Gravitational Compression and Ice ```Name: Scott Status: other Grade: other Location: NV Country: N/A Date: N/A ``` Question: Can gravitational compression liquefy water ice? Replies: 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? S Ross Scott, 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? Matt Voss Scott, 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) Click here to return to the Astronomy Archives

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