The four large moons of Jupiter are streaked and patched in red, orange, yellow and brown, the first moons in the solar system known to possess color surfaces.
Photographing the four moons, called the Galilean moons, from the relatively short distance of less than 1 million miles, the Voyager spacecraft rapidly approaching Jupiter today gave the world its first look at moons not colored gray and white. The pictures sent back by Voyager today revealed the moon Io to be orange and red, Ganymede to be a rich brown streaked with yellow, Callisto a darker brown patched with yellow, and Europa to be pale yellow streaked with tans.
"To planetary geologists, this is Christmas Eve," Dr. Laurence A. Soderblom of the U.S. Geological Survey said today at the Jet Propulsion Laboratory here, where Voyager is directed. "We had never seen these worlds before except as points of light."
Looking a little like harvest moons on earth, the four Galilean moons (so named for Italian astronomer Galileo, who discovered them in 1610) really resemble no other bodies in the solar system. They even appear different from each other, separated in space by a different chemistry and a different enviroment that gives each one of them a different color.
At 7 o'clock tonight, Voyager was less than 500,000 miles from Jupiter and closing toward the planet at more than 50,000 miles an hour. The 1,800-pound unmanned spacecraft will fly by Jupiter at 7:42 a.m. EST Monday, speeding past the planet at 81,000 miles an hour, 173,000 miles from the tops of the Jovian clouds.
The spacecraft will photograph the Galilean moons again Monday morning, catching Europa from a distance of 600,000 miles' and Ganymede and Callisto from less than 100,000 miles. Voyager will fly less than 12,000 miles from Io, photographing features on its surface a half-mile across. Jupiter has nine other moons but they are so small that Voyager will not study them.
Even today, the photographs of the Galilean moons surpassed any ever taken by earthbound telescopes at any time in history. Features were visible on all four moons that had not been hinted before. What appeared to be huge canyons showed up on Europa, what looked like craters and a giant volcanic basin stood out on Io.
Ray-like, yellow streaks suggesting volcanic flows appeared on Ganymede. Callisto, the outermost of the Galiean moons, looked like a frozen ball of brown and green ice patched with yellow.
Geologists were reluctant to speculate on what it is that gives color to the Galilean moons, except to say that the chemistry of their surfaces is not the chemistry of the Earth's moon, the planet Mercury or the two moons, of Mars called Phobos and Deimos.
The reds and oranges that swirl across the surface of Io apparently come from the surfur and salt beds believed to cover it. The yellows and browns of the three other moons could come from sulfates and nitrates embebbed in their surfaces.
"I am struck by a similarity between Ganymede and Callisto," said the University of Hawaii's Dr. David Morrison, a geochemist who has spent his life observing Jupiter's moons through telescopes on Earth. "Europa appears stuck in some super ice age. It looks like it might have had oceans that all bolied away at one time and left salt beds behind."
The fact that Io is a closest to Jupiter and lies trapped in the planet's most intense radiation fields almost surely has something to do with its red and orange color. Hardold Masursky of the U.S. Geological Survey thinks the protons and electrons bombarding the surface of Io enhance the colors that appear naturally in its surface.
"We've seen how radiation in the laboratory can bring out color in rocks," Masursky said. "There's no reason to think it's not doing the same thing on Io."
Voyager has already found an aurora of sulfur cicling Jupiter in the orbit of Io, which scientists believe is sputtered off that moon's surface by the constant bombardment of highspeed protons and electrons. Today, Voyager flew into a swarm of highspeed sulfur particles moving away from Jupiter at a rate of 5,000 miles a second.
"We think these evry energetic sulfur particles come from Io," Voyager Project scientist Edward Stone said today. "Somehow these ionized [electrified] suflfur particles are grabbed by the magnetic field of Jupiter and speeded up to these terrific speeds we are seeing in the outer limits near the planet."
If the four large moons were streaked with color, the striped clouds of Jupiter itself showed up in today's photographs in what Cornell University's Dr. Carl Sagan called a "riot of color." In the clouds of Jupiter were reds, browns, oranges and yellows of almost every hue, and deep inside the southern clouds the color blue was observed for the first time.
The chemistry causing these colors is still unknown but Sagan and the University of Arizona's Dr. Bradford Smith said speculation centers on red phosphorous, sulfur and its polymer compounds, ammonia mixing and reacting with hydrogen sulfide and a brilliant yellow chemical called ammonium hydrosulfide.
"We think the rich reds we see are complex organix molecules being made deep below the clouds and arried to the tops by an upwelling of warm air," Smith said. "We don't yet understand the convective mechanism that gets it up there to where we can see it."
One of the biggest mysteries about Jupiter is still why its Great Red Spot is red. The color could come from red phosphorous, hydrogen cyanide or some far more complicated polymer of sulfur. Just as mysterious is its age. Soviet scientist G.S. Golitsyn has speculated that the Red Spot has been there at the tops of Jupiter's clouds for at least one million years.