THE SOLAR system, taken as a whole, is a very cool place.

We mean that quite literally. Spacecraft exploring the planets and moons of the solar system beyond the Earth have found worlds ruled by ice—from the water ice on Mars to the icy moons and planets that are farther from the Sun.

Science fiction writers have long been aware that ice is very interesting stuff, and not just in drinks on the rocks. Science fiction has featured worlds of ice—from the planet Winter in Ursula K. Le Guin's Left Hand of Darkness to Hoth in The Empire Strikes Back.

And then there's our favorite science fictional use of ice—Kurt Vonnegut's "ice 9" in the novel Cat's Cradle. This fictional ice has a melting point of 114 °F and a unique property. A single crystal of ice 9 dropped in liquid water will seed the crystallization of the water, even at a temperatures well above the melting point of ordinary ice. If a crystal of ice 9 ever touched the Earth's ocean, the results would be catastrophic. The entire ocean would freeze solid along with every body of water in contact with the ocean.

We'll get back to ice 9. But before we do, we're going to explore worlds of ice—starting on planet Earth, venturing into the solar system to discover just how strange ice can get, and returning to the safety of home, where you can make your own personal ice world, using your household freezer.

You've probably seen photographs of snowflakes that reveal the hexagonal symmetry of the crystalline structure of water ice.

The ice in each snowflake is made of water molecules arranged in hexagons.

Why hexagons? That has to do with the shape of the water molecule. Being savvy sf readers, you all know that water is H₂O. Each water molecule has an oxygen atom and two hydrogen atoms connected to the oxygen. The molecule is shaped like the head of Mickey Mouse—with the oxygen as the head and the two hydrogens as the ears. The three atoms join to make a V shape, with an angle of 105° between the ears.

The bonds that hold each water molecule together are made when each hydrogen atom shares an electron with the oxygen atom. But oxygen doesn't share well with others—at least not when the others are lightweights like hydrogen. In water, oxygen hogs the electrons, pulling on them so they spend more time over by the oxygen than they do by the hydrogens. Since electrons are negatively charged, this means that Mickey's head (the oxygen) is negatively charged and the ears (the hydrogens) are positively charged.

The molecule as a whole is neutral, but it's what chemists call polar—that is, the molecule's charge is unevenly distributed. This has some important consequences. Four billion years ago (give or take half a billion), water served as a medium for the great mashup of organic compounds that eventually led to life on this planet. Water could act as a solvent for these compounds because it's polar and attracts molecules like sugars, nucleic acids, and many amino acids.

That polarity is also responsible for some unique qualities of water. Since positive charges attract negative charges, the hydrogens of one water molecule are attracted to the oxygens of another. That makes water molecules tend to stick together with what is called a hydrogen bond. That's why water stays liquid at temperatures where similar molecules that aren't polar are gaseous.

In liquid water, all the little mouse-head molecules are jiggling around. Cool the water to below its freezing point, and the molecules form into a regular pattern—with the hydrogen-ears of one molecule sticking to the oxygen-mouse-head of another. A regular hexagon that's perfectly flat has edges with angles of 120 degrees between them. The water molecules, with their angle of 105 degrees, get distorted. The angle between the hydrogen mouse ears opens up to 109.5 degrees, and the molecules then come together to make a hexagon that's not quite flat—the water molecules crinkle up and down as they form the hexagonal shapes that make up the snowflake.

If you have ever made the mistake of putting a glass bottle full of water in the freezer, you have already seen first hand a consequence of this hexagonal structure. Water expands when it freezes, busting the bottle and making a frozen mess surrounded by broken glass.

You owe that mess to the hexagonal structure of ice. Each of those hexagons has a fairly large open region in the center. In liquid water, molecules crowd together without such open spaces. That's why ice is less dense than liquid water. Because it's less dense, ice floats on water, and bodies of water freeze at the surface, leaving the lower water liquid. Fish and other aquatic animals can live below the ice in liquid water through the winter.

Knowing this characteristic of ice has helped astronomers deduce what's going on inside some of the ice worlds they have observed. The surface may be covered with ice—but what's underneath that hard shell? If it's a liquid water ocean that is freezing, the volume of the planet must increase. That means the surface must stretch and crack.

Astronomers look for these stretch marks in the surface of ice worlds and find them. Take Ganymede, the largest moon of Jupiter, for example. The bright icy terrain on Ganymede is covered with grooves, evidence of the freezing ocean underneath the ice. Europa, another of Jupiter's icy moons, has a beautiful pattern of cracks, formed by pressure created by ice expanding below the surface.

On Enceladus, an icy moon of Saturn, the Cassini spacecraft documented cryovolcanoes that shoot out geysers of water vapor, salt water from the under-ice sea propelled by internal pressure. You may have seen a similar phenomenon in your own ice cube tray. An ice cube, like a planet, freezes from the outside in, resulting in a center filled with water under pressure. Sometimes, the pressure creates a crack in the surrounding ice and then pushes water out the crack through the top surface of the ice cube. The result is a "unicorn horn" or ice spike growing upward out of the ice cube.

