While the two stories come from separate research teams, the possibilities are sure to fuel interest in the moon and its potential habitability for life, says Lorenz Roth, a postdoctoral researcher at the Southwest Research Institute and one of the authors on the plume discovery.
"Once confirmed, Europa likely will once more be shifted into the focus of planetary research, and also of the search for potential habitable places away from Earth," Roth says.
For years scientists have suspected Europa has an ocean lurking approximately 12 miles below the ice crust of the planet. In 1979 Voyager 1 showed an icy world with a network of cracks, now called "chaos terrain," and the relative lack of cratering led to the speculation of an ocean beneath the icy crust. Further studies by the Galileo probe in the 1990s and 2000s refined this view. And, as scientists often point out, where there's water, there's life—even at vents at the bottom of the seafloor or under miles of Antarctic ice.
Roth and his team used spectral observations on the south pole of Europa to find traces of hydrogen and oxygen, indicating water activity at the pole. Roth says it could be an indication of a plume at the pole connected to the subsurface ocean, as is the case on Enceladus, a small moon of Saturn with geysers that spew plumes of water out of cracks in the surface (making it another object of speculation for life in our solar system). Or Europa's plumes could be the result of the sublimation of ice into water vapor. Unlike on Enceladus, water from the proposed plumes on Europa would mostly fall back to the surface, similar to the volcanic activity on the comparably sized Io.
The plumes on Europa occur in a narrow 7-hour window during the moon's 85-hour orbital period around its gas giant parent. Some of the plumes may be as high as 124 miles, arising out of two distinct areas in the southern region. They occur when Europa is at its furthest point from Jupiter.
The , which found evidence of the Hadley cells, comes from a team led by Krista Soderlund of the University of Texas Institute for Geophysics and Britney Schmidt of Georgia Institute of Technology. They examined the possible ways Europa's subsurface ocean might circulate by running numbers on the way fluids move, and incorporating the rotation rate, ocean depth, size of the moon, inertia, and other factors. They used a 62-mile ocean depth in their model, as that is thought to be the depth from the bottom of the ice shell surface to the rocky crust seafloor down below.
The scientists found that there is a potential heat exchange through the water in which warmth emanates from the seafloor in the equatorial regions and moves to the poles from there. In addition, this creates a sort of exchange between the water at the bottom and the water at the top, potentially ensuring a continual nutrient exchange and opening up more regions to habitability below the ice crust.
"Because we think the ocean is vigorously convecting; it implies that there is an active hot mantle below," Soderlund says.
Schmidt says that rather than moving like the gases of Jupiter—which are freewheeling and uninterrupted by pesky things like continents—the Europa ocean instead moves like the ice giants, Uranus and Neptune, whose flows aren't quite so smooth, thanks to more interruptions from ice materials and inertial forces.The model helps explain why most of the chaos terrain skews toward the equator. Through this process, and the movement of ice, while there may not be a direct line to the surface of the moon, materials can still accrete from the ocean to the ice shell and vice versa.
"What we've shown here is that Europa's ocean mixes in the very same way [as the ice giants], and it tells us that most of the heat that's transferred from the interior of the planet to the surface is transferred near the equator," Schmidt says.
The heat transfer opens up more doors to life as well.
"On Earth marine ice is usually a habitat for creatures underneath the ice shell," Soderlund says, "so we think that if there is life, there is the potential that it's located at lower latitudes rather than high."