If life exists on the icy ocean moons of Enceladus and Europa, detectable trace molecules could survive just below their frozen surfaces.
Scientists have long theorized that both Enceladus, one of Saturn's 146 known moons, and Europa, one of Jupiter's four large Galilean moons among its total 95 moons, could host vast liquid water oceans that harbor life. If this is the case, then complex organic molecules like amino acids and nucleic acids, the building blocks of life as we know it, could serve as "biosignatures" of life on the worlds.
The problem, however, is that both Europa and Enceladus are bombarded by harsh radiation from the sun that could potentially destroy complex organic molecules at their surfaces. But new research offers some hope on this front, suggesting that those biosignatures could indeed survive if they're preserved in the icy shells of the moons. And if that's true, these molecules could sit so close to the surface that future robotic landers may be able to dig them free. At Enceladus, in fact, this digging might not even be needed; biosignature molecules could survive in shallower ice than on Europa.
"Based on our experiments, the 'safe' sampling depth for amino acids on Europa is almost 8 inches (20 centimeters) at high latitudes of the trailing hemisphere, the hemisphere opposite to the direction of Europa's motion around Jupiter, in the area where the surface hasn't been disturbed much by meteorite impacts," research leader Alexander Pavlov of NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. "Subsurface sampling is not required for the detection of amino acids on Enceladus — these molecules will survive radiolysis, breakdown by radiation, at any location on the Enceladus surface less than a tenth of an inch (under a few millimeters) from the surface."
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Dramatic plumes that erupt through the icy shell of Enceladus could also mean orbiting robotic missions will be able to snatch these biosignature molecules from around the Saturnian moon without the need to visit the surface.
Life would run deep on icy moons
Though Europa and Enceladus are often cited as two of the most likely worlds to harbor life elsewhere in the solar system, this life is very unlikely to dwell at the surface of these moons. That is because not only are they practically atmosphere-less and frigid, but they are also belted by energetic particles and radiation from the sun and cosmic rays from powerful events like supernovas beyond the solar system.
Yet, both Europa and Enceladus are believed to have liquid water oceans beneath their thick surfaces, which are like icy shells. Those oceans would be therefore protected from such particles and warmed by geothermal heat generated by the gravitational tug these moons' parent planets and their sibling moons exert on them.
This would mean that, as long as these subsurface oceans have the right chemistry and a source of energy, life could dwell on them.
To investigate this, Pavlov and colleagues tested amino acids as they underwent radiolysis. Though amino acids can be created by both living things and non-biological processes, spotting them on Europa or Enceladus would be a potential sign of life simply because they are important to life on Earth as a key component of protein building. Amino acids could be brought about from the deep oceans of these moons, thanks to geyser activity, or by the churning motion of the icy outer shells themselves.
The team took amino acid samples, sealed them in airless vials, and chilled them to around minus 321 degrees Fahrenheit (minus 196 degrees Celsius). The researchers then bombarded the amino acids with high-energy light called "gamma rays" at various intensities to test the molecules' survival capabilities.
The researchers also tested how well amino acids could survive in dead bacteria sealed in the ice of Europa and Enceladus, and explored what effects their mixing with meteorite material would have on their survival.
Factoring in the age of ice on Europa and Enceladus, in addition to considering the radiation environments around both moons, the team was able to calculate drilling depth and locations where 10% of the amino acids would survive radiolytic destruction.
Experiments of this type have been done before, but there were two firsts this particular test delivered.
It was the first time researchers had considered lower doses of radiation on these molecules, that don't completely break apart the amino acids, with the team reasoning that damaged or degraded molecules could still serve as biomarkers. And, it was also the first time such a test had considered amino acid survival in conjunction with meteorite dust.
The team found that amino acids degraded more rapidly when mixed with silicas, similar to those found in meteorite dust. However, the amino acids in dead microbacteria degraded at a slower pace than average. This may be because bacterial cellular material shields amino acids from reactive compounds created by the radiation bombardment that would otherwise speed up their degradation.
"Slow rates of amino acid destruction in biological samples under Europa- and Enceladus-like surface conditions bolster the case for future life-detection measurements by Europa and Enceladus lander missions," Pavlov said. "Our results indicate that the rates of potential organic biomolecules’ degradation in silica-rich regions on both Europa and Enceladus are higher than in pure ice and, thus, possible future missions to Europa and Enceladus should be cautious in sampling silica-rich locations on both icy moons."
The team's paper was published on Thursday (July 18) in the journal Astrobiology.
