From surface to seafloor, there鈥檚 a lot going on on Europa. New evidence points to a great deal of activity just below the ice shell of this ocean world.
Europa is the second innermost of Jupiter鈥檚 massive Galilean moons and believed to be one of the most聽promising places to search for alien microbes.
For decades, we鈥檝e spotted signs of a massive ocean beneath its cracked crust. Some of this water makes its way to space via geysers.
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Between the crust and the ocean, there are another couple of layers of ice, a study based on data from the Galileo mission and previous models of the ice crust by Eli Tziperman at Harvard University and his colleagues has found. The team estimates that the rigid crust is 5 kilometres deep and that a transitional layer less than 1 km deep is sandwiched between the rigid crust and a more soluble layer of convecting ice. This transports heat and matter from the moon鈥檚 north pole to its south pole.
This transitional layer is more than just a subsurface heat highway 鈥� it could be driving convection throughout the world.
It never quite reaches the surface, says Tziperman. 鈥淭he upper hard layer cannot convect as it is made of ice that鈥檚 just too hard 鈥� brittle in technical terminology.鈥�
鈥淪o convection, which is just vertical motions that mix the lower part of the ice, cannot access the upper layer and would not be a way to replenish the surface ice,鈥� he says. There鈥檚 still a large-scale global effect, though.
These heat transports would happen globally, affecting the volume of ice. And if there鈥檚 microbial life in the ocean 鈥� and indeed, Europa鈥檚 ocean is the most promising place in our solar system for life beyond Earth 鈥� it could be pushed around by these flows down below.
Going with the flow
鈥淸If] the microbes are in the ocean, which is only speculation at this point, they would certainly be subjected to the currents within the ocean,鈥� says Jessica Noviello at Arizona State University. 鈥淭hey would literally go with the flow. The velocities of those currents, and how they change over time, is something that is still unknown.鈥�
This transportation of materials could mean that ice at the equator is thinner than ice at the poles, according to one model in the study.
Tziperman and his team doubt convection currents would affect the brittle upper layer of ice on Europa. But Noviello says other studies 鈥渉ave suggested ice convection as a way to transport material from the subsurface to the surface, especially since the materials that get transferred 鈥� mostly salts 鈥� end up in the same places that surface features are鈥�.
Future flybys
Two planned space missions will hopefully tell us more. In 2022, NASA plans to launch a craft called Europa Clipper, and the European Space Agency will send up its JUpiter ICy moon Explorer, aka JUICE.
JUICE鈥檚 radar should be able to penetrate the upper ice layers and find these heated ice flows in its two flybys of Europa, while Europa Clipper has instruments that may be able to confirm whether the flows under Europa鈥檚 shell are a result of the moon鈥檚 surface moving around like continental plates, a phenomenon suggested by earlier work.
鈥淚 think so little is known about Europa and its substructure in particular that any reasonable answer could be at least partially true,鈥� Noviello says. 鈥淚 can鈥檛 wait for Clipper to give us some answers.鈥�
Journal Reference: Nature Astronomy, DOI: 10.1038/s41550-017-0326-7
Read more: Joint mission to Europa could seek life under the ice
