Using its on-board radar to penetrate the clouds over Venus, the Pioneer spacecraft in orbit around the planet found a rift valley that is deeper and longer than any such feature on Earth.

The radar on the 810-pound Pioneer found the valley about 1,000 miles south of the equator. The radar altimeter measured the depth of the rift at four miles and its length at least at 150 miles. The radar scanned only that part of the surface that covered 150 miles of the rift.

"We don't see rift valleys of this magnitude on Earth," Dr. Gordon Pettingill of the Massachusetts Institute of Technology said yesterday at a Pioneer press conference at the Ames Research Center in Mountain View, Calif. "They don't exist on Earth."

Unlike canyons, which are carved by flowing water, rift valleys are caused by movements in the planet's crust. Areas of the crust move away from one another, causing the surface in between to drop. The largest rift valleys on Earth are in Africa, but they are nowhere near the size of the one found on Venus.

Pettingill said the bottom of the Venusian rift appears to have collapsed as much as one mile below the surrounding surface. The radar aboard the Pioneer spacecraft measures the height of the crested sides of the rift at between four and six miles.

The MIT scientist sprang a second surprise at yesterday's press conference when he disclosed that Pioneer troubleshooters were able to reactivate the altimeter attached to the radar instrument after sclentists had given it up for dead just after Christmas.

"The conventional wisdom about instruments like this is that they don't like to be turned on and off and so we left ours on continuously," Pettingill said. "Well, that was the apparent reason it failed, because when we turned it off to cool it down, the problem began to cure itself.

"We now keep it on for two hours every day and turn it off for 22. I am happy to report that the altimeter has just about fully recovered."

Scientists whose instruments are aboard the orbiting Pioneer and were aboard the four probes that descended to the surface of Venus also said yesterday they had discovered what appears to be a steady discharge of lightning in the atmosphere of Venus.

"We think we see continuous electrical activity in the clouds of Venus," said Dr. Frederick Scarf of TRW Inc., "of the kind that produces a steady stream of lightning bolts."

The ultraviolet telescopes on the orbiting Pioneer have found a pair of bright rings that circle the north and south polar regions down to a latitude 50 degrees from each pole. The polar rings were described yesterday as the brightest things on the disc of the planet that is the brightest in the heavens.

Curiously, inside the bright polar rings the ultraviolet telescopes have found wells of darkening where the upper atmosphere is sinking down, pushing aside the sulfuric acid clouds at the top of the planet's 11-mile cloud deck.