Until 1969, the search for extraterrestrial life in site was limited to meteorites. But with the progress of space exploration, the study was extended. The Apollo missions to the Moon to Earth allowed to bring hundreds of kilos of lunar samples. To avoid potential contamination of the Moon by soil organisms, the U.S. space agency, NASA, the Apollo spacecraft was sterilized before launch. But, conversely, to protect our planet from aliens living germs that might be present on the Moon and in the samples collected, imposed a quarantine of ships and returning astronauts to the lunar flights. However, the analysis showed that lunar samples contain no living germ, not even any organic compound complex. Another life? The diversity of terrestrial living matter suggests that any extraterrestrial life should also be extremely varied.
Their appearance may differ significantly from those of the life around us, as were the conditions of the global environment. But, what can be said at the molecular level? Extraterrestrial life is based on organic chemistry and in liquid water. The carbon atom is a very abundant element in the universe. And water is a molecule also widespread in the cosmos. Alternative solutions have been proposed with other elements. Silicon, like carbon, can not produce a chemical complex.
Ammonia, nitrogen analogue of water, can not replace it, since it is only liquid in a temperature range much closer (-78 to -33 C under conditions of normal pressure) than water (to 100 C). Thus, exobiologists can not display much originality: extraterrestrial life should be a carbon-based life and in an aqueous medium. The carbon hypothesis is also supported by the presence of numerous organic compounds in very varied in the cosmos. Biothermic area of life development can only occur in a few ways in which the temperature moderate, consistent, namely the presence of liquid water. But life also requires a shield to protect you from energetic radiation, especially in the ultraviolet radiation from the sun, and cosmic rays, which are particles of a planetary atmosphere can exert this protective role, therefore, a planet possesses a dense atmosphere is an ideal place to harbor life. But it is also necessary that the planet offers moderate temperatures, consistent with the presence of liquid water on its surface. This condition is satisfied, in particular, on the planets which revolve in a well determined around its star. In the solar system, this area called biothermic area is located between the orbits of Venus and Mars. Earth, located practically in the middle, is a privileged place. And this area also includes our natural satellite, the Moon. However, due to the absence of a substantial atmosphere, its surface is completely inhospitable to life.