Page:Concepts for detection of extraterrestrial life.djvu/14

4 By studying the radioactive decay of minerals, scientists have determined that the surface of the Earth hardened into something like its present form about 4.5 to 5 billion years ago. Life itself probably arose during the first billion years of the Earth’s history.

Although the planets now have differing atmospheres, it is believed that 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. Among the most abundant elements in our galaxy are hydrogen, oxygen, nitrogen, and carbon. These were present in the primitive atmosphere of the early Earth in the form of water, ammonia, methane, and hydrogen. Later, this reducing primitive atmosphere was altered to our present oxidizing atmosphere by the escape of hydrogen and by the formation of oxygen through the photodissociation of water vapor in the upper atmosphere and through plant photosynthesis. 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.

On other celestial bodies different things happened to the atmospheres. The Moon with feeble gravitational attraction was unable to retain any atmosphere at all. Jupiter and Saturn, large in size and with a much more powerful gravitation and cooler atmospheres, retain hydrogen, hydrogen compounds, and helium.

Scientists believe 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, supporting their theory by laboratory experiments. In these experiments, a mixture of gases similar to the primitive atmosphere is prepared and energy is applied; i.e., energy in the form of an electric spark or ultraviolet light. These and other forms of energy were available on the primitive Earth.

By this action, simple organic molecules are formed. This is not to say that the molecules are alive, although they are constitutentsconstituents [sic] of living things on Earth.

Among these molecules are the building blocks of proteins (amino acids) and the building blocks of DNA. The latter is the genetic material which contains information for the development of the individual organism and which is passed from generation to generation.