After all, Europa may not be the best place to search for extraterrestrial life in the solar system.
A new study modeling what the soil represents Jupiter The Moon’s Hidden Ocean appears to conclude that tectonic activity – and the complex chemical reactions it facilitates – is probably negligible.
Europe shelters a deep ocean under a shell of ice several tens of kilometers thick. This ocean wraps around a rocky core, but little is known about the interface between the ocean and the core. If life is to exist in Europa’s ocean, it must somehow gain energy, most likely through interactions on the sea floor between water and rock. Access to fresh rock is vital for producing more nutrients.
On Earth, tectonic faults in the seafloor allow water to plunge miles into the rock, and as new faults are opened by the movement of tectonic plates, new rocks are exposed, thus maintaining the supply of nutrients released into the ocean through hydrothermal vents.
Byrne’s team assessed the potential for tectonic activity on Europa’s seafloor with a new model that took into account constraints due to gravitational tides experienced by Jupiter, the long-term contraction of the Moon as its interior gradually cools, and the convection of thermal energy through the mantle.
However, they found that none of these factors would be strong enough to produce tectonic activity. For example, tidal constraints occur because Europa’s orbit around Jupiter is not perfectly circular but rather eccentric, consistent with Johannes Keplerthe first law of orbital motion. This means that at certain points in each of its 84-hour orbits around Jupiter, Europa is closer to the planet than at other times, and the resulting gravitational differential results in tides. However, for the tides to be strong enough to induce sufficient tectonic activity, the eccentricity of Europa’s orbit would need to be larger – more elongated – than it is (an eccentricity of 0.441 compared to the actual value of 0.009). Although repeated tidal stress weakens the upper part of Europa’s seafloor, creating shallow fractures, it is not intense enough to extend these faults deep to new rocks.
Likewise, while theoretical models suggest that Europa’s rocky core contracted over billions of years as its interior cooled, it would need to shrink several kilometers to fracture the bedrock and create deep tectonic faults. It would be more extensive than the process on Earth moonwhich is estimated to have contracted by several tens of meters throughout its history of four and a half billion years, but less than on Marchwhich would have contracted by up to 7 kilometers (4.3 miles).
The lack of tectonics is bad news for the possibility of life, because life would need new chemical nutrients to survive. One of the main sources of these nutrients deep in Earth’s oceans are hydrothermal vents, such as the famous black smokers. But according to the new modeling, black smokers spewing nutrient-filled hot water are not possible on Europe.
“But it turns out there are other types of hydrothermal systems,” Byrne said. These other species seep through the bedrock to shallower depths and are therefore colder.
“In fact, these other types are the most common on Earth,” Byrne added. “Such relatively colder hydrothermal vents could exist on Europa, but they would be much less energetic than the traditional images we have in mind when we think of hydrothermal vents. And it is very far from certain how long such colder hydrothermal systems could last and support chemosynthetic microbial life.”
If hydrothermal vents and tectonic faults are not on the menu for Europa, are there other possible sources of chemical energy and nutrients that could support life on the Oceanic Moon? Maybe, Byrne said, but there are still too many unknowns to be sure. For example, radioactive decay could be an alternative energy source, but we do not know the figures for this process in Europe. Alternatively, perhaps nutrients enter the ocean not from below, but from above – meteorites which strike the surface ice and are swallowed up and carried into the ocean. However, it is unclear whether there are routes through the thick ice cover to the ocean and vice versa. This is one of the unknowns that NASA Europe Clipper mission, currently en route to Europe, aims to discover.
These findings are also potentially bad news for other ocean moons around the world. solar systemand Byrne’s team is currently preparing a new study that delves deeper into this question.
“Without saying too much, I can say that the overall results for Europa are applicable to most other moons of this type, with the probable exception of (Saturnit’s the moon) Enceladus“Byrne said.
However, despite these pessimistic outlooks, Byrne is keen to emphasize that we should not stop searching for life on these icy moons with hidden oceans.
“We are not saying, and we cannot say, that there is no life in Europe,” Byrne said. “What we are saying is that it is a more difficult proposition, given our results.”
The results were published on January 6 in the journal Natural communications.
