Concepts for detection of extraterrestrial life/Chapter 1
Evidence Relevant to Life on Mars
by Dr. Carl Sagan
The difficulty of directly detecting Martian life can be easily understood if you imagine yourself on Mars, peering through a large telescope at Earth. Detecting life on Earth—particularly intelligent life—from such a vantage point would be extremely difficult. In view of this, it is not surprising that the question of life on Mars is as yet unresolved. In general, there are three approaches which can be taken to this problem.
The Origin of Life
In the past decade, considerable advances have been made in our knowledge of the probable processes leading to the origin of life on Earth. A succession of laboratory experiments has shown that essentially all the organic building blocks of contemporary terrestrial organisms can be synthesized by supplying energy to a mixture of the hydrogen-rich gases of the primitive terrestrial atmosphere. It now seems likely that the laboratory synthesis of a self-replicating molecular system is only a short time away from realization. The syntheses of similar systems in the primitive terrestrial oceans must have occurred—collections of molecules which were so constructed that, by the laws of physics and chemistry, they forced the production of identical copies of themselves out of the building blocks in the surrounding medium. Such a system satisfies many of the criteria for Darwinian natural selection, and the long evolutionary path from molecule to advanced organism can then be understood. Since nothing except very general primitive atmospheric conditions and energy sources are required for such syntheses, it is possible that similar events occurred in the early history of Mars and that life may have come into being on that planet several billions of years ago. Its subsequent evolution, in response to the changing Martian environment, would have produced organisms quite different from those which now inhabit Earth.
Simulation Experiments
Experiments have been performed in which terrestrial micro-organisms have been introduced into simulated Martian environments, with atmospheres composed of nitrogen and carbon dioxide, no oxygen, very little water, a daily temperature variation from +20° to —60° C, and high ultraviolet fluxes.
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Figure 2.—International Astronomical Union map of Mars. In the astronomical convention, south is toward the top. The extent of the polar ice caps in summer can be seen at the top and bottom of the picture. The area Syrtis Major, at +10° latitude, 290° longitude, is a site of strong seasonal darkness and polarization changes, and is a suspected site of hydrocarbons and aldehydes. The dark area, Solis Lacus, at —30° latitude, 90° longitude, is a site of strong secular changes which occur erratically and cover areas up to 1000 kilometers in extent. These two sites are among those of greatest interest for early exploration of Mars.
It was found that in every sample of terrestrial soil used there were a few varieties of micro-organisms which easily survived on “Mars.” When the local abundance of water was increased, terrestrial micro-organisms were able to grow. Indigenous Martian organisms may be even more efficient in coping with the apparent rigors of their environment. These findings underscore the necessity for sterilizing Mars entry vehicles so as not to perform accidental biological contamination of that planet and obscure the subsequent search for extraterrestrial life.
Direct Searches for Life on Mars
The early evidence for life on Mars—namely, reports of vivid green coloration and the so-called “canals”—are now known to be largely illusory. There are three major areas of contemporary investigation: visual, polarimetric, and spectrographic.
As the Martian polar ice cap recedes each spring, a wave of darkening propagates through the Martian dark areas, sharpening their outlines and increasing their contrast with the surrounding deserts (fig. 2). These changes occur during periods of relatively high humidity and relatively high daytime temperatures. A related dark collar, not due to simple dampening of the soil, follows the edge of the polar cap in its regression. Occasional nonseasonal changes in the form of the Martian dark regions have been observed and sometimes cover vast areas of surface.
Observations of the polarization of sunlight reflected from the Martian dark areas indicate that the small particles covering the dark areas change their size distribution in the spring, while the particles covering the bright areas do not show any analogous changes.
Finally, infrared spectroscopic observations of the Martian dark areas show three spectral features which, to date, seem to be interpretable only in terms of organic matter, the particular molecules giving rise to the absorptions being hydrocarbons and aldehydes.
Taken together, these observations suggest, but do not conclusively prove, that the Martian dark areas are covered with small organisms composed of familiar types of organic matter, which change their size and darkness in response to the moisture and heat of the Martian spring. We have no evidence either for or against the existence of more advanced life forms. There is much more information which can be garnered from the ground, balloons, Earth satellites, Mars flybys, and Mars orbiters, but the critical tests for life on Mars can only be made from landing vehicles equipped with experimental packages such as those discussed on the following pages.