If life exists on the icy ocean moons Enceladus and Europa, detectable traces of molecules could survive just below their frozen surface.
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 95 moons, could contain vast oceans of liquid water that harbor life . If this is so, then complex organic molecules such as amino acids and nucleic acids, the building blocks of life as we know it, could serve as “biosignatures” of life on worlds.
The problem, however, is that both Europa and Enceladus are bombarded by intense radiation from the sun that could potentially destroy complex organic molecules on their surfaces. But new research offers some hope on that front, suggesting that these biosignatures may indeed survive if preserved in the icy shells of moons. And if that’s true, these molecules could be sitting so close to the surface that future robotic landers might be able to dig them up. On Enceladus, in fact, this mining may not even be necessary; biosignature molecules can survive in shallower ice than Europa.
“Based on our experiments, Europa’s ‘safe’ amino acid sampling depth is almost 8 inches (20 centimeters) at high latitudes in the posterior hemisphere, the hemisphere opposite to Europa’s direction of motion around Jupiter, in the area where the surface has not been disturbed much by meteorite impacts,” study leader Alexander Pavlov of NASA’s Goddard Space Flight Center in Greenbelt, Md., said in a statement. “Subsurface sampling is not necessary to detect amino acids on Enceladus—these molecules will experience radiolysis, breakdown by radiation, at any location on Enceladus’ surface less than a tenth of an inch (under a few millimeters) from the surface.”
Connected: If extraterrestrial life exists on Europa, we may find it in hydrothermal vents
The dramatic jets erupting through Enceladus’ icy shell may also mean that orbiting robotic missions will be able to pick up these biosignature molecules around Saturn’s moon without the need to visit the surface.
Life will flow deep on icy moons
Although Europa and Enceladus are often cited as two of the most likely worlds to host life elsewhere in the Solar System, it is highly unlikely that such life resides on the surface of these moons. This is because not only are they virtually atmosphereless and cold, but they are also surrounded by energetic particles and radiation from the sun and cosmic rays from powerful events such as supernovae outside the solar system.
Yet both Europa and Enceladus are thought to have oceans of liquid water beneath their thick ice-shell-like surfaces. These oceans would therefore be shielded from such particles and warmed by geothermal heat generated by the gravitational pull that the parent planets of these moons and their sibling moons exert on them.
This would mean that as long as these subsurface oceans have the right chemical composition and energy source, life can live in them.
To investigate this, Pavlov and colleagues tested amino acids while undergoing radiolysis. Although 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 building block for proteins. Amino acids can be obtained from the deep oceans of these moons, thanks to geyser activity or by the stirring of the icy outer shells themselves.
The team took samples of amino acids, sealed them in airless vials, and cooled them to about minus 321 degrees Fahrenheit (minus 196 degrees Celsius). The researchers then bombarded the amino acids with high-energy light called “gamma rays” at varying intensities to test the molecules’ survival abilities.
The researchers also tested how well the amino acids could survive in dead bacteria sealed in the ice of Europa and Enceladus, and studied what effects mixing them with meteorite material would have on their survival.
By accounting for the ice age of Europa and Enceladus, in addition to considering the radiation environment around the two moons, the team was able to calculate the penetration depth and the locations where 10% of the amino acids would survive radiolytic destruction.
Experiments of this type have been done before, but there were two firsts conducted by this particular test.
It was the first time the researchers considered lower doses of radiation on these molecules that did not fully degrade the amino acids, with the team reasoning that the damaged or degraded molecules could still serve as biomarkers. Furthermore, this was the first time such a test looked at the survival of amino acids in relation to meteorite dust.
The team found that amino acids break down faster when mixed with silica, similar to those found in meteorite dust. However, the amino acids in dead bacteria are broken down at a slower rate than average. This may be because the bacterial cell material protects the amino acids from reactive compounds created by the radiation bombardment that would otherwise accelerate their degradation.
“The slow rates of amino acid decay in biological samples under surface conditions similar to Europa and Enceladus support the case for future life-detection measurements from the Europa and Enceladus landers missions,” Pavlov said. “Our results show that the rate of degradation of potential organic biomolecules in the silica-rich regions of both Europa and Enceladus is higher than in the pure ice, and therefore potential future missions to Europa and Enceladus should be cautious in sampling silica-rich sites on both icy moons.”
The team’s report was published Thursday (July 18) in the journal Astrobiology.
