Hmm. I did not think that the temperature of Europa's oceans (below their probable icy crust) has been well-constrained. To the extent that it has, I think the subsurface temperature is expected to be much greater than -50C in many places (the mechanism is tidal heating).
There is a general belief that Europa is likely to have significant liquid water below its icy surface. The phase diagram for water (first image at http://www1.lsbu.ac.uk/water/phase.html) does not allow liquid water at -50C = 223K, no matter what the pressure.
Atmospheric pressure on Earth is about 0.1 MPa, and gravity on Europa is about 1/6 of Earth. The highest pressure in Earth's oceans is about 1000x atmospheric pressure (100 MPa). Across this range (3 orders of magnitude), the melting point of H2O is pretty constant, 0C or a little below (see the linked phase diagram).
My conclusion: liquid water implies temperatures of at least 0C in the relevant pressure range.
Pure liquid water does indeed freeze at 0C, but the water in Europa's oceans is unlikely to be pure. The presence of salts can significantly depress the freezing point of water; ocean water on Earth freezes at about -2C or so. With just sodium chloride alone, it's possible to knock that down to about -20C. Europa's oceans are known to contain magnesium chloride (http://www.jpl.nasa.gov/news/news.php?release=2013-082), and probably have other chlorides as well. It's definitely possible, indeed likely, that the temperatures involved are sub-zero, but I agree that -50C sounds unlikely. I'm not sure what the actual lower bound would be though, given the largely unknown chemistry of the water.
From your link: A liquid below its standard freezing point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form creating a solid.
Europa's ocean is surrounded by a vast sheet of actual ice. You can't keep such an ocean at supercooled state.
Comments
Hmm. I did not think that the temperature of Europa's oceans (below their probable icy crust) has been well-constrained. To the extent that it has, I think the subsurface temperature is expected to be much greater than -50C in many places (the mechanism is tidal heating).
There is a general belief that Europa is likely to have significant liquid water below its icy surface. The phase diagram for water (first image at http://www1.lsbu.ac.uk/water/phase.html) does not allow liquid water at -50C = 223K, no matter what the pressure.
Atmospheric pressure on Earth is about 0.1 MPa, and gravity on Europa is about 1/6 of Earth. The highest pressure in Earth's oceans is about 1000x atmospheric pressure (100 MPa). Across this range (3 orders of magnitude), the melting point of H2O is pretty constant, 0C or a little below (see the linked phase diagram).
My conclusion: liquid water implies temperatures of at least 0C in the relevant pressure range.
Pure liquid water does indeed freeze at 0C, but the water in Europa's oceans is unlikely to be pure. The presence of salts can significantly depress the freezing point of water; ocean water on Earth freezes at about -2C or so. With just sodium chloride alone, it's possible to knock that down to about -20C. Europa's oceans are known to contain magnesium chloride (http://www.jpl.nasa.gov/news/news.php?release=2013-082), and probably have other chlorides as well. It's definitely possible, indeed likely, that the temperatures involved are sub-zero, but I agree that -50C sounds unlikely. I'm not sure what the actual lower bound would be though, given the largely unknown chemistry of the water.
Not really. https://www.youtube.com/watch?v=Fot3m7kyLn4
Also. http://en.wikipedia.org/wiki/Supercooling
From your link: A liquid below its standard freezing point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form creating a solid.
Europa's ocean is surrounded by a vast sheet of actual ice. You can't keep such an ocean at supercooled state.
It's not likely that an entire large planetary body will maintain a state out of equilibrium for geological periods of time. Supercooling is unstable.
So, I'm assuming you don't have any basis for your assertion above? Because this is nothing to do with Europa.