If life exists on the icy ocean moons of Enceladus and Europa, it could be that detectable trace molecules exist just beneath their frozen surfaces.
Scientists have long suspected that both Enceladus, one of Saturn’s 146 moons, and Europa, one of Jupiter’s four large Galilean moons, are total The 95 moons could harbor vast liquid oceans that could support life, and if so, complex organic molecules such as amino acids and nucleic acids — the building blocks of life as we know it — could act as a “biosignature” of life on these planets.
The problem, however, is that both Europa and Enceladus are subject to intense radiation from the Sun, which could destroy complex organic molecules on their surfaces. However, a new study offers some hope in this regard, suggesting that these biosignatures could indeed survive if preserved in the moon’s icy shell. And if that’s true, these molecules could be very close to the surface, where a future robotic lander might be able to dig them up. In fact, on Enceladus, this digging might not be necessary: ​​biosignature molecules might be able to survive in shallower ice than on Europa.
“Our experiments suggest that a ‘safe’ sampling depth for amino acids on Europa is about 8 inches (20 centimeters) at high latitudes in the trailing hemisphere, the hemisphere opposite the direction Europa moves around Jupiter, in an area where the surface has not been significantly disturbed by meteorite impacts,” said study leader Alexander Pavlov of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. It said in a statement“Detecting amino acids on Enceladus does not require subsurface sampling; these molecules survive radiolysis (breakdown by radiation) anywhere within less than a tenth of an inch (less than a few millimeters) of Enceladus’ surface.”
Related: If extraterrestrial life exists on Europa, it might be found in hydrothermal vents
The dramatic plumes erupting from Enceladus’ icy shell also mean that orbiting robotic probes may be able to collect these biosignature molecules from around Saturn’s moon without ever visiting its surface.
Life is deeply rooted on the icy moon
Although Europa and Enceladus are often cited as the planets most likely to support life elsewhere in the solar system, the likelihood of life harboring them on their surfaces is extremely low: not only are they virtually atmosphereless and frigid, they are also covered in high-energy particles and radiation from the Sun, and cosmic rays from powerful events such as supernova explosions outside the solar system.
But both Europa and Enceladus are thought to have oceans of liquid water beneath their thick, icy surfaces, which protect them from such particles and warm them from geothermal heat generated by the gravity of their parent planet and its sibling moons.
This means that as long as these subsurface oceans have the right chemical reactions and energy sources, life could survive there.
To investigate this, Pavlov and his colleagues tested amino acids, which undergo radiolysis. Although amino acids are produced by both biological and abiotic processes, their discovery on Europa or Enceladus could be a sign of life simply because they are important for life on Earth as key building blocks of proteins. Amino acids could be brought from the deep oceans of these moons by geyser activity or by churning movements in the icy crust itself.
The team took samples of the amino acids, sealed them in airless vials, and cooled them to about minus 321 degrees Fahrenheit (minus 196 degrees Celsius). The researchers then exposed the amino acids to various intensities of high-energy light called “gamma rays” to test the molecules’ viability.
The researchers also tested how well amino acids survived in dead bacteria trapped in the ice of Europa and Enceladus, and investigated how mixing with meteorite material might affect bacterial survival.
Taking into account the age of the ice on Europa and Enceladus, as well as the radiation environment surrounding both moons, the team was able to calculate how deep to drill and where 10% of the amino acids would survive radiolysis.
While these kinds of experiments have been done before, this particular test yielded two firsts.
This is the first time that researchers have considered exposing these molecules to low doses of radiation that do not completely break down amino acids, leading the team to reason that damaged or degraded molecules could still function as biomarkers. It was also the first time that they had considered the survival of amino acids in the context of meteorite dust.
The team found that amino acids broke down more quickly when mixed with silica, like that found in meteorite dust, but the amino acids from the dead microbes broke down at a slower-than-average pace, likely because the bacterial cellular material protects the amino acids from reactive compounds produced by radiation, which speeds up their decomposition.
“The slow decomposition rates of amino acids in biological samples under surface conditions like those on Europa and Enceladus provide a basis for future life-detection measurements from Europa and Enceladus landing missions,” Pavlov said. “Our results indicate that the decomposition rates of potential organic biomolecules in the silica-rich regions of Europa and Enceladus are higher than in pure ice. Future missions to Europa and Enceladus should therefore be careful when sampling silica-rich sites on both icy moons.”
The team’s paper was published in the journal Neuroscience on Thursday (July 18). Astrobiology.
