Friday, March 16, 2007

Hot Ice

Riddle me this: How do you make ordinary water turn to ice while heating it above the boiling point?

It's all about pressure. At 0 degrees Celsius (or 32 degrees Fahrenheit, for you American, non-science types), water freezes to ice, but if you increase the atmospheric pressure, even a little, it melts back to a liquid. Increase the pressure a great deal, and it turns into ice again, but not the same sort of ice you're used to.

Take the planet Neptune, for instance. If you were to teleport a glass of water into its atmosphere, that water would suddenly be exposed to very high pressures, above 70,000 Pascals. At that pressure, it is more thermodynamic for water molecules to go to a close-packing arrangement, forming ice, even at high temperatures. Under these conditions, it only takes nanoseconds for the ice to form. This was recently demonstrated using the Z machine at Sandia National Laboratory:

The Z Machine was built and has been maintained by the Department of Energy for experiments on nuclear fusion. It can produce temperatures greater than the sun and magnetic rays comparable to neutron stars. How would you like to be technicians working on that rig? And to think their moms scolded them for standing too close to the TV! When fired in its usual configuration, Z releases in X-rays, for a fraction of a second, about 80 times the entire Earth’s electrical generating capacity. You can find out more about this gigantic machine HERE. The Z machine made ice by passing electric pulses of twenty million amperes through an aluminum container containing water. The magnetic field created by these pulses created a pressure of more than 70,000 atmospheres, forming ice at that pressure level, even though the temperature was hotter than boiling water. That will give you one hell of a headache!

What really blows my mind is that there are many forms of ice. Thirteen found so far, depending the temperature and pressure. Each form has its own lattice structure and thermodynamic properties. For instance, the sort of ice we are familiar with expands when it freezes, but all the other forms contract. The ice created by the Z machine is called "Ice VII". When you go home at night and have a refreshing glass of your favorite beverage, the ice you hear clinking around is what physicists call "Ice Ih". But if you increase the atmospheric pressure, the ice will melt back to a liquid. If you continue to exert more pressure, the liquid will freeze to a solid again, but this time it will be "Ice VI". Increase the pressure even more, and you get "Ice VII". HERE is a link to a page with an interactive graph showing all these different forms of ice and their temperature and pressure conditions, including notes on where these conditions match the ambient conditions of Earth, Mars, and Venus, with links to further information on each kind of ice.

Even though it is considered a gas giant, the center two-thirds of Neptune is solid, including water, and there is water in the considerably thick, gaseous atmosphere around the planet. You'll find two different water ices there, Ice II and Ice VI.

Of course, you'll never get the pleasure of cooling your drink or making Sno-cones out of these different types of ice, since in order to do so you would have to be willing to be frozen to death or crushed under incredible pressures, and you would either burn off or freeze off the insides of your mouth!

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