Although the planets now have differing atmospheres, in their early stages the atmospheres of all the planets may have been essentially the same. The most widely held theory of the origin of the solar system states that the planets were formed from vast clouds of material containing the elements in their cosmic distribution.
It is believed that the synthesis of organic compounds preceding the origin of life on Earth occurred before its atmosphere was transformed from hydrogen and hydrides to oxygen and nitrogen. This theory is supported by laboratory experiments of Calvin ([ref.16]), Miller ([ref.33]), and Oró ([ref.34]).
The Earth's present atmosphere consists of nitrogen and oxygen in addition to relatively small amounts of other gases; most of the oxygen is of biological origin. Some of the atmospheric gases, in spite of their low amounts, are crucial for life. The ultraviolet-absorbing ozone in the upper atmosphere and carbon dioxide are examples of such gases.
Significant in the search for extraterrestrial life are the data (e.g., planet's temperature) transmitted by Mariner II, which was launched from Cape Canaveral on August 27, 1962, and flew past Venus on December 14, 1962. Mariner II's measurements showed temperatures on the surface of Venus of the order of 800° F, too hot for life as known on Earth.
The question "Is life limited to this planet?" can be considered on a statistical basis. Although the size of the sample (one planet) is small, the statistical argument for life elsewhere is believed by many to be very strong. While Mars is generally considered the only other likely habitat of life in our solar system, Shapley ([ref.35]) has calculated that more than 100 million stars have planets sufficiently similar in composition and environment to Earth to support life. Of course, yet unknown factors may significantly reduce or even eliminate this probability.
SPACECRAFT STERILIZATION
The search for extraterrestrial life with unmanned space probes requires the total sterilization of the landing capsule and its contents. Scientists agree that terrestrial organisms released on other planets would interfere with exobiological explorations (refs. [ref.36]-[ref.43]). Any flight that infects a planet with terrestrial life will compromise a scientific opportunity of almost unequaled proportions. Studies on microbiological survival in simulated deep-space conditions (low temperature, high ultraviolet flux, and low dose levels of ionizing radiation) indicate that these conditions will not sterilize contaminated spacecraft (refs. [ref.44]-[ref.48]). Furthermore, many terrestrial sporeformers and some vegetative bacteria, especially those with anaerobic growth capabilities, readily survive in simulated Martian environments (refs. [ref.49]-[ref.54]). It has been estimated that a single micro-organism with a replication time of 30 days could, in 8 years of such replication, equal in number the bacterial population of the Earth. This potential could result not only in competition with any Martian life, but in drastic changes in the geochemical and atmospheric characteristics of the planet. To avoid such a disaster, certainly the first, and probably many succeeding landers on Mars, must be sterile—devoid of terrestrial life ([ref.55]). Since the space environment will not in itself kill all life aboard, the lander must leave the Earth in a sterile condition.
The sterility of an object implies the complete absence of life. The presence of life or the lack of sterility may be proven; but the absence of life or sterility cannot be proven, for the one viable organism that negates sterility may remain undetected. Many industrial products which must be guaranteed as sterile cannot be tested for sterility in a nondestructive manner. A similar situation exists in determining the sterility of a spacecraft. Certification of sterility—based on experience with the sterilizing process used, knowledge of the kinetics of the death of micro-organisms, and computation of the probability of a survivor from assays for sterility—is the only accurate approach to defining the sterility of such treated items.
Macroscopic life can be readily detected and kept from or removed from the spacecraft, but the detection and removal of microscopic and submicroscopic life is an extremely difficult task. The destruction of micro-organisms can be achieved by various chemical and physical procedures. Sterilizing agents have been evaluated not only for their ability to kill microbial life on surfaces and sealed inside components, but also for the agents' effects on spacecraft reliability as well (refs. [ref.56]-[ref.59]). Of the available agents, only heat and radiation will penetrate solid materials. Radiation is expensive, hazardous, difficult to control, and apparently damages more materials than does heat. Heat, therefore, has been selected as the primary method of spacecraft sterilization and will be used, except in specific instances where radiation may prove to be less detrimental to the reliability of critical parts ([ref.60]).
The sterilization of spacecraft is a difficult problem if flight reliability is not to be impaired. The development of heat-resistant parts will enable the design and manufacture of a heat-sterilizable spacecraft. Without careful microbiological monitoring of manufacture and assembly procedures, many bacteria could be trapped in parts and subassemblies. To permit sterilization at the lowest temperature-time regimen that will insure kill of all organisms, the microbiological load inside all parts and subassemblies must be held to a minimum.