Gravitational Compression and Ice
Can gravitational compression liquefy water ice?
I have been reading about the possibility that Jupiter's 3 ice moons
probably harbor large amounts of liquid water. Offered reasons
include tidal forces and elliptical orbits and electromagnetic
induction by the moons' orbital paths taking them through Jupiter's
magnetic field somehow warming up the ice beyond the melting
point. But what about plain old gravity: what does the melting
point of water as a function of temperature and pressure look
like? I do not know what the ambient pressure near the core of
Europa is like, but with the mass of a whole moon heaped on it, I
bet there is a lot of pressure.
How hot/massive does something like an ice moon have to be to have a
liquid water core?
Oxygen is a pretty common element in the Universe. Hydrogen is the
most common element in the Universe. Water is a pretty common
compound. Perhaps liquid water (and life?!) is fairly common, too?
It is great that there is one simple (well, not really simple)
diagram that can answer all of your questions. It is called a phase
diagram and basically is a plot of temperature versus
pressure. There are different zones on this plot that indicate the
different phases of water: solid, liquid, gas and supercritical
fluid. Here is a link to a good diagram that shows the information
that you wanted to know
about: http://www.lsbu.ac.uk/water/phase.html Here you can see
that above 647K and 22MPa water is a supercritical fluid--a
supercritical fluid is a liquid that is at a temperature at which it
should be a gas, but the pressure is so great that a liquid state is
maintained. This does not mean that a supercritical fluid has the
same properties as regular liquid water. Water as a supercritical
fluid can only exist under high temperatures and pressures and has a
density much greater than liquid water at STP.
I will let you find the pressure and temperature information about
what the core of Jupiter's moons might look like, then you can
compare this against the phase diagram. Also, just to note, all of
the different areas marked as solid are actually different crystal
forms of ice.
I will comment on your indication about the amount of hydrogen and
oxygen and water in the universe. Based upon the web site that I
found (http://www.seafriends.org.nz/oceano/abund.htm) Hydrogen is
87% of the universe and oxygen is 0.06%. To assume that water would
be a common molecule may or may not be correct, but water and oxygen
are only two components that are needed for the life forms that we
have. There are certainly anaerobic bacteria that can live without
oxygen (O2) (though they need water). But temperature is very
important as well. If you look at the abundance of the elements in
the universe versus what animals contain, I think that it is pretty
amazing that the concentration of those particular elements in a
very particular arrangement could happen at all. But hey, with all
the time in the universe, almost all possible combinations should
happen at some point eh?
Start by looking up a phase diagram of water. You will notice that
the triple point (the temperature and pressure where solid, liquid
and gaseous water exist in equilibrium) is at 4.58torr and 0.0098C.
The solid-liquid line for water is skewed slightly backward so that
at the lower temperature of 0C, the pressure has to be 1atm
(760torr) in order for there to be an equilibrium between liquid and
solid water phases. This means that at 0.0098C, drawing a perfectly
vertical line, any pressure higher than 4.58 torr will convert the
water into the liquid phase. Likewise, at the lower temperature of
0C, any pressure higher than 1atm will change the water to a liquid.
Since the mean temperature on Ganymede and Europa approximately
-165C one can imagine how much more pressure is required in order to
have a liquid layer underneath the ice. However, considering that
the ice mantle can be as much as 100km thick, even with a surface
gravity that is tenths lower than that of Earth, it is entirely
possible that the water will be liquid underneath all that ice.
Greg (Roberto Gregorius)
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Update: June 2012