant within signs could be detectable. “The point is that all life is very powerful in driving the chemistry of surface environment,” he says.

Some diehards hold out hope for the outskirts of Io's volcanic regions. Unlikely as the wild temperature variations may make this, Io does have the advantage that such life might be easier to find than at the bottom of the Europan Ocean.

Closer to Home
Comets are far too cold for functioning life, but advocates suggest they might be a good place to find traces of life from other places, distributing amino acids or even living things from planet to planet. Even bolder suggestions include the carrying of life from one star system to another. While we don't expect to find anything alive on a comet, the more complex the organic compounds we can detect, the more common life is likely to be elsewhere. 

Amazingly, some people are still promoting that old favorite, Venus. It's hot enough to melt lead at the surface and the mid atmosphere rains sulfuric acid, but the temperature at the top of the clouds is almost civilized. Anything living in Venus would have the same problem as the life forms Carl Sagan proposed for Jupiter.

 

 

If your habitable zone is made of gas and sits above territory that will fry or crush, you life's pretty insecure. Survival is hard, and the long slow march to getting started harder still.

Finally, physicist Professor Paul Davies of Arizona State University has pondered, "Life as we know it appears to have had a single common ancestor, yet, could life on Earth have started many times? Might it exist on Earth today in extreme environments and remain undetected because our techniques are customized to the biochemistry of known life?" Discovering this “shadow life,” would increase confidence of life being common elsewhere in the universe, as well as providing us with an alternative evolutionary path that would be utterly fascinating, while also exceptionally cheap and easy to study.