Natural smog on Uranus? Scientists piqued by Voyager data on atmosphere, its circulation, moons

As images of Uranus grow clearer on their monitors, scientists at the NASA Jet Propulsion Laboratory (JPL) have only to look out the window for an earthly analogy. The orange-brown haze they now can see on Uranus may have some similarity to ``the sort of thing that hangs over L.A. from time to time,'' says Bradford Smith, leader of the Voyager imaging scientific team.

He says the haze over the Uranian south polar region could be produced by the action of ultraviolet sunshine on methane in the planet's atmosphere. This would produce acetylene. And acetylene, in turn, could produce a weak photochemical smog. A planet, which Dr. Smith says has been ``of almost unfathomable mystery,'' is beginning to reveal itself. Yet, as Smith notes, ``Uranus is not going to give up its secrets easily.''

Astronomers for the first time are seeing some clouds on the planet. But the clouds' extremely low contrast against the general Uranian background makes them hard to distinguish. Their motion shows there are winds on Uranus. But, so far, the scientific team here has been unable to see how, in general, the planet's atmosphere circulates.

Images sent back by Voyager 2 have shown a number of new moons. But these are unexpectedly -- and so far, unexplainably -- as dark as charcoal.

One of these moons, called 1985U1, which was found Dec. 31, has begun to show an irregular shape and a large enough image to confirm that it reflects only about 5 percent of the incident light. Six similar moons were later found. They too are probably very black, Smith says. He explains that the darkness of 1985U1 ``came as a bit of a surprise.''

All seven new moons lie between the outermost of Uranus's nine rings and Miranda, the innermost of the five previously known satellites. The new moons' positions and dark surfaces ``must be telling us something about the [Uranian] system and how it evolved,'' Smith says. But that message so far is unclear.

Two small satellites also have been found close to, and on either side of, the outmost ring -- the Epsilon ring. Their gravitational influence helps keep ring particles from dispersing. Other such ``shepherd'' satellites probably will be found. And, Smith says, they too are likely to be quite dark.

The five major previously known satellites are a different kind of puzzle. They began to show distinct individ-ualities earlier in the week, while Voyager was still several million miles away. Smith told a press briefing that when Voyager was at a similar distance from Saturn, that planet's icy moons all looked alike. They didn't have their own distinctive surface markings and degrees of brightness as do the Uranian satellites.

Clearly, the Uranian complex is a planetary system different from anything studied before. But to scientists, who don't yet know what to make of it, it's as though the planet were teasing them with hints of discoveries yet to come as Voyager closes in.

This fed the scientists' anticipation as the spacecraft neared its Uranian rendezvous at 10 a.m. Pacific standard time today. They realize that, for planetary experts, this is the opportunity of a lifetime. ``We know less about Uranus in advance than we knew about Jupiter and Saturn, so the opportunity for surprises is much greater,'' observes project scientist Edward Stone.

Also, Dr. Stone notes, no other mission to Uranus is likely to be launched until well into the next century. It would take 30 years to reach the planet if a spacecraft were launched toward it directly from Earth. Voyager has gotten there in less than nine years thanks to a rare alignment of planets. This, in effect, allowed Voyager controllers to use the gravitational influence of Jupiter and Saturn to pull the spacecraft onto a new course and give it extra energy. Now Uranus is about to ``refuel'' the spacecraft and fling it onto a course for a closeup inspection of Neptune on Aug. 25, 1989.

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