Concepts for detection of extraterrestrial life/Introduction
Introduction
The mystery of his own origin has intrigued man since earliest antiquity. Throughout the ages he has puzzled and theorized over the question of how he began, where and when.
But man is like a detective arriving at the scene some millions or billions of years after the event and trying to reconstruct the event. The principals have long since departed; most of the clues have disappeared; even the scene itself has changed.
Of equal fascination today is the question of life on other worlds—extraterrestrial life. Do the seasonal changes in the darkening on the Martian surface mean that plant life blooms, withers, and dies there? Are there living things beneath the covering clouds of Venus despite the great heat this planet is subjected to? Did life on the Moon go underground eons back when the atmosphere departed; and does life, or its residue, still exist there? Is Jupiter actually ice encrusted beneath its hydrogen shroud; and if it is, does this preclude some form of life undreamed of by man?
Now, for the first time, man is beginning to grasp the key which may solve the question of whether or not life in some form exists on the other celestial bodies of our solar system. The key is, of course, the technology of space exploration. The search for life in space now being planned by the National Aeronautics and Space Administration is part of that technology.
The question of extraterrestrial life and the question of the origin of life are interwoven. Discovery of the first may very well unlock the riddle of the second.
The oldest form of fossil known today is that of a microscopic plant similar in form to common algae found in ponds and lakes. Scientists know that organisms like it flourished in the ancient seas over 2 billion years ago. (See fig. 1.) However, since algae are a relatively complex form of life, it is obvious that life in some simpler form originated much earlier. Organic material similar to that found in modern organisms can be detected in these ancient deposits as well as in much older Precambrian rocks.
Figure 1.—Representative microfossils, three-dimensionally preserved in chert from the Gunflint iron-formation of the north shore of Lake Superior. This formation is of middle Precambrian age and has been dated by K40-Ar40 as approximately 2 × 106 years. All figures are from thin sections of the chart photographed in transmitted light. Published here for the first time (from Dr. E. S. Barghoorn).
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 constitutents 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.
Scientists, in fact, can duplicate many of the individual steps through which they think life first arose. But they cannot (or cannot yet) reconstruct the actual process by which life originated, a process which may have occupied Nature for hundreds of millions of years.
At some point energy and chemical materials combined under the right conditions and life began. Nucleic acid molecules were probably formed as well as other complex molecules which enabled the nucleic acids to replicate. Due to errors in replication, or mutations, evolution occurred, and in time many different life forms arose. Since this happened on Earth, it is possible that it also happened on other planets.
The NASA program of space exploration for the next few decades holds great promise of solving one, and of throwing light on the other, of these great twin mysteries—extraterrestrial life and the origin of life. American space technology is now developing the capability of exploring the Moon and the planets of our solar system to search there for organic matter and living organisms.
Spacecraft have flown past, and crashed on, the Moon. Mariner II, launched from Cape Kennedy on August 27, 1962, flew past Venus on December 14, 1962, taking readings and transmitting data which are significant in the search for extraterrestrial life. Mariner’s measurements showed temperatures on the surface of Venus in the order of 800° F, too hot for life as known on Earth.
Other flights past Venus and to Mars are planned. Later, instruments will be sent to Mars in search of extraterrestrial life or biologically significant molecules. Culture media, microscopes, and chemical detecting devices will search out micro-organisms and life-related substances. Eventually, television cameras will look for foliage—and, who knows, footprints?