If life exists on the icy ocean moons 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 95 total moons, could host vast oceans of liquid water that harbor life. If so, then complex organic molecules such as amino acids and nucleic acids, the building blocks of life as we know it, could serve as “biological signatures” of life on worlds.
The problem, however, is that both Europa and Enceladus are bombarded with harsh solar radiation that could potentially destroy the complex organic molecules on their surfaces. But new research offers some hope on that front, suggesting that these biological signatures could indeed survive if preserved in the icy shells of moons. And if true, these molecules could be sitting so close to the surface that future robotic landers could dig them up. In Enceladus, this digging wouldn’t even really 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 nearly 8 inches (20 centimeters) in the high latitudes of the posterior hemisphere, the hemisphere opposite to Europa’s direction of motion around Jupiter, in an area where the surface has not been greatly disturbed by meteorite impacts, Alexander Pavlov of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. “Subsurface sampling is not required to detect amino acids on Enceladus—these molecules survive radiolysis, decay by radiation, anywhere on Enceladus’ surface less than a tenth of an inch (less than a few millimeters) from the surface.”
Related: If extraterrestrial life exists on Europa, we may find it in hydrothermal vents
The dramatic plumes that erupt through Enceladus’ icy shell could also mean that robotic missions in orbit will be able to retrieve these biosignature molecules from around Saturn’s moons without having to visit the surface.
Life would run deep on icy moons
Although Europa and Enceladus are often cited as two of the most likely worlds for life elsewhere in the Solar System, it is highly unlikely that such life resides on the surface of these moons. Not only are they virtually atmosphereless and frigid, they are also belted with energetic particles and radiation from the sun and cosmic rays from powerful events such as supernovae outside the solar system.
Still, both Europa and Enceladus are thought to have liquid water oceans beneath their dense surface, which are like ice shells. These oceans would therefore be shielded from such particles and warmed by geothermal heat generated by the gravitational pull exerted on them by the parent planets of these moons and their sibling moons.
This would mean that if these subsurface oceans have the right chemistry and energy source, life could live in them.
To investigate this, Pavlov and colleagues tested amino acids when they underwent radiolysis. Although amino acids can be made by both living things and non-biological processes, finding them on Europa or Enceladus would be a potential sign of life simply because they are important to life on Earth as a key part of making proteins. Amino acids could be obtained from the deep oceans of these moons through geyser activity or the swirling motion of the icy outer shells themselves.
The team took amino acid samples, sealed them in airless vials, and cooled them to temperatures around minus 321 degrees Fahrenheit (minus 196 degrees Celsius). The scientists then bombarded the amino acids with high-energy light called “gamma rays” at varying intensities to test the molecules’ ability to survive.
The researchers also tested how well the amino acids could survive in dead bacteria encased in the ice of Europa and Enceladus, and examined what effects mixing them with meteorite material would have on their survival.
Taking into account the age of the ice on Europa and Enceladus, in addition to considering the radiation environment around both moons, the team was able to calculate the depth of the boreholes and the location where 10% of the amino acids would survive radiolytic destruction.
Experiments of this type have been done before, but this particular test produced two firsts.
It was the first time researchers considered lower doses of radiation to these molecules that don’t completely break down 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 considered the survival of amino acids in association with meteorite dust.
The team found that amino acids break down faster when mixed with silica, similar to meteorite dust. However, the amino acids in the dead microbacteria degraded more slowly than average. This may be because the bacterial cell material protects the amino acids from reactive compounds created by radiation bombardment that would otherwise accelerate their degradation.
“The slow rates of amino acid destruction in biological samples under conditions similar to the surface of Europa and Enceladus support future life detection measurements by the Europa and Enceladus landers,” Pavlov said. “Our results suggest that the rate of potential degradation of organic biomolecules in the silica-rich regions of both Europa and Enceladus is higher than in pure ice, and therefore potential future missions to Europa and Enceladus should be careful when sampling silica-rich sites . both ice moons.”
The team’s work was published Thursday (July 18) in the journal Astrobiology.