Alien Moons’ Global Oceans Might Not Host Life After All
For over twenty years, experts have wondered if extraterrestrial life could be thriving deep below the icy coatings on the moons in our outer solar system.
Both the Galileo mission to Jupiter and the Cassini mission to Saturn have come across proof that some of their moons are home to global oceans, warmed by the pull of each giant planet they orbit. Additionally, spacecraft exploring much closer to home have found vibrant communities living in the darkness around geologic features on the ocean floor. Together it’s easy to be swept away by dreams of alien seafloors thriving with microbes. However, new research is delving deeper, into the rock itself, and it’s possible that these worlds are actually dead inside, both biologically, as well as geologically.
Paul Byrne, the lead author and planetary geologist at North Carolina State University told Space.com last month at the annual conference of the American Geophysical Union in Washington that they were wondering, what would it look like if you were in a submarine and you could fly over the surface of the ocean floor on Jupiter’s moon Europa.
It is in these seafloors that astrobiologists hoped to discover heated, mineral-packed seawater spitting out into the ocean, similar to hydrothermal vents and black smokers on our planet. In Earth’s oceans, those features support buzzing communities centred around microbes that can feed themselves with chemicals generated where hot rock and seawater continuously mix. If similar structures are found on extraterrestrial ocean worlds, the prospect of seeing life on planets far from the sun becomes slightly more probable.
Byrne explained that they were hoping to characterize what the chain of volcanoes would look like and what the rift zones would look like but then they realized they’re not going to be there.
To figure that out, the team focused on the rock itself, determining how much force would be required to break the rock in two ways we see on Earth: normal faults, which happen when rock is pulled apart, and thrust faults, which happen when rock is pushed together and where more force is needed to make this happen. The more force needed to break rock, the less geological activity is taking place — and that means less of the interactions between fresh rock and alien seawater that could theoretically host life.
Byrne and his team looked at four ocean worlds: Jupiter’s moons Europa and Ganymede and Saturn’s Enceladus and Titan. For each of these moons, the team calculated the strength of the rock. Although there’s a myriad of questions we can’t answer about these worlds yet, rock strength calculations, which are commonly done on Earth for mining operations, are quite feasible.
Those calculations are based on the thickness of the cold, solid rock layer, which sits on top of a warmer, softer layer that can’t break. Byrne explained it as a Milky Way bar or a Mars bar, where the chocolate and caramel meet. That depth is like the thickness of the brittle, rigid layer. The thicker it is, the more difficult it is to break.
Next, the team added other values, such as the body’s gravity at a set depth and the weight of water and ice on top of the moon’s rocky surface. Even after they included a range of plausible values for unknown inputs, the final calculations were in the same general range for each moon.
Byrne expressed those outcomes, which he was presenting at the conference, propose that the rock is so strong that there’s no known force on these moons powerful enough to regularly crack it. That’s due to the sheer weight of the water and ice sitting above the rock. As Byrne explained, when it comes to an understanding how strong the rock is, it’s pretty strong, and it gets that strength because even though the gravity’s pretty low, there’s a lot of water on top of it.
Each moon the team analyzed demonstrated a different calculated rock strength. However, the results aren’t exactly promising for geological showstoppers or possible alien life. Byrne added that for Europa, it appears to be almost impossible to make any fractures or faults, and then when you look at Titan and Ganymede, these numbers are stupidly high, so really nothing is happening at all on those worlds.
Enceladus’ rock strength numbers aren’t as dire because this moon is much smaller than the other three, which lessens the weight of the water and ice above its rocky surface. The picture also looks a little different with Enceladus since its rocky core is more porous. If those pores happen to line up, they could carry water deep into the moon. Thus, it’s plausible that Enceladus could be wet and soggy all the way through.
Unlike the other moons, scientists do have evidence suggesting that rock and water are interacting at Enceladus, due to Cassini’s flight through seawater plumes shooting through the icy crust and into space, which found organic compounds. Byrne said it is quite encouraging, but it’s hard to explain that it’s not rock and water touching.
Byrne also explained that if the rock at all these seafloors is too strong to be broken regularly, it’s difficult to believe anything could be happening down there. Two significant factors shaping Earth’s seafloor are soil washing off the continents, and the bodies of sea creatures sinking to decay, and none of these factors appear to be likely on these ocean worlds. Spacecraft haven’t seen scars left behind by any impacts that appear to be large enough to make a dent on the seafloor plausibly. What’s more, based on Byrne’s calculations, the rock is too strong to enable the moons to shrink like the planet Mercury or to host volcanic chains or rift zones.
However, these are not the team’s final results. Byrne and his colleagues are still seeing how other scientists respond to the calculations, and the research is not yet published. Moreover, it will likely be decades before scientists could collect the data they might need to really test the idea — which would call for seismometers on these extraterrestrial seafloors. People are just now starting to get their first good seismometer data from another planet, as a result of the InSight lander on Mars.
Byrne was a little sad about his own team’s discovery stating that it would be awesome if they discovered interesting stuff since these moons are cool and maybe there’s life there. But if his team is right, it means they do need to reconsider these worlds as habitable destinations or destinations for exploring habitability.
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