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Why Do We Think Aliens Are Made of Water?If it's good enough for us, it's good enough for them.

By Daniel Engber
Posted Friday, March 10, 2006, at 7:09 PM ET

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The Cassini spacecraft has found evidence of what may be geysers of liquid water on a moon of Saturn, project scientists said on Thursday. "If we are right," said one of the Cassini researchers, the moon "might possibly have conditions suitable for living organisms." Liquid water is generally considered one of the likely preconditions for extraterrestrial life, along with sources of heat and organic materials. But why are we so sure water is crucial for the development of life?

Every living thing on Earth needs water to survive. That doesn't mean life on other planets would necessarily be based on liquid water, but it gives us one of our best clues as to what to look for. Since water works so well for us, we may as well focus our attention on planets or moons that have it, too.

What makes water so useful? First of all, it serves as a substrate for all the chemical reactions you need to make a living thing. To get something as complicated as biology, you've got to have a system that allows a wide variety of molecules to interact in a wide variety of ways. Water, which is a polar molecule—i.e., it has both positively and negatively-charged ends—acts as a "universal solvent." That means it can dissolve many chemicals—including the organic compounds that are the building blocks of life on Earth—and allow them to recombine or attach to one another in various arrangements.

It also helps that water remains liquid at a wide range of temperatures. That's important because solids are too rigid to allow for the necessary chemical reactions and gases aren't stable enough to maintain them. If you started to mix up ingredients for living things in a liquid that's not as stable as water, climate changes on your planet might send the whole experiment down the toilet.

Water has the added advantage of being self-insulating. That's because ice is lighter than water and floats on its surface. If a lake or ocean froze over, the sheet of ice on top could allow the water beneath the surface to stay liquid—which would in turn preserve the right conditions for life. Another liquid that doesn't share water's peculiar properties might freeze from the bottom up.

Some scientists have proposed other liquid substrates that might foster life. You could imagine a life-form based on a different set of chemicals interacting in a substrate of liquid ammonia, which, like water, is polar. (Ammonia isn't a liquid at the same temperatures as water, but its properties could be similar on a planet with the right atmospheric pressure.) The discovery of liquid methane (and methane rain) on another of Saturn's moons has led some biologists to imagine a methane-based biology. Similar thought experiments consider the possibility of life based on elements other than carbon, like silicon or boron.

Got a question about today's news? Ask the Explainer.

Explainer thanks David Grinspoon of the Denver Museum of Nature & Science and Michael Shara of the American Museum of Natural History.

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Daniel Engber is an associate editor at Slate. He can be reached at .
Photograph of Saturn moon by UPI Photo/NASA/JPL/Space Science Institute.
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