Life on other planets?
From our print archive: In spite of the hours clocked at super-spyglasses, astronomers have yet to spot any little green men on Mars or any other planet. But they have made some pretty convincing observations that life does exist on other planets.
Is there life on other worlds? If other planets can support life chemically as we know it here on earth, how does this relate to the origin of life itself? Scientists have long speculated on the theory that life in its most primitive form may be the next step in cosmic evolution after the formation of planets. While this is still only a theory, new ideas on planetary origin and recent discoveries in chemistry have given it support. For example, forty million miles from Earth, at this writing, is Mars, a planet colder than the earth, with no oxygen in its atmosphere, and little water on its surface. A man transported to Mars would gasp and die—and most other familiar organisms would also perish. Yet, for over half a century astronomers have observed slight seasonal color variations on the planet; variations apparently coinciding with the availability of water. These have been interpreted as evidence for plant life on Mars, life specifically adapted to the rigors of the Martian environment. If the reported color changes are real, there seems to be no other reasonable interpretation. Further, marginal spectroscopic observations by W. M. Sinton suggest that there may be molecules with C-H bonds on the surface of Mars. Carbon and hydrogen are fundamental elements for all terrestrial organisms, and the chemical bond combining them is essential for the structure of proteins, nucleic acids, and other biological building blocks. Is it possible, then, that the same sort of life, similar in its basic chemical makeup, has originated twice in the same solar system? While speculative in some of its details, the general pattern of cosmic evolution is fairly well established. Cosmic evolution begins with an enormous cosmic dust cloud, such as exists today between the stars. Such a cloud has a “cosmic” abundance of the elements, being composed primarily of hydrogen and helium, with only a small admixture of heavier elements. Here and there matter will be somewhat more dense than in nearby regions. The more diffuse regions will be gravitationally attracted to the denser region, which, in consequence, will grow in size and mass. As matter streams in towards the condensing central nucleus, conservation of angular momentum will cause the whole region, nucleus and streaming matter, to rotate faster and faster. In addition, as large amounts of matter continue to collide with the nucleus, its temperature will steadily rise. After perhaps a hundred million years, the temperature at the center of the cloud will have risen to about fifteen million degrees. This is the ignition temperature for thermonuclear reactions, (such as the conversion of hydrogen to helium in the hydrogen bomb). At this time the nucleus of the cloud will become a star, “turning on” and radiating light and heat into nearby space. If the rotation is sufficiently fast, the forming star will separate under certain conditions into smaller parts, producing a double or multiple star system. Now as the star forms, there still is a large dust cloud surrounding the star and rotating with it. In this cloud, the solar nebula, small, denser regions begin attracting nearby matter, as in star formation. However, the protoplanets that grow from these regions, (in the gravitational field of the nearby star), never rise by collisional heating to the thermonuclear ignition temperature, and so become planets and not stars. Gerard P. Kuiper, professor of astronomy at Yerkes Observatory, has described how planets are formed in this manner in recent years. In the forming protoplanets, there would be a tendency for the heavier elements to sink to the center, leaving the much more abundant hydrogen and helium as the principal constituents of the atmosphere surrounding the new planets. When the newly formed star “turns on,” radiation pressure will tend to blow away this atmosphere. However, if the protoplanet is very massive, or very far from the sun, the gravitational attraction of the protoplanet for a gas molecule may be greater than the force of radiation trying to blow it away, and the protoplanet may retain an atmosphere. This atmosphere can be residual from the proto-atmosphere, or may be due to gaseous exhalations from the planetary interior. For example, the earth’s present atmosphere is due to exhalations; Jupiter’s present atmosphere is residual. In such a way, one can understand, generally, the atmospheres of the planets in this solar system:
- Mercury: Not massive, close to the sun, retains negligible atmosphere.
- Venus: More massive than Mercury, further from the sun, retains only the heavy gas, carbon dioxide.
- Earth: Retain s the lighter gases, nitrogen, oxygen, and water vapor, but has lost almost all hydrogen and helium.
- Mars: Although further from the sun, is less massive than Earth or Venus, and so retains principally only the heavy gas, carbon dioxide.
- Jupiter, Saturn, Uranus, Neptune: Much further from the sun and very massive, they retain much hydrogen and helium, while the other planets have lost theirs.