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If life exists on the icy ocean moons Europa and Enceladus, it is possible that trace molecules could survive just below their frozen surfaces.
Scientists have long hypothesized that both Enceladus, one of Saturn's 146 known moons, and Europa, one of Jupiter's four large Galilean moons, might be among the planets orbiting them. the total The 95 moons may host vast oceans of liquid water that could harbor life. If so, complex organic molecules like amino acids and nucleic acids, the building blocks of life as we know it, could serve as “biomarkers” of life on these worlds.
The problem is that both Europa and Enceladus are exposed to intense solar radiation that could destroy the complex organic molecules on their surfaces. But new research offers some hope on this front, suggesting that these biosignatures might survive if they are preserved in the moons’ icy shells. If that’s true, the molecules could remain so close to the surface that future robotic rovers could freely drill through them. In fact, there might be no need for such drilling on Enceladus; the biosignature molecules could survive in the shallower ice than on Europa.
“Based on our experiments, the ‘safe’ sampling depth for amino acids on Europa is about 8 inches (20 centimeters) at the high latitudes of the trailing hemisphere, the hemisphere opposite to the direction of Europa’s motion around Jupiter, in the region where the surface has not been disturbed much by meteorite impacts,” said Alexander Pavlov, lead researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. He said in a statement:“Detecting amino acids on Enceladus doesn’t require sampling the interior of the Earth—these molecules would survive radioactive decay, or radiation-induced decomposition, anywhere on Enceladus less than a tenth of an inch (less than a few millimeters) from the surface.”
Related: If there is alien life on Europa, we might find it in hydrothermal vents.
The dramatic plumes erupting from Enceladus's icy shell could also mean that robotic orbiting missions will be able to pluck these biosignature molecules from all over Saturn's moon without having to visit the surface.
Life will be 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, the odds of life on these moons are extremely low. This is because not only are they virtually atmosphere-free and cold, 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.
However, both Europa and Enceladus are thought to have oceans of liquid water beneath their thick, icy shell-like surfaces. These oceans would be protected from such particles and warmed by geothermal heat generated by the gravity of their parent planets and their sister moons.
This means that as long as these subsurface oceans have the right chemistry and energy source, life can persist there.
To investigate, Pavlov and his colleagues tested amino acids as they underwent radioactive decay. Although amino acids can be created by living organisms and non-biological processes, their detection on Europa or Enceladus could be a potential sign of life simply because they are essential to life on Earth as building blocks of proteins. The amino acids could originate in the deep oceans of these moons, thanks to geyser activity, or through the churning motion of the icy outer shells themselves.
The team took samples of amino acids, placed 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' ability to survive.
The researchers also tested how well amino acids could survive in dead bacteria trapped in the ice of Europa and Enceladus, and explored what effects their exposure to meteorite material might have on their survival.
By taking into account the age of the ice on Europa and Enceladus, as well as looking at the radiation environments around the moons, the team was able to calculate the depth of the craters and the locations where 10% of amino acids could survive radiation damage.
Experiments of this kind have been done before, but there are two early ones that this test in particular provides.
This was the first time researchers had considered lower doses of radiation on these molecules, which did not completely break down the amino acids, and the team concluded that the damaged or degraded molecules still functioned as biomarkers. It was also the first time such a test had considered whether amino acids could survive alongside meteorite dust.
The team found that amino acids decomposed more quickly when mixed with silica, similar to that found in meteorite dust. However, amino acids in dead microbacteria decomposed 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 speed up their decomposition.
“The slow rates of amino acid destruction in biological samples under surface conditions similar to those on Europa and Enceladus strengthen the argument for future life-detection measurements by landing missions to Europa and Enceladus,” Pavlov said. “Our results suggest that the degradation rates of potential organic biomolecules in silica-rich regions on both Europa and Enceladus are higher than those in 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 paper was published Thursday (July 18) in the journal Astrobiology.
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