One of the best nearby candidates for finding extraterrestrial life is believed to be Jupiter’s moon Europa, one of the four Galilean moons (Galileo having discovered them). Europa’s surface is water ice, and it’s now clear that beneath that ice lies a very large ocean in which it’s conceivable that life could have established itself. Eventually, NASA would like to send a probe there and dig down under the ice to see if it’s possible to find any evidence of life. This gives rise to a pertinent question: How far down would a probe have to dig to find evidence of life? This is a crucial question that has real implications regarding the design of such a probe.
The issue here is not trying to get beneath the ice to the liquid water. The surface of Europa is geologically active with occasional water geysers guaranteeing regular turnover. Under the right circumstances, it shouldn’t be hard to find the molecules of life (such as amino acids) just below the surface. However, there’s another problem: radiation. Europa orbits within Jupiter’s radiation belts, which baths the surface of the moon with a hefty dose of high-speed charged particles that would blow apart these fairly delicate molecules.
The results of a recent study have shown that despite the radiation, a probe wouldn’t have to dig too deep to find complex molecules.
NASA scientist Tom Nordheim and his colleagues modeled Europa's radiation environment in detail, laying out just how bad things get from place to place. They then combined these results with data from laboratory experiments documenting how quickly various radiation doses carve up amino acids (a stand-in here for complex biomolecules in general).
The researchers found significant variation, with some Europan locales (equatorial regions) getting about 10 times the radiation pounding of others (middle and high latitudes).
At the most benign spots, the team determined, a lander would likely have to dig just 0.4 inches (1 centimeter) or so into the ice to find recognizable amino acids. In the high-blast zones, the target depth would be on the order of 4 to 8 inches (10 to 20 cm).
This is one of those questions that had been floating in my mind for some time regarding how deep evidence of life would be buried on celestial bodies without atmospheres. How deep do you have to go to find something? It turns out, less than a foot ought to do just fine.
Now all that has to be done is to launch a landing probe to do the observations—which, unfortunately, isn’t planned to happen anytime soon. But when it does, we’ll know how deep to look.
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