New Delhi, India

Europa and Enceladus are the two moons in our solar system where astronomers believe life can exist. Now they say that complex organic molecules like amino acids and nucleic acids, the "biosignatures" of life, on these worlds could be present just below their frozen surfaces. Enceladus is one of Saturn's 146 known moons, and Europa is one of Jupiter's four large Galilean moons of the 95 moons it has. Several studies have suggested that there could be vast liquid water oceans harbouring life on these moons. 

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They hope that future robotic landers can dig the surface of the moons and set these biosignatures free. This is if they are caught within the icy shells of the moons, according to a study. These biosignatures of life cannot survive if present on the surface, because Europa and Enceladus are constantly bombarded by harsh radiation from the sun. This radiation can kill complex organic molecules. 

Talking about landers digging up the surface of the moons, the study says that on Enceladus digging might not even be needed to release these biosignature molecules since they probably survive in shallower ice than on Europa. 

"Based on our experiments, the 'safe' sampling depth for amino acids on Europa is almost 8 inches (20 centimetres) at high latitudes of the trailing hemisphere, the hemisphere opposite to the direction of Europa's motion around Jupiter, in the area where the surface hasn't been disturbed much by meteorite impacts," research leader Alexander Pavlov of NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. 

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"Subsurface sampling is not required for the detection of amino acids on Enceladus — these molecules will survive radiolysis, breakdown by radiation, at any location on the Enceladus surface less than a tenth of an inch (under a few millimetres) from the surface," he added. 

In fact, humans might not even have to land a rover on Enceladus to catch the biosignature molecules. Its icy shells throw up dramatic plumes that can be directly captured by an orbiting mission. 

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Problems on the moons

The hope for life on these moons is marred by the fact that they sustain quite a few harsh incidents regularly. They are cold and frigid, have no atmosphere and are regularly hit by energetic particles and radiation from the sun. Supernovas happening beyond the solar system release powerful cosmic rays that also bombard the moons. 

But the likelihood of the presence of liquid water oceans beneath the thick icy shell surfaces of Europa and Enceladus has excited scientists for a long time. The icy shells protect the oceans against harsh particles and radiation, with geothermal heat generated by the gravitational tug of their planets and the moons able to heat them. This combination of factors has instilled hope about the survival of life on these moons.