CONDITIONS FOR LIFEThree
CONDITIONS are important when considering the possibility of life on other
planets: temperature, the existence of water, and the existence of an atmosphere. Although these conditions need not be precisely the same as those on Earth for life to
exist, certain tolerance limits for life as it is normally understood can be established. (Totally different life chemistries lie only in the realm of conjecture.) An ecosphere for any
star is that small range of distances from the star within which
temperatures are suitable for life, water may exist in liquid form, and an atmosphere may be retained. Within the
Solar SystemThe ecosphere of the Sun includes the Moon and the planets Earth and Mars. The Moon and Mars present striking evidence that even within a star's ecosphere, planetary development may provide conditions inhospitable to life. The Moon lacks an atmosphere and (possibly) water and is alternately scorched and frozen by temperatures ranging from 117¡ C (240¡ F) to ç190¡ C (ç310¡ F). Although small amounts of water may exist on Mars and temperatures at the equator may reach a comparatively mild 17¡ C (62¡ F), the average atmospheric pressure is only 6 millibars, compared to the standard atmospheric pressure of 1,013.25 millibars (about 14.7 psi) at mean sea level on the Earth, and the atmosphere is primarily in the form of carbon dioxide. Arguments exist, however, for the possibility that simple life forms could have arisen and then died out in the long geological past of Mars.Outside of the ecosphere of the Sun, Mercury and Venus, with temperatures of 430¡ C (800¡ F) and 485¡ C (900¡ F) respectively, are ruled out as abodes of life, as are the outer planets, where temperatures fall below ç140¡ C (ç220¡ F) even in the upper cloud layers of the closest gas giant, Jupiter. Although internal heat sources might generate milder temperatures in the lower cloud layers of Jupiter or on the surface of its volcanic satellite Io, life under such conditions is difficult to conceive. The chance is equally faint that life or protolife might exist below the clouds of Saturn's satellite Titan, although some astronomers are hoping for the potential of life in an ocean, should it exist, below the frozen surface of Jupiter's satellite Europa. Outside the Solar SystemA large number of
factors must be considered in attempting to estimate how many intelligent civilizations might have arisen throughout the Galaxy. One famous attempt at assessing these factors, originated by the radio astronomer Frank Drake, expresses the number of technical civilizations in the Milky Way Galaxy in the form of an equation, where each of the letters on the right side of the equation represents a factor necessary to sustain intelligent life.The first three factors may be characterized as physical and take into account the rate of star formation, the fraction of stars with planetary systems, and the number of planets in each system with conditions favorable to life. The next two factors estimate the fraction of those planets on which life develops and the fraction with intelligent life. The last two factors incorporate societal evolution and represent the fraction of those planets with intelligent life that evolve technical civilizations capable of interstellar communication, and the lifetime of such a civilization.Each of the factors in the Drake equation is extremely uncertain. Among the physical factors, for example, the most widely accepted theory of the origin of planets and planetary systems asserts that planets condense out of a rotating nebula, of which the central star is itself the most prominent remnant, a circumstance that scientists have no reason to believe was unique to our solar system. This possibility is in contrast to an early rival theory, which held that our solar system was the chance result of the collision of another star with the Sun. Despite these theories, no Earthlike planets have as yet been detected with ceinty outside the solar system. Clouds of material discovered around various other stars, however, indicate that planetary systems may be forming there, and a number of relatively nearby Sunlike stars appear to be accompanied by planets in the size range of Jupiter.Theories of the origin of life are equally elusive. In 1953, Stanley Miller demonstrated that if hydrogen, ammonia, methane, and water vapor were irradiated, organic molecules would be produced. Later experiments showed that components of DNA and RNA, the basis of life on Earth, could be produced in a similar manner. Some organic molecules have since been discovered floating in space (see astrochemistry). The limits of their formation and the subsequent evolution of life, however, are not well understood.Even more conjectural is societal evolution. The only known example of a technical civilization capable of radio communication is our own; whether a scientist believes a civilization would take 50 years or millions of years to develop such a technology depends on the individual scientist's prejudices. As a result of such uncertainties, widely different numbers are used in the Drake equation, and scientists have estimated that as many as 10,000,000 technical civilizations exist in our Galaxy or as few as 1, represented by the Earth.
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