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,
