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If life exists on the icy ocean moons of Enceladus and Europa, detectable trace molecules could live just below their frozen surfaces.
Scientists have long theorized that Enceladus, one of Saturn’s 146 known moons, and Europa, one of Jupiter’s four major Galilean moons among its 95 moons in total, could host vast oceans of liquid water which sustains life. If this is the case, then complex organic molecules such as amino acids and nucleic acids, the building blocks of life as we know it, could act as “biosignatures” of life on life.
The problem, however, is that both Europa and Enceladus are threatened by harsh solar radiation that could destroy complex organic molecules on their surfaces. But new research offers some hope in this area, suggesting that those biomarkers may indeed survive if preserved in the moon’s icy shells. And if that’s true, these molecules could sit so close to the surface that future robotic landers might be able to dig them up for free. At Enceladus, in fact, this excavation might not even be necessary; biosignaling 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) at high latitudes of the trailing hemisphere, the hemisphere opposite the direction of Europa’s motion around Jupiter, in the area where the meteorite impacts have not significantly affected the surface,” research leader Alexander Pavlov of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. “Detecting amino acids on Enceladus does not require subsurface sampling – these molecules will survive radioactivity, decay by radiation, at any location on the surface of Enceladus less than a tenth of an inch (a few millimeters) from the surface.”
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Dramatic plumes sprouting through the icy shell of Enceladus could mean that orbital robotic missions will be able to pluck these biosignature molecules from around Saturn’s moon without the need to visit the surface.
Life would run deep on icy moons
Although Europa and Enceladus are often mentioned as two of the most likely worlds to support life elsewhere in the solar system, life is unlikely to live on the surface of these moons. That’s because not only are they nearly atmospheric and frigid, but they’re also belted by energetic particles and radiation from the sun and cosmic rays from powerful events like supernovae outside the solar system.
However, both Europa and Enceladus are believed to have oceans of liquid water beneath their thick, icy shell-like surfaces. Those oceans would thus be protected from such particles and warmed by geothermal heat generated by the gravitational pull exerted by the parent planet of these moons and their sibling moons.
This would mean that as long as these oceans have the right chemistry and an energy source below the surface, life could stay on them.
To investigate this, Pavlov and his colleagues tested amino acids while undergoing radiology. Although living things and non-biological processes can create amino acids, they could be seen on Europa or Enceladus as a sign of life simply because they are important to life on Earth as a key part of protein building. Amino acids could be brought in from the deep oceans of these moons, thanks to geyser activity, or through the churning motion of the icy shells themselves.
The team took amino acid samples, 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 different intensities to test the molecules’ ability to survive.
The researchers also tested how well amino acids could survive in dead bacteria sealed in the ice of Europa and Enceladus, and explored the effect of mixing them with meteorite material on their survival.
Taking into account the age of the ice on Europa and Enceladus, as well as considering the radiation environments around the two moons, the team was able to calculate drilling depths and locations where 10% of the intact amino acids would be destroyed by radiation.
Experiments of this nature have been done before, but this particular test was offered for the first time.
This was the first time that researchers considered lower doses of radiation on these molecules, which do not completely break down the amino acids, with the team saying that damaged or degraded molecules could still function as biomarkers. And, it was the first time such a test evaluated the survival of an amino acid in association with meteorite dust.
The team found that amino acids degraded faster when mixed with silicas, such as those found in meteor dust. However, the amino acids in dead microbacteria degrade at a slower rate than average. This could be due to bacterial cellular material shielding amino acids from reactive compounds created by the radiation bombardment that would otherwise accelerate their degradation.
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“Slow rates of amino acid destruction in biological samples under surface conditions similar to Europa and Enceladus support the case for future life detection measurements by the Europa and Enceladus lander missions,” 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 in pure ice and, therefore, potential future missions to Europa should and Enceladus be careful when sampling silica-rich sites on. the two icy moons.”
The team’s paper was published Thursday (July 18) in the journal Astrobiology.
