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Hydrothermal vents are common in areas subject to volcanic activity at the bottom of Earth's oceans. Meanwhile, hydrothermal vents are believed to exist on Jupiter's moon Europa and maybe in other moons of the Solar System. Vents on Earth are beginning to be understood and the complex ecosystems associated with them provide many new and fascinating discoveries. My question is what are differences between vents in the two systems?

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    $\begingroup$ The acceleration of gravity on these moons is a fraction of Earth's gravity. "Other-things-being-equal" a much lower gravity could reduce the: 1) Rayleigh number in the fluid-convective system 2) depth of the fluid's critical point, and 3) escape velocity for plume material. $\endgroup$ – Mark Rovetta Jan 9 '15 at 13:57
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At the moment, the question is unanswerable, because nobody knows if the vents exist - or even if there is an ocean under Europa's icy exterior (although the idea is reasonably well-accepted). At the moment, they're hypothetical, and until a mission is sent to Europa to observe the depths of its oceans, we won't know much about them.

Well, there actually is one difference. On Earth, hydrothermal vents are the byproducts of tectonic activity. They occur near where tectonic plates meet - for example, in the Mid-Atlantic Ridge, or the Pacific Ring of Fire. On Europa, however, the source of the vents is thought to be tidal interactions with Jupiter, which provide enough heat to keep the ocean from freezing solid, like the moon's exterior.

There is one interesting difference regarding the impacts of vents, described in part of this paper (starting at the bottom of page 7, then jumping in more detail to page 10), which is that Earth's oceans are stratified (i.e composed of distinct layers, stemming from effects of heating, cooling, and convection), whereas Europa's ocean may be unstratified and uniform. On page 10, the authors explain this by saying:

Earth's ocean is stratified because it is both heated and cooled at different locations along the upper surface. Water cooled at the poles slides beneath warm tropical water, forming stable stratification. If the dominant source of buoyancy in Europa's ocean is heat input at the base, the situation is more reminiscent of a pot of water on a stove, or of convection in the Earth's liquid core. The fluid should be convectively unstable everywhere, and stable stratification should not occur.

The authors say that they can apply this unstratified model to plumes from vents and therefore better explain ocean dynamics on Europa.


Sources:

National Oceanic and Atmospheric Administration

Woods Hole Oceanographic Institution

"Hydrothermal Plume Dynamics on Europa: Implications for Chaos Formation" by Goodman et al.

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