What appears to be a permanent jet stream of frozen ammonia moving at a speed of 350 miles an hour has been photographed above the clouds of the giant planet Jupiter.

The white-colored jet stream stands out in some of the most recent photographs of Jupiter taken by the approaching Voyager spacecraft, which yesterday was only 7.5 million miles from the planet and moving toward it at a rate of more than 600,000 miles a day. The atmopheric movement has astonished scientists by its speed and the changes it produces in the clouds.

"It's clear to us that Jupiter is far more complex in its atmospheric motions than we ever imagined," Dr. Bradford A. Smith of the University of Arizona, one of the main Voyager scientists, said yesterday at a press briefing. "What we're seeing is a rapidly changing planet."

One thing showing up in Voyager photographs is the puzzling phenomenon of huge cyclones traveling across the face of the planet, then overtaking and attaching themselves to other giant cyclones at the tops of the clouds. Smith said some cyclones speeding across the planet at almost 200 miles an hour "stick" to other cyclones for as long as 12 days, then break apart again.

"We don't understand the mechanism where one of these storms overtakes another," Smith said. "We don't know why a lot of things happen that we're seeing."

Voyager's photographs of the Great Red Spot on Jupiter in the last few months show that it undergoes dramatic change. Now actually a dark brown instead of red, this permanent hurricane in the southern hemisphere of Jupiter has altered shape as well as color.

"It's not as uniform as it was four years ago when Pioneer photographed it," Smith said. "And there are atmospheric currents swirling around it that weren't there before."

Some storms are moving clockwise around the Great Red Spot, which itself is as large as the Earth. The storms are traveling so fast they circle the Red Spot in six days. Another "air" current can be seen traveling north from the Red Spot, then disappearing just south of the Jovian equator.