Life in the Solar System

Ancient process / modern laboratory.
Electric discharge apparatus is used to simulate conditions in the primitive atmosphere of the Earth that may have produced the chemicals of life.

From molecules to Man

It now is generally accepted that the origin of life on Earth was closely tied to the origin of the solar system itself. From this concept, combined with experimental findings, a general model for the beginnings of life on Earth is emerging. It starts, almost immediately after the original accretion of the Earth, with a period of chemical evolution which resulted in the formation of small organic molecules that are essential for life. These molecules were synthesized from the gases that made up the primitive atmosphere. This first atmosphere, which surrounded the newly formed Earth, is believed to have been reducing in nature. That is, it contained no free oxygen and was composed mainly of gases such as hydrogen, methane, ammonia, and water vapor.

As time passed, these gases were activated by energy sources like lightning discharges and ultraviolet radiation from the Sun. The original molecules broke apart, and their atoms recombined to make new, more complex molecules, thus beginning the prebiotic synthesis of organic matter, meaning the reactions that preceded the development of life.

Under the influence of primitive planetary environmental conditions the organic molecules combined to form larger, even more complex molecules. Finally, thousands and millions of molecules were assembled into structures exhibiting key properties of living things: metabolism, respiration, reproduction, and the transfer of genetic information. This stage was reached on Earth about 3.5 to 4 billion years ago, about 0.5 to 1 billion years after the Earth and the solar system formed. From that point on, the record preserved in Earth's rocks constitutes the raw material for laboratory studies because it is the repository for all evolutionary information from the time of the first living cells through the evolution of today's complex life forms. As the details of this process are being unravelled in terrestrial laboratories by scientists using sophisticated instruments and experiments, we find ourselves also exploring an important related question: Are the same or similar processes producing life elsewhere, beyond the Earth?

Sequence of cosmic evolution.
Sequence of cosmic evolution. This scheme places the theory of the chemical origin of life on Earth into the broader context that similar events may occur elsewhere in the universe.

From theory to search

The first opportunity to search directly for extraterrestrial life was provided by the Apollo missions to the Moon. Rock and soil samples returned from the Moon were analyzed extensively, but neither living nor fossil life forms were found. Further studies show that the Moon, which has no water, no atmosphere, and no protection from the radiations of space, probably never had the physical characteristics needed to begin the synthesis of organic matter, a required precursory step to the origin of life.

Mars provided our second opportunity. With its obvious atmosphere and polar caps, Mars was deemed (in both scientific papers and the popular press) a much more likely location in the solar system to harbor extraterrestrial life. The search for Martian life began in earnest when the Viking Project landed two robot spacecraft on the planet in 1976 to photograph the surface material and analyze it in detail. These missions increased our knowledge of the physical and chemical properties of the remarkable red planet tremendously. Perhaps the most unexpected and fascinating findings, however, were returned by the two instruments designed to search for life and for life-related molecules. One instrument showed that, quite unexpectedly, the Mars soil samples contained no organic materials, not even traces of carbon from meteorites that must hit the planet's surface.

This is probably due to the destructive power of the intense ultraviolet radiation from the Sun, which destroys any exposed organic material. The second instrument, intended to detect biological reactions in the Martian soil, did not detect life on Mars. It did uncover an intriguing chemical property of the soil that, at least in part, does mimic some simple lifelike reactions. Among these are the breakdown of nutrient chemicals and the synthesis of organic matter from gasous substances.

diagram of experiments conducted by the Viking Lander

Outline of a search for life.
Three life detection experiments, schematically described here, were conducted by the biology instruments on the Mars' Viking landers.

All in all, Mars remains a planet of extreme interest to exobiologists. Although Viking did not detect life, Mars continues to tantalize us. Why were no organic molecules detected? Were conditions in Mars' past history more favorable for prebiotic chemical syntheses than those present today? Are there other locations on Mars today where conditions are more conducive to life or to life-related chemistry? These questions and more require continued exploration of Mars. After all, the Earth is not the most favorable place to look for clues about life's origin, since life itself has altered the planet so drastically that much neces sary information has been obliterated. Mars, then, provides us an essential point for comparison with Earth: an environment not extensively modified by widespread life, and perhaps still harboring secrets about the relationship of the origin of life to the origin of the solar system.