So far, we've been talking about the density of ice. As anyone who has been ice skating can tell you, another attribute of frozen water is its hardness.

Geologists measure hardness on the Mohs scale. On the Mohs scale, diamonds have a hardness of 10, the top of the scale. Talc, the softest of stones, has a hardness of 1. At 0 °C, ice has a hardness of 1.5, about as hard as lead. But as temperatures drop, ice gets harder.

On a visit to the dry valleys of Antarctica, Paul saw granite carved into fantastic shapes by blowing snow. The snow was below -40 °C and had a hardness of greater than 6. It was hard enough to chip away at the softer minerals in granite.

Remember this when you look at images of other planets. Images from the Cassini spacecraft show that Saturn's moon Titan has water ice mountains. With a surface temperature averaging about -179 °C, Titan's ice mountains are as hard as granite. They are eroded by rivers of liquid methane.

New Horizons sent us amazing images of Pluto that show water icebergs floating in a "sea" of nitrogen ice. You can think of nitrogen ice as Silly Putty—it flows slowly even at Pluto's estimated surface temperature of about -233 °C.

The water ice we've described above is just one form of ice—the one you're most likely to encounter in day-to-day life on Earth. You just call it ice, but when talking about the many forms of ice, scientists call it ice Ih (pronounced: "ice one h").

Each one of the many forms of water ice has its own geometry and each has been given its own number. What type of ice forms from water depends on temperature, pressure, the rate of cooling, and radiation it receives.

Other than ice Ih, the only form of ice you'll find on Earth is ice Ic (pronounced "ice one c"). The c stands for cubic, because water molecules come together with cubic symmetry in this form. Ice Ic forms in cold high altitude clouds at temperatures between -140 and -50 °C. When ice Ic falls to lower warmer regions, it turns to ice Ih.

Many of the other kinds of water ice require extreme pressures to form. Ice III or ice 3 forms at a pressure of 3000 atmospheres and at temperatures below -23 °C (250 K or -10 °F) . That's not a pressure you're going to find on Earth—not even in the deepest part of the ocean (the Marianas Trench in the Pacific). Given the temperature and pressure requirements, this type of ice will never be found on the outside surface of a moon or planet. The most likely place to find it is inside one of the ice worlds in our solar system—like Callisto, a moon of Jupiter.

Ice III is denser than liquid water. If it formed in a planet's ocean, it would immediately sink to the bottom. This means there could be liquid water sandwiched between two layers of ice—ice I at the surface and ice III at the bottom.

There are also forms of ice that are not crystalline. Instead, these forms of solid water are amorphous—the molecules don't form a regular geometric pattern. To make amorphous ice, the water must cool so quickly that the molecules do not have time to move around and find their place in a crystal. To make amorphous ice, you have to cool the water at a rate of 100,000 degrees Celcius per second down to a temperature below 136 K (-137 °C).

Amorphous ice is biologically useful. When water in a cell freezes slowly, it can form sharp crystals that can puncture the cell wall, killing the cell. However, amorphous ice does not form crystal spikes. If you want to preserve cells by freezing them, a technique known as cryopreservation, amorphous ice freezing techniques are the way to go.

That's how people "bank" human sperm—creating amorphous ice by adding compounds that lower the freezing point of water and by carefully controlling the rate of cooling. It's relatively easy to freeze a sperm cell. It's tiny and can be cooled rapidly. Large and complex collections of cells—like a functioning human brain, for instance—present a significantly greater challenge.

Amorphous ice doesn't naturally form on Earth, but it may well be the most common form of ice in the universe. Water molecules striking cold grains of dust in the vacuum of space can make amorphous ice, freezing into a random position wherever they hit the dust. So the geysers from Enceladus's cryovolcanoes are adding to the Solar System's supply of amorphous ice.

Amorphous ice can also form when crystalline ice is blasted with radiation that knocks water molecules out of their positions in the crystal. The Near-Infrared Mapping Spectrometer (NIMS) on the Galileo spacecraft detected varying amounts of amorphous ice on the moons of Jupiter. Europa and Ganymede orbit in a heavily irradiated region around Jupiter and have more amorphous ice than Callisto, which orbits outside Jupiter's radiation belts.

Here on Earth, you can make amorphous ice by spraying tiny drops of water into liquid propane cooled to 80 K (-193 °C). But if you manage to make some, you need to keep it cold. If amorphous ice reaches a temperature above 136 K (-137 °C), the water molecules jiggle about enough to vibrate into a crystalline form. Give those polar molecules half a chance and they'll snap into a configuration that brings the positive bits closer to the negative bits.

