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.
Thanks for turning up some real info. The paper you found is speculative, but the options they consider all have a temperature range within a few degrees of zero C.
They entertain some scenarios whereby stratification could allow the deep temperature to get as high as ~4C without breaching the icy surface.
According to this[1] the pressure on Europa is "one hundred billionth that of the Earth". Your graph uses a log scale, so maybe I'm measuring this wrong, but I'm pretty sure under so little pressure, water can be vapor at -50ºC. So the pressure is hardly negligible in this case.
Comments
As warm as the word vapor may sound it is actually really cold. Europa's oceans, if they exist, are -50°C and lower.
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.
Upon further research, where are you getting this number from?
http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1824.pdf
Thanks for turning up some real info. The paper you found is speculative, but the options they consider all have a temperature range within a few degrees of zero C.
They entertain some scenarios whereby stratification could allow the deep temperature to get as high as ~4C without breaching the icy surface.
Doesn't water freeze and become ice at 0°C?
You can make supercooled water in your freezer.
http://en.wikipedia.org/wiki/Supercooling
Good luck finding distilled water with no contact with rocks/ice on Europa...
That requires very difficult circumstances that almost certainly can't occur at the planetary scale.
Only at one atmoshpere of pressure.
The difference is negligible: http://i.stack.imgur.com/n6LXj.gif
According to this[1] the pressure on Europa is "one hundred billionth that of the Earth". Your graph uses a log scale, so maybe I'm measuring this wrong, but I'm pretty sure under so little pressure, water can be vapor at -50ºC. So the pressure is hardly negligible in this case.
[1]http://hubblesite.org/newscenter/archive/releases/1995/12/te...