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Chapter 2: Biochemistry

Why Water-Based Life?

Dear Dr. SETI:
All contemporary astrobilogy research seems to focus on water-based life. For example, a planet is described as being in the habitable zone of its star if its surface temperature is consistent with the existence of liquid water. Just because life on Earth is water-based doesn't mean all life in the Universe will be. Why are you restricting your studies to water planets alone?

A student

The Doctor Responds:
There are three possible answers to this very important question: a short one, a long one, and a flippant one. The short answer is that we are water-based life, therefore we know for certain that water-based life is possible. That some other basis for life might be possible is, at this point, only speculation.

The long answer involves the need for a solvent to sustain life, if for no other reason than to act as a transport mechanism for fuel (i.e., to carry an energy source throughout the organism, and to help dispose of waste products). There are at least five desirable characteristics of such a solvent, for the purpose of supporting life:

  1. It should maintain a liquid state over a wide range of temperatures. That is, the spread between its freezing and its boiling points should be as great as possible. (Water, as you know, remains liquid over a 100-degree C range. That's among the widest liquid-state ranges of known sovents, which seems ideal for the purpose stated.)

  2. It should be rather polar. A molecule with a "positive" and a "negative" side can form bonds more easily than an electrically symmetrical one. (Water is indeed quite highly polar.)

  3. It should have high surface tension. The ability to glom together in drops, to climb plant stalks through capillary action, and to pool together and flow are all useful for biological processes. (Water does have high surface tension, as you can easily observe any time it is raining.)

  4. It should be less dense in the solid than in the liquid state. An ocean that freezes from the bottom up will likely kill all life that may have emerged within it. (Water ice floats on top of liquid water, permitting life to flourish below frozen lakes.)

  5. It, or its building materials, should be readily available. (Water is made from hydrogen and oxygen. Hydrogen is the most abundant element in the Universe, and oxygen is right up there among the top ten.)

The flippant answer actually raises another question. Water-based life is, in essence, life as we know it. We know exactly how to look for life as we know it. But how do we look for life as we don't know it? We just don't know!

Of course, all three of these answers apply to organic life. If we start talking about inorganic (non-biological) lifeforms, such as intelligent machines, then all bets are off.

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