Most of us aren't equipped to make our own amorphous ice. But that doesn't mean you can't do some experimenting with ice. Paul likes to create his own ice world by filling a balloon with water and putting it into the freezer for twenty-four hours or longer.

Once you have a balloon that's frozen solid, Paul has a few experiments to suggest. First, take the balloon out of the freezer and measure the temperature of the surface by touching a thermometer to it. You'll find it is colder than 0 °C—often as cold as -18 °C or 0 °F.

Now you can experiment with the real world equivalent of Vonnegut's ice 9. Using an eyedropper, put six small drops of water on the surface of the balloon. Watch them and you will eventually see one or two of the drops freeze solid. Once a couple of drops have frozen, you are ready to crystallize those unfrozen drops. Just touch them with a bit of frost from the freezer or a chip of ice with a dry surface.

That unfrozen drop will turn solid almost instantly. In this experiment you made supercooled water—water that is colder than the freezing point, 0 °C. If supercooled water comes in contact with solid ice, the ice can initiate crystallization in the water just like the ice 9 did in Cat's Cradle.

Why does this happen? Because it is hard for an ice crystal to get started in liquid water. When water molecules come in contact with the polar surface of a pre-existing ice crystal the crystal will help line the molecules up so they can snap into position.

Water ice isn't the only thing that can serve as a seed crystal. In cloud seeding, small particles of silver iodide are released into cold moist air to seed the formation of ice crystals. These crystals, once started, can grow large enough to fall. They become raindrops when they melt. (And for those of you who wonder where science fiction writers get our ideas, here's an interesting aside. Kurt Vonnegut's brother Bernard was an expert at cloud seeding and discovered that silver iodide crystals could be used to induce ice crystal formation in clouds. Coincidence? We don't think so.)

If you want to experiment with the super cool but don't have the patience to wait for a water balloon to freeze, Paul offers an alternate method. Stretch plastic wrap over the mouth of a plastic tumbler. Use an eyedropper to put 6 drops of water (each about the size of a small pea) on the plastic wrap. Place the tumbler and drops into the freezer.

Every few minutes, check on the drops. You want to catch the moment when one or more have frozen solid, but the rest are still liquid. The water in those liquid drops is supercooled. Touch them with an icicle or bit of frost and watch them instantly harden.

If you are the patient sort and have a frozen water balloon, Paul offers some other experiments to try. Cut and remove the balloon material. While the ice is still colder than 0 °C, use an eyedropper to place drops of water on top of the ice balloon. Because these drops are in contact with the crystalline ice, they will not supercool. But they will freeze into ice bumps. This is what happens on Earth during "freezing rain," rain that falls when surface temperatures are below freezing. Unlike sleet or hail, the raindrops are supercooled water. They crystallize into ice as soon as they hit a surface, like a road or a tree or a power line.

You can also take a look at the ice that's inside your miniature ice world. If a coating of opaque white frost prevents you from seeing inside, just wait until the surface reaches 0 °C. The frost will melt, giving you a clear view.

If you want to speed the process, put your ice balloon in a tank or bucket or bathtub of water. Let it float, and you'll see that most of the ice is beneath the water. In fact 91 percent of the ice ball is underwater. Most people are surprised by just how large a fraction of the ice balloon (or an iceberg) is underwater. (The passengers on the Titanic certainly were surprised.) As a bonus, putting the ice into water melts the frost on the outside.

As you look inside, you will see clear ice on the outside of the balloon and air bubbles inside. As ice freezes, it forms a crystal structure that is made of nearly pure water. Impurities in the water—like the dissolved air that's in tap water—do not get included in the ice. As the water freezes from outside of the balloon in, the dissolved air is squeezed out of the ice into the remaining liquid water in the center of the ice balloon.

Eventually the water inside becomes saturated with air and bubbles begin to form. Once one bubble forms, it seeds the growth of more bubbles. Often you can see lines of bubbles that formed along a radial line pointing toward the center of the balloon as the water froze from the outside-in.

In 1968, five years after Kurt Vonnegut published Cat's Cradle, scientists discovered a type of water ice that they named ice 9—or actually ice IX. (Scientists prefer Roman numerals to designate different types of ice.) If you want to make ice IX, you need to start with ice III, which will require pressures over 200 million pascals (2000 atmospheres of pressure). Once you have ice III, cool it rapidly to below 140 K, and keep it there. Ice IX has a melting point near 140 K (-133 °C), so it doesn't present the danger of global freezing. And we're grateful for that.
 

Paul Doherty works at The Exploratorium, San Francisco's museum of science, art, and human perception—where science and science fiction meet. For more on Paul's work and his latest adventures, visit www.exo.net/~pauld. Pat Murphy is a science educator, a science fiction writer, and occasionally a troublemaker. She works at Mystery Science, developing hands-on lessons for elementary school. You can learn more about what she's up to at www.brazenhussies.net/Murphy.