Astronomy: Storms on a Mixed-Up Planet

When astronauts eventually soar be yond the moon to explore the distant planets, they will find Jupiter a dangerous place to visit. Even if they manage to withstand the tremendous pull of Jupiter’s gravity and survive the frigid atmosphere of ammonia, methane, hydrogen and helium, they may well perish in the gigantic storms that sweep the planet every decade or two.

Because these violent outbursts never occur at the same point on the planet’s eccentric orbit, some astronomers have suggested that they are caused by the erratic release of internal heat. This would indicate that Jupiter is behaving like a star as well as a planet—radiating not only energy absorbed from the sun but also giving off heat generated in its own interior. Jupiter’s split personality has now been confirmed by a University of Arizona astronomer.

More than the Sun. Working with the new 61-in. reflecting telescope at the university’s Catalina Observatory near Tucson, Dr. Frank J. Low focused Jupiter’s image on a germanium bolometer —an infrared measuring device of his own invention that is more sensitive than any other now in use. After analyzing the feeble radiation, he determined that Jupiter’s effective temperature is — 225 °F.—much warmer than the — 274 °F. that Jupiter would register if it were an ordinary planet radiating only the heat it received from the sun. To reach the higher temperature, Low calculated, the heat output of Jupiter would have to be roughly 2-. 5 times as great as the amount of heat it absorbs from the sun.

Neither Low nor anyone else can say for certain how Jupiter manufactures its heat. Its internal pressures are not large enough to cause the thermonuclear reactions that occur in a true star. And because less than 1% of Jupiter’s mass consists of the heavier elements that are the source of the radioactive isotopes believed to heat the earth’s core, radioactivity can contribute only a small fraction of the heat that is apparently generated in the interior.

A Small Star. To Astronomer Gerard Kuiper, who directs the University of Arizona’s Lunar and Planetary Laboratory, there is only one satisfactory answer. “Like a small star,” he says, “Jupiter is still contracting somewhat under the force of its own gravity.” As the planet contracts, Kuiper speculates, the compressed and solid hydrogen mantle that envelops its molten core occasionally cracks open, releasing the vast amounts of heat that brew Jupiter’s mysterious storms.