WHO'S THE LEADER OF THE GANG?:

Seeking Life as We Know It: To all appearances, it has to start with water -- but does it? What is the likelihood of an ammonia-based alien somewhere in space? (K.C. Cole, March 5, 2004, LA Times)

On the face of it, water seems a rather silly molecule — two hydrogen atoms attached to an oxygen atom in a way that looks like the head of Mickey Mouse. Even children know its chemical formula: H2O.

But the bonds it forms with itself and other molecules are anything but ordinary.

Atoms normally bond by sharing the negatively charged electrons that buzz around their positively charged nuclei, like people sharing popcorn at a movie.

In water, the oxygen shares one electron with each of its hydrogens, leaving four extras. These clump together as "lone pairs" that can grab onto other molecules like prehensile feet.

At the same time, the two positive hydrogen nuclei stick out the other side like arms. The "feet" of one water molecule grab the "arms" of the other, forming abnormally strong networks. Where one water molecule goes, the others tend to follow. Thus, water can climb tall trees — hand over foot, as it were — in defiance of gravity, carrying nutrients from the soil to the leaves.

Chemists say they would expect water to be a gas at room temperature because it's made up of just a few light atoms. But the strong bonds make the molecules stick together in a liquid form.

Luckily, the bonds aren't so sticky that they form a viscous gel — something that Boston University physicist Eugene Stanley initially found perplexing. Water flows freely, he and others discovered, because water molecules stick to each other only briefly, let go, grab another partner — whirling an ever-changing cast of partners around in a molecular square dance.

The upshot is that water stays watery over a remarkable range of temperatures (32 to 212 degrees Fahrenheit, to be exact).

This is a liquid bonanza for life, which seems to need some form of fluid to transport things from place to place. In solids, molecules stick together and can't go much of anywhere. In gases, the molecules don't get close enough to interact.

Water's unbalanced geometry — positive charges on one side, negative on the other — also gives it a distinctively schizophrenic personality (although chemists, like psychiatrists, prefer the term bipolar). This makes it an excellent solvent.

One side of a molecule grabs on to negative charges; the other side grabs the positive. This pulls most things apart, so water can dissolve almost anything. (If things didn't dissolve, they'd sink to the bottom, or rise to the top — not good for a free flow of chemical reactions.)

Why doesn't life just disintegrate altogether in water then? While water is one of the most strongly bipolar molecules, it is not the most reactive — meaning it can make things fall apart (dissolve) without changing their composition (react). So the parts can be endlessly rearranged.

And as it turns out, the few things water doesn't dissolve are equally important in assembling life's building blocks. Water hates fat. "It won't dissolve a spot of grease on my nice silk tie," Stanley said.

Water herds these hydrophobic (water-hating) and hydrophilic (water-loving) molecules into structures such as cells. The hydrophobes point away from each other, while the hydrophiles look inward. "It's like circling the wagons," McKay said.

Water, in other words, gives living things outsides and insides. The hostile outside is kept at bay, while inside, the proteins behind nearly all of life's mechanisms go about their business.

"You have 3,000 proteins, minimally, in every cell," said University of Massachusetts biologist Lynn Margulis, "and every reaction requires water. Everything else is negotiable."

What's the water doing with the proteins exactly? "Everything," Margulis said. "It's like a loom that you can do the weaving in. It's the matrix that's holding things in place. Nothing can go on without it."

The magical molecule does a whole lot more: For example, it absorbs heat slowly, and holds on to it for a long time. This stabilizes temperatures not only in the oceans, but also inside living things — which, lest we forget, are made mainly of water.

Finally, water expands when it freezes, contrary to nearly every other substance known. That's why ice floats, allowing it to form an insulating blanket on lakes and ponds for life beneath. Without it, fish would freeze before they hit the grocer's shelves.

Of course, it's hard to ignore one obvious reason life may depend on water. Hydrogen is the most abundant element in the universe. Helium is the second, but it's inert — so standoffish it doesn't bond with other atoms at all. Oxygen comes third. Maybe life is made of water simply because it's there.

But some otherwise habitable worlds just don't have water. Are they out of luck?

Not necessarily. "Water's a wonderful molecule," McKay said, "but there are other wonderful molecules."

Are molecules really perfectly round and do the H's and the O really line up in a V?

Posted by Orrin Judd at March 15, 2004 5:18 PM