Ultraviolet radiation could serve as an extraterrestrial energy source for organic synthesis. Research shows that, while an atmosphere is important, living systems can survive a wide range of ambient pressures and are little affected by a wide range of magnetic field strengths. Oxygen is not a prerequisite for all living systems. While it is sometimes concluded that free oxygen is needed for all but the simplest organisms, less efficient metabolic processes coupled with higher food collection efficiency—or a more sluggish metabolism—would seem to do just as well. Earth is the only planet in the solar system on which molecular oxygen is known to be present in large amounts. Since plant photosynthesis is the primary source of atmospheric oxygen, it seems safe to infer that no other planet has large-scale plant photosynthesis accompanied by the production of oxygen. The possibility of the existence of extraterrestrial life raises the important question of man’s being able to detect it. Research on extraterrestrial life detection is predicated on the ability to develop ways to detect it even when the living systems are based on principles entirely different from those on Earth. The substitution of various molecules for those of known biological significance to living organisms as we know them has been investigated; the substitution of NH₂ for OH in ammonia-rich environments leads to a diverse, and biologically very promising, chemistry. The hypothesis that silicon may replace carbon does not support the construction of extraterrestrial genetics based on silicon compounds. (Silicon compounds participate in redistribution reactions which tend to maximize the randomness of silicon bonding, and the stable retention of genetic information over long time periods is thus very improbable.) Evidence relevant to life on Mars has been summarized by Sagan (ch. 1 of [ref.10]): _The Origin of Life_