Voyager 2 expected to solve some of Saturn's riddles

Saturn's mysteries are legion, but space scientists here expect to unravel at least a few of them in the coming week. Last November, America's supersophisticated space probe, Voyager 1, loped within 80,000 miles of the planet, the second largest in the solar system, and gave mankind its closest look to date at the spectacular rings and superficially bland face that Saturn presents to the solar system. The television pictures and scientific data beamed back the 950 million miles to Earth by Voyager 1 multilied Saturn's mysteries.

Now a twin spacecraft, Voyager 2, is speeding across the interplanetary void and closing in on the giant gaseous sphere. Planetary scientists here at the Jet Propulsion Laboratory have had nine months to analyze and puzzle over the earlier pictures and data on Saturn. They have programmed the instruments in the second spacecraft to concentrate on gathering clues to the various riddles which the planet presents. Teh scientists will be aided by substantially sharper TV cameras on the second spacecraft than those on its twin. Also, its path will carry it much closer to the planet -- within 63,000 miles -- and the rings will be better illuminated than on the previous flyby.

As a result, "We anticipate an even better look at Saturn," summarized Ed Stone, a voyager project scientist. In a preliminary press briefing he summarized the outstanding puzzles of Saturn and how scientists hope Voyager 2's electronic eyes and ears will shed light on them.

the rings, of course, are Saturn's most dramatic feature and its biggest puzzle. They were perplexing enough before the first Voyager cameras profiled them. If a scale model were made from the thinnest airmail paper, the rings would be about 30 feet wide with an outer diameter of 135 feet, and almost perfectly flat.

The best scientific explanation for how such a fragile structure could be stable enough to exist for millions of years involved Saturn's numerous moons. Somehow, scientists theorized, the gravitational forces of these moons trapped the ring material, keeping it from falling in toward the plant or spiraling out into space. But the first Voyager shocked scientists with pictures of incredible complexity. Instead of a smooth surface with a few major divisions as they appear from Earth, the pictures showed the rings to be made up of thousands of ringlets. Not only are there thousands of these smaller divisions, but a number of them are complex in themselves. Some are braided in a complicated fashion, while others are eccentric. Also, the appearance of radial features, quickly nicknamed "spokes," were totally unexpected.

After months of study, scientists have come up with two basic explanations for the baffling array of ringlets, Dr. Stone reports. One theory holds that they are not actually separate rings, but waves superimposed on the ring material, ice-covered debris that averages 1 to 10 meters (3.3 to 33 feet) in diameter. The second theory requires the existence of a large number of "moonlets" up to 10 kilometers (62 miles) across which would, by their gravitational influence, tend to clear the finer material out their orbits and concentrate it into bands.

To scrutinize the ring structure, Voyager 2 will be measuring the light of the star Delta Scorpii a thousand times a second as it passes behind the rings. The spacecraft's cameras will also be photographing the gaps between ringlets, trying to locate moonlets.

The so-called spokes represent a deeper mystery. "We know they are clouds of fine material that appear and disappear, but there is no satisfactory explanation for their origin that I know of," Dr. Stone says.

One suggestion is that these strange clouds are levitated above the ring plane because they somehow build up a charge of static electricity. How they would become electrically charged is unclear, but the spacecraft is going to snap an edge-on picture of the rings to look for elevated clouds. Also, Voyager 1 detected radio noise characteristic of electric discharges, and Voyager 2 is going to attempt to pinpoint its source.

Another major mystery is Saturn's muted appearance. The gaseous giant Jupiter, by contrast, is a rich riot of color. Unless computer-enhanced, pictures of Saturn's cloudtops are quite bland.

Because it is farther from the Sun, Saturn is a cooler plant than Jupiter, and its cloudtops are buried deeper in its atmosphere. One speculation during Voyager 1 was that the planet was obscured by a high-altitude haze which obscured a more Joavian-appearing atmosphere. Preliminary Voyager 2 pictures of the planet show no indication of such a visible haