Pluto's best portrait yet comes from Hubble images Space telescope's photos show unexpected contrasts

March 08, 1996|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Pluto: It's not just a dot anymore.

The solar system's ninth planet, visible only as a speck of light since its discovery in 1930, has finally had its picture taken by NASA's Hubble Space Telescope, from a distance of 2.7 billion miles.

The snapshots don't look like much, even after computer enhancement. Pluto's surface appears blurry and mottled with light and dark areas, much like the early photos of Mars.

But those blotches show contrasts sharper than almost anything else in the solar system, scientists say, and they are as delighted as new parents.

"These images are just fabulous," said Dr. Bruce Margon, a professor of astronomy at the University of Washington. He has ties to Hubble, but not to the Pluto project. "The amount of contrast is something no one could have predicted. Pluto never fails to surprise."

Astronomers believe the bright areas in the photos are frozen nitrogen, while the dark regions are hydrocarbons produced by the breakdown of methane in Pluto's thin atmosphere under bombardment by the sun's radiation.

But Dr. Marc W. Buie of the Lowell Observatory, near Flagstaff, Ariz., cautioned against "over-interpretation" of the pictures. "It's difficult to say precisely what they [the light and dark regions] are," he said.

Dr. Buie produced the Hubble images with Dr. S. Alan Stern of the Southwest Research Institute in Colorado and Dr. Laurence Trafton of the University of Texas. During the summer of 1994, the team snapped more than a dozen photos in visible light as Pluto rotated on its axis. The pictures were later assembled into a single image of the planet's entire surface, and computer enhancement then sharpened the portrait.

Dr. Stern said other duties delayed release of the work until yesterday. "We apologize for sitting on the data," he said.

Pluto is the only planet not yet visited by a spacecraft. NASA is preparing to end that distinction with the Pluto Express mission, due for launch after 2000.

The trip will take 10 to 12 years, and Drs. Buie and Stern said only then will anyone get a clearer picture of the planet's surface than the one released yesterday. They said their portrait of the tiny planet, at such extreme distances, pushed Hubble's capabilities to their limits.

Pluto is the smallest planet, only 1,400 miles in diameter. It is two-thirds the size of Earth's moon, but 12,000 times farther away. And it is the only planet whose discoverer -- 90-year-old Clyde Tombaugh -- is still alive.

It was named for the Roman god of the dead and ruler of the underworld. But the moniker was chosen because the first two letters are the initials of 19th-century astronomer Percival Lowell, who first predicted Pluto's existence based on irregularities in the orbits of Neptune and Uranus.

Pluto orbits the sun once every 248 years, and rotates once every six days and nine hours. It has one known moon -- Charon. The moon, only 789 miles in diameter, was detected in 1978 but not photographed as a separate object until Hubble did it in 1990.

During most of its trip around the sun, Pluto is more distant from the sun than is Neptune, as far as 4.6 billion miles out. But for 20 years of each circuit it wanders inside Neptune's orbit, as close as 2.8 billion miles from the sun, making it the eighth planet. That's where it has been since 1979.

When it is closest to the sun, scientists say, some of Pluto's frozen nitrogen, carbon monoxide and methane warms to minus 350 degrees Fahrenheit and vaporizes enough to produce a tenuous atmosphere first detected in 1988.

As Pluto moves farther from the sun and grows colder in the coming years, Dr. Stern said, its atmosphere will likely "collapse," or "snow down," until all of it has been redeposited on the surface. That may cover the dark regions and make the planet uniformly bright.

It's possible, he said, that "by the end of Hubble's life, we will see a very different planet from this one."

Some scientists believe Pluto is one of the last "ice dwarfs," a class of ice-covered rocky objects common in the early solar system, which were swept up eons ago by the bigger planets.

Because Pluto's moon is nearly as big as Pluto itself, and because the two orbit around a common center of gravity, they are regarded as a double planet system.

Pub Date: 3/08/96

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