Casting Some (Star)Light on Jupiter's Atmosphere

Abstract

Jupiter expedition .sc ientists Tull, Van Citters, Nather, Evans and Wells. Astronomers have studied the solar system for centuries, but they still cannot agree on how the planets originated. Many theorists assume that the planets were formed from material thrown off by the sun, either as a result of a near collision with some other star or by another means, but even those who agree on this cannot agree on whether the planets were ever hot bodies or whether they were formed by cold accretion and were never much warmer than they are now. Jupiter is the largest planet of the solar system, and it has been suggested that it may not be made of solar material at all. Some astronomers believe that Jupiter is a dead, or almost dead. star that the sun has captured into orbit. Such a situation would be by no means unique in the universe. In our Milky Way galaxy there are many examples of binary or multiple stars, in which two or more stars are bound together by gravitational forces and revolve around each other. In many of these cases bright stars have dark, cold companions. In Jupiter's case the planet's composition tends to support the idea that it may be a star. Hydrogen and helium are the two most prominent constituents of the planet's mass. These elements are abundant in stars, but they are almost lacking in the terrestrial planets, Mercury, Venus, earth and Mars, which are almost certainly not originally stars. The other large planets, however, Saturn, Uranus and Neptune, have compositions that are in many ways similar to Jupiter's, and they may share a common origin with it. Jupiter's radiation also tends to support the notion of independent stellar origin. The light received from planets is reflected sunlight, but Jupiter's radio emission shows the planet to be a few degrees hotter than heating by the sun can account for. This circumstance ,as led some astronomers to suggest that in the center of Jupiter, under layers of frozen hydrogen and helium, is the dying remnant of a stellar core that is still producing a little surplus energy. Those who would argue that Jupiter is a planet made from accretion of material thrown off by the sun point out that the elements hydrogen and helium are abundant in the sun and would be abundant in any cloud expelled by it. They are abundant in the larger planets, this argument goes, because the outer planets are large enough to have the gravity to retain them. The terrestrial planets, whose main constituents are the heavier elements iron and silicon, were too small to retain the light gases. Nobody explains why the terrestrial planets are small and the outer planets large. Pluto, the farthest planet, re-