THE MYSTERIOUS MOONS OF JUPITER.

• Author: BRAFFMAN-MILLER, JUDITH

As various space missions examine their surfaces, Io, Europa, Ganymede, and Callisto are beginning to give up their secrets.

THE DISCOVERY of the four great moons of Jupiter is usually attributed to Italian astronomer Galileo Galilei, who published The Starry Messenger, describing the results of observations he made at Padua on Jan. 10, 1610. Galileo had climbed to the roof of his house, aimed his telescope at the giant planet Jupiter, and spotted three bright fuzzy balls dancing near it. A few days later, he discovered a fourth.

In the sun's family of nine planets, Jupiter far outdazzles the other eight. It is larger than all of them and their numerous and sundry moons combined.

The Solar System was born about 4,500,000,000 years ago, when a small section of a huge cloud of gas and dust collapsed under its own gravitational weight, shrinking into a rotating disk called the solar nebula, a churning, extremely hot place. The turbulence and heat eventually caused the sun and its array of planets, moons, asteroids, and comets to form.

The Solar System is made up of nine planets--(in order of their distance from the sun) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Orbiting at a mean distance of 482,000,000 miles from the sun, Jupiter has a volume about 1,000 times that of Earth. Jupiter is not simply a lone, enormous planet, but the central object of a very complex system, involving four great moons, at least 12 smaller satellites, and a majestic magnetic field (the magnetosphere) that influences a vast area of space filled with charged particles of all varieties.

At its birth, Jupiter twinkled like the star it might well have become. The energy released by infalling material heated its interior. The larger Jupiter grew, the hotter it became. When the nebula material was at last exhausted, Jupiter probably had a diameter of more than 10 times what it currently has, a central temperature of about 50,000 Kelvin (the Kelvin scale is an absolute scale of temperature, in which zero is equal to minus 459.4 [degrees] F), and a luminosity roughly one percent as great as that of the sun today.

Had Jupiter been about 20 times more massive than it was, it would have continued to shrink and increase in temperature, until self-sustaining nuclear reactions caused its interior to ignite. If this had happened, Jupiter would have been a star, and the sun would have been part of a double-star system. Under such conditions, Earth and the other planets might not have formed. However, Jupiter failed as a star--after one brief, shining moment of glory, it began to cool. So immense for a planet, Jupiter was too puny to become a star.

Within the first 10,000,000 years of its life, the planet dwindled down to its present size, with just a small percentage of additional shrinkage during the past few billion years. Its luminosity also diminished. After 1,000,000 years, Jupiter sent forth a mere .00001 as much radiation as the sun, and currently, its luminosity is .0000001 that of the sun.

However, the interior of Jupiter still glows. The central temperature is about 30,000K, which is quite adequate to maintain the interior in a molten state. Jupiter is believed to be an entirely fluid planet, with no solid core.

The Voyager mission of the 1970s revealed the Jupiter system to Earthlings when it flew by this mighty planet. The hours that Voyager I swept past each moon for the first time on March 5 and 6, 1979, dramatically revealed the true nature of Io, Europa, Ganymede, and Callisto, which have now been added to the planets Mercury, Venus, and Mars and Earth's moon as important bodies inhabiting the Solar System.

The Galilean moons vary in size from just smaller than Earth's moon (Europa) to nearly as large as Mars (Ganymede). Today, they are being brought into even sharper focus by high-resolution images, magnetic measurements, and spectra gathered by the NASA craft appropriately named Galileo.

The four great moons are a study in extremes, ranging from burningly hot Io, which disgorges more lava per unit than any other known body in the Solar System, to Callisto, a frigid ball of mud...