The seafloors of Europa and Enceladus may not be prone to fracturing. Such fissures are thought to be important for the prospect of life beneath these moons’ icy shells, so if there isn’t enough stress to cause them, there might also be a shortage of the energy and chemicals that any potential living organisms would need.
We can’t observe the cores of these frigid worlds directly, so we know very little about them. If they fracture often, the fresh rocks revealed by these breaks could interact with the waters overlying them in reactions that would provide energy and nutrients for any potential life in those oceans.
at Washington University in St. Louis, Missouri, and his colleagues modelled some of the stresses on the icy moons’ rocky cores to see if those stresses could crack the rock.
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The researchers focused on three main sources of stress to the rock of the seafloors. The first was tidal stress, which occurs because the gravity of the planet – Jupiter for Europa and Saturn for Enceladus – pulls on one side of the moon more intensely than the other, stretching it slightly.
We know from surface features on Enceladus and Europa that this effect is strong enough to crack ice, but the researchers found that it was far from powerful enough to crack the moons’ rocky cores. “Even with the weakest rock we can really find, we are still a factor of three lower than we would require [for tidal stress to cause fracturing],” Dawson told the Lunar and Planetary Science Conference (LPSC) in Houston, Texas, on 17 March.
The second source of stress the researchers considered was contraction of the moons as they cool. This is common across many planets and moons, which are born relatively hot because of the chaotic environment in which they form. Then, as that heat is radiated away, they shrink and their surfaces – and seafloors – wrinkle up.
The researchers calculated that the solid interior of Europa would have to contract by more than 1 kilometre in diameter in order to cause fissures, which is so significant that it is unlikely to have occurred at all. Even if it did, it wouldn’t continue to crack over time, so that isn’t a good way to continuously provide the ocean with nutrients. Enceladus is much smaller than Europa, so it might have shrunk enough to cause faults, but only if it cooled extremely quickly since its formation.
The final source of stress was pressure on the rock from below due to rising magma. The researchers found that this was a plausible way to cause faulting, but we know so little about how magma forms and moves in these sorts of environments that it is impossible to tell for sure.
The work is preliminary, but so far it isn’t looking good for fissures on the seafloor. “If there aren’t enough stresses to produce regular faulting of the rock, that would lead to a less nutrient-rich ocean,” said Dawson. “It would be leaning against the possibility of life.”
As members of the audience at LPSC pointed out, this is still a bit pessimistic – there may be other ways to weaken the seafloor and create fracturing that Dawson and his colleagues have not yet investigated, such as expansion of the cores due to reactions between the rocks and the water. Life in the oceans of Europa and Enceladus may not seem as likely as it did before, but it isn’t ruled out yet.
