NASA's battle cry behind the small armada of orbiters, landers and rovers dispatched to Mars is “follow the water!" Where there's water, there could be life, which needs a solvent like water to assemble the complex macromolecules needed for living systems.
Mars is covered with geological evidence that it was once a soggy planet. But no longer. One of the most exciting findings to date from the roving field geologist, the Mars Science Laboratory Curiosity, was the detection of a dried up ancient stream where water once flowed billions of years ago.
The irony is that if you travel a couple hundred million miles beyond Mars' orbit you cross the solar system's frost line, the boundary beyond which there is plenty of water preserved from the planets' birth.
At least six outer moons have subsurface oceans that could potentially be cozy places for life: Europa, Ganymede, Callisto, Titan, Enceladus and Triton. Each of them could have as much if not more water than found in all of Earth's oceans. In fact Earth is a comparatively dry world.
The idea of a stellar habitable zone, where water can remain stable on a planet's surface, was scientifically spelled out and popularized by Michael Hart in the late 1970s. Since such a zone is a narrow slice of the solar system's real estate, Hart used his widely cited research paper to support the Rare Earth hypothesis: that the evolution of complex life would be hard to replicate in the cosmos.
Today, the concept of a habitable zone is old fashioned says Ken Hand of NASA's Jet Propulsion Laboratory. “The Goldilocks scenario is outdated. There are new ways to mediate habitability via tidal interactions."
This new paradigm is further bolstered by the emerging realization that there is a tremendous diversity of life on Earth in extreme environments. In fact, the so-called “exteremeophiles" were probably the first inhabitants of Earth — and will be the last survivors 1 billion years from now.
Finding samples of life in extraterrestrial oceans is no small task. It requires burrowing through miles of a thick ice shell. But in actuality that would be far less difficult than sending an industrial drilling rig and astronaut crew to Mars to penetrate deep into subsurface aquifers.
More importantly, finding life in a Europa ocean would unequivocally prove that a Genesis II took place in the solar system. And that would mean that life is an inevitable spinoff of an evolving universe.
Even more profoundly, if Europan microbes incorporated RNA and DNA into their biological machinery it would demonstrate that the concept of convergent evolution beats out contingent evolution that favors a purely random sequence of events (as in the Rare Earth hypothesis).
Convergent evolution predicts that the universe defaults to the same molecular template for life regardless of the initial starting conditions and biological constraints. No doubt creationists would embrace such news as evidence for intelligent design.
Why can't finding Mars microbes lead us to the same solution? The problem is that if Martians were found to use DNA and RNA, it would be tempting to think that they are really our cousins. The early solar system may have seen planetary cross-fertilization via dispersal of hitchhiking microbes between Earth and Mars meteorites. Or, less likely, Mars may have been contaminated by poorly sterilized spacecraft from Earth.
This would not be the case for any of the outer solar system oceans that have been encapsulated for billions of years.
The Saturnian moon Enceladus is one of the most promising places to go “microbe fishing" though it is a staggering one billion miles away. The Enceladus ocean “jumped out at us," says Hand. Geyser-like plumes spewing off the moon from slush fill surface cracks contain water and organics. The moon is tidally heated and this has been brewing an ocean for billions of years.
At half Enceladus' distance, the Jovian moon Europa seems a better destination for astrobiology hunting. Europa has two to three times more water than Earth. Where Earth's oceans average a depth of a few miles, Europa's ocean is at least ten times deeper.
The European Space Agency's planned Jupiter Icy moons Explorer (JUICE) will tour all three Jovian ocean worlds, Europa, Ganymede and Callisto beginning in 2030. Looking beyond 2030 the mother of all sample returns would be to land on Europa and dispatch a nuclear-heated cryobot probe to melt its way though a thin portion of the ice shell. Ultimately, samples of the Europan ocean would be returned to Earth for study at a class 5 biocontamination lab.
Sterilization would be no problem because the probe would be irradiated in Jupiter's seething radiation belts. The moon's hydrogen peroxide would further sterilize the probe as it burrowed through the ice.
However, a Europa lander and penetrator presents numerous engineering challenges and no doubt would be costly. A less expensive proposed mission called the Live Investigation for Enceladus (LIFE) would be a descendent of the Stardust mission that captured dust grains from comet Wild-2 during a flyby. The $200 million NASA Stardust spacecraft trapped the grains in a disk of aerogel. LIFE would use a similar method to gather samples from zooming thorough the geysers (shown above) of Enceladus and then return to Earth.
The solar system is so rich and varied in ocean worlds, that finding Genesis II is probably more a matter of when, not if.
Image credit: NASA, ESA