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One Thing Comet Probe Hubbub Has Right: Water Is Vital to Life Chemistry

Daniel Bakken


Editor’s Note: We are pleased to present a new series at ENV, "Exoplanets." Daniel Bakken is an engineer who teaches astronomy at the college level, and an entrepreneur in compound semiconductor crystal growth. In a series of articles he will critically examine recent claims about exoplanets beyond our solar system, asking whether our own planet Earth is a rarity, or common, in the cosmos.

With the landing of a European probe yesterday on comet 67P/Churyumov-Gerasimenko, excitement in the media and scientific communities has centered in part on the idea that life arose on Earth in a water medium carried here by comets. The Boston Globe, for example, reported:

For scientists, one of the key mysteries that Rosetta will explore is whether Earth’s oceans are filled with melted comets. Since the rocky bits that came together to form the planet were dry, water has to have come from somewhere else. One possibility is that comets slamming into the Earth early on seeded it with water.

The Wall Street Journal similarly explained:

Because comets carry water and organic molecules, scientists also hope that the Rosetta mission will provide insights into whether comets could have brought water to Earth and possibly kick-started life here.

exoplanet2.jpgThe rapid appearance of life on Earth has indeed suggested to many researchers that life may be common in the cosmos, provided a suitable liquid water habitat is available.1 So liquid water is often touted as a requirement, many times the only one, for a life-supporting body.

When faced with the origin-of-life problem, some will argue that liquid water is not necessarily the only option for life. They’ll ask, what about life as we don’t know it? In his book, Life as We Do Not Know It: The NASA Search for (and Synthesis of) Alien Life, author Peter Ward speculates on these possibilities.2 On Earth, all known life is based on carbon atoms with water for a solvent. Yet chemists have suggested alternatives to both. Silicon is chemically similar to carbon, another abundant element in the universe, and by far the best option besides carbon to build complex molecules life requires.3 It fails, however, when compared to carbon. Silicon forms very strong bonds with oxygen, as exemplified in granite, for example. Carbon’s bonds to oxygen are gentler, making it much more bio-friendly. Silicon molecules don’t hold together well enough to form long molecules analogous to proteins.4 For these and other reasons, most serious origin-of-life researchers do not feel silicon could work.5 Besides, carbon is common in the galaxy, more so than silicon, its closest competitor.

Replacements for water as a solvent are a little more promising. Water’s nearly unique characteristics make it an ideal solvent for life, yet its temperature range over which it is liquid limit the possibilities for life. Several alternatives are imagined with liquid temperature ranges both higher and lower than water. Water is very abundant in the universe, so liquid competitors will be less available to varying degrees. In hotter environments sulfuric acid has been proposed6, yet organic compounds would have a difficult time holding together at these temperatures. Colder than water environments could utilize methane or liquid nitrogen, and it is speculated that these liquids may exist in the outer solar system moons, like on Titan or Triton.7 Unlike water, however, these liquids are denser in their solid phase, and sink when solids form in liquid pools. When water freezes, it floats above the liquid, acting as an insulator, and is exposed to solar radiation enabling it to melt more efficiently when warmth returns. A more important problem is cell walls of Earth life will not work in these liquids8, and it is not known if suitable realistic alternatives could be found. In colder environments molecular activity is necessarily much slower. Therefore any imaginable life forms in colder solvents would likely not be very active, so they would not have much evolvability.9

It is clear that these candidates are not serious challengers to water for life chemistry. As exobiologist Fran�ois Forget says,

It is difficult to imagine any alternative chemistry approaching the combination of diversity, versatility and rapidity afforded by liquid water-based biochemistry. This results from the unique ability of carbon to form complex species, and the unique characteristics of water as a liquid solvent (a large dipole moment, the capability to form hydrogen bonds, to stabilize macromolecules, to orient hydrophobic-hydrophilic molecules, etc.).10

Next up: Defining habitable space bodies.

References Cited:

(1) Forget, "On the Probability of Habitable Planets," 3.

(2) Peter Ward, Life as We Do Not Know It: The NASA Search for (and synthesis of) Alien Life (New York: Viking, 2005).

(3) Ward, Life as We Do Not Know It, 62.

(4) Ben Bova, Faint Echoes, Distant Stars: The Science and Politics of Finding Life Beyond Earth (New York: HarperCollins, 2004), 28-29.

(5) Iris Fry, The Emergence of Life on Earth: A Historical and Scientific Overview (Piscataway NJ: Rutgers University Press, 2000), 246.

(6) Ward, Life As We Do Not Know It, 65.

(7) Ibid., 65-67.

(8) Ibid., 73.

(9) Ibid., 74.

(10) Forget, "On the Probability of Habitable Planets," 3.

Image: Artist’s depiction of probe Philae touching down on comet 67P/Wikipedia.

Daniel Bakken