8.2.1 - The Limits of Carbon Aqueous

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

Carbon chemistry in terrestrial organisms proceeds by chemical reactions in the medium of water -- an amazing substance with a whole set of properties which make it ideal for our kind of life. Some have even contended that "water is the only possible candidate material."

In 1913, Harvard University biochemist Lawrence J. Henderson published a little book entitled The Fitness of the Environment in which he assembled for the first time the many points in favor of water as a life-fluid.⁸⁷⁹ Henderson’s analysis extends to the other molecules of life as well, and his main contribution is to show that the very chemical properties of the elements gives each of them a certain unique status and irreplaceability.

Among the many advantages of water, Henderson notes that it is an excellent solvent for countless substances, making it quite useful as a mediator of chemical activity in the liquid phase. Water, too, is an ionizing solvent, which means that an acid-base chemistry is permitted and an ever wider range of reactions can take place. (Acid-base chemistry is fundamental to Earth life but is not necessarily a requirement for all life.¹⁰⁷⁴)

Hydrogen bonding between water molecules gives the liquid a high heat capacity -- the ability to store lots of heat without changing temperature very much. Organisms which use water are thus at a distinct advantage in an environment in which sudden swings between hot and cold are common. This same bonding force also holds biomolecules together so that reaction rates are enhanced⁶⁴ (although it has been pointed out that H-bonding may not be absolutely essential for life²³⁵³).

Furthermore, water has a comfortably wide liquidity range -- a full 100 K under normal terrestrial conditions. However, the extent to which this temperature span may be broadened is not generally appreciated. Saturated salt water may freeze as low as 250 K; under 100-200 atm of pressure, the boiling point may be elevated to as much as 640 K.

In the proper environment, water could remain a liquid over a range of 400 degrees. It is not unreasonable to conclude that H₂O may well be the solvent of choice from 250-500 K, particularly in view of its extremely high cosmic abundance.

Serious laboratory work aimed at defining and measuring the limits of carbon-based, aqueous biochemistries has just gotten under way in earnest in the 1970s. Consequently, direct evidence is only beginning to emerge from the scanty data.

In spite of this handicap, there are early signs that many alternatives are possible even within the confines of a carbon-water system.

Dr. Peter M. Molton at the University of Maryland has suggested that simple changes in the early prebiotic environment may drastically affect the chemical species which later turn up as the dominant actors on the biochemical stage of evolution.¹⁰⁹⁴ His example is drawn from Miller-type experiments involving the prebiotic synthesis of amino acids, the building blocks of proteins.

In the lab, chemists have learned that there are two common structural forms taken by amino acids. They are called alpha and beta.

The basic layout of an amino acid molecule is a chain of carbon atoms with a small -NH₂ ("amino group") stuck on somewhere. In the alpha form, the amino group appears near the tail end of the molecule. In the beta form, the amino group is displaced more towards the front of the chain.

All amino acids used in terrestrial biochemistry, with one minor exception, are of the alpha variety. The beta forms are absent. Why?

Molton shows that this peculiarity may be due to nothing more complicated than the order in which water is introduced during the early stages of chemical evolution. If H₂O enters into the prebiotic reactions when the first simple compounds are being synthesized, then life will evolve with proteins consisting exclusively of alpha amino acids.

But what if the initial products of chemical evolution never come into contact with water at all in the early stages? According to Molton, when water is thus absent the beta amino acids will predominate. The proteins comprising the resulting extraterrestrial lifeforms would then be of the beta, rather than the alpha, variety.*

The next step, says Molton, is to try to synthesize plausible alternative nucleotides in the laboratory, simply by altering the prebiotic conditions under which they arise. Scientists are just beginning to see the myriad possibilities that may be open to carbon-water biochemistry on other worlds.

* Proteins made from the beta forms would probably not be edible by humans. Indeed, they might even be poisonous -- a fact of considerable importance for future interstellar astronauts and colonists.