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May 1 , 2006
The rarest element

Sidney Perkowitz is Charles Howard Candler Professor of Physics.

Several years ago a construction crew rolled into my backyard and built a pond that was more than a pond. It lies at the bottom of a slope and is fed by water rushing down a gently curving artificial streambed lined with four tons of rocks. From my deck I can watch water still and serene, and water noisily cascading among the rocks. I’m seeing two aspects of the remarkable state of matter called liquid, which on our planet is mostly inhabited by water, H2O.

Water has drawn thinkers for millennia. In the sixth century B.C.E., Thales of Miletus considered water the elemental substance behind all things. A hundred years later, when the philosopher Empedocles proposed that all things are made of four elements, he retained water along with earth, air and fire.

Both were right—but also wrong, for the water we find so readily on Earth is in short supply elsewhere. The other planets circling our sun are either too hot or too cold to support liquid water (though water probably once flowed on Mars). Go further out, to the 150 extrasolar planets we know, and you’ll find that they also are too hot to retain liquid water.

We can point to only a few cosmic sites for liquids, and only two for water. Liquid hydrogen and helium are thought to lie deep within Jupiter and Saturn; lakes of hydrocarbons—compounds of hydrogen and carbon—exist on Saturn’s largest moon, Titan, the only extraterrestrial liquid under open skies like Earth’s oceans; and Jupiter’s moon Europa shows evidence of an ocean of water hidden under a layer of ice. In March of this year, scientists reported that NASA’s Cassini space probe spotted geysers of ice and water vapor, thought to come from buried reservoirs of liquid water, erupting from Saturn’s moon Enceladus.

So the universe seems to be virtually bone dry, except for the water that covers more than 70 percent of the Earth’s surface and is integral to life, because it is a prime medium to support complex chemical reactions. This is why molecules essential for life, like DNA, chlorophyll and hemoglobin, could have formed in the sea.

For this reason, you might think that 21st century science fully understands water. Indeed, the water molecule, a tiny boomerang with a hydrogen atom at each end and oxygen in the middle, has been scanned to within an inch of its life.

But many fascinating questions remain unanswered. Some are characteristic of liquids in general, such as why it is that water can generate seemingly random swirls of turbulence; some are water’s alone. Unlike other substances, water does not contract as it freezes—it expands, making ice less dense than water, with surprising consequences. It is the reason the Titanic sank, since it is the reason that icebergs float, and it is the reason that marine life survives winter, since water freezes from the top down.

This and other anomalous properties arise from the dynamic interactions of water molecules. Despite the adage “still waters run deep,” there is no “still” water. Far below the limits of human perception, inconceivable numbers of molecules in my quiet pond perform an endless thermal dance. Solids and gases also contain swarms of atoms or molecules, but in solids, they are more or less frozen in place, like a child’s interlocked Lego blocks, and the resulting properties are relatively easy to explain. In gases, simplicity comes because the molecules hardly interact at all. Each can be treated separately, like a billiard ball that only rarely encounters another billiard ball.

The molecules in a liquid, however, are neither fully free nor fully bound. They twist, turn and vibrate, and affect their neighbors. Water molecules come together in evanescent clusters that separate and reform in fractions of nanoseconds. Researchers must resort to computers that follow individual actions molecule by molecule, cluster by cluster, until they are summed up to simulate a drop of water. But even big computers can examine only a few molecules, giving unrealistic results.

At least the behavior of water at rest can, in principle, be computed. Water in motion is worse. It partakes of randomness. In some places in my backyard brook, the water streams over rocks in an orderly manner, mounding up or dipping down to follow their underlying shapes. This is laminar flow, where water molecules trace parallel paths to form layers. But elsewhere, the motion is pure chaos, with water unpredictably swirling or splashing, now here, now there.

Such turbulent behavior has engaged scientists for centuries, and artists as well; Leonardo da Vinci loved to draw the characteristic eddies of moving water. The great 18th century Swiss mathematician Leonhard Euler analyzed fluid motion but omitted the effects of friction. Unfortunately, when Euler used his equation to design a fountain for Frederick the Great, it failed to work.

Now we know the correct equation, but it is brutally difficult to solve, and it leaves unanswered the central question: How does deterministic laminar flow break into random whorls? This commonplace effect that we experience daily is a great unsolved problem of classical physics, in some ways as challenging as quantum mechanics.

Nevertheless, amid the puzzles, studies of water continue, and they continue to surprise. In 2003 researchers examined water in rapid snapshots that captured its atoms in motion and concluded that, at this time scale, water is not H2O after all, but more nearly H3O2, though recent results contest this. Other researchers have used tiny cylinders made of carbon to muster water molecules into one-dimensional arrays, like soda in a drinking straw. This seemingly artificial arrangement is thought to mimic the way water migrates from soil to plants and the way proteins are carried across membranes.

The study of water often relates back to its importance for life. It would be dismaying if the apparently arid universe around us meant that life has not evolved elsewhere, but the geysers on Enceladus suggest that the cosmos may be wetter than we thought. In any case, life has a way of appearing when least expected. No matter what the rest of the universe holds, we can be thankful to exist in a world enlivened by the rarest element and its mysteries.

This essay, adapted from Perkowitz’s “The Rarest Element” (published in Writing On Water, MIT Press, 2001), first appeared in the April/May 2006 Academic Exchange and is reprinted with permission.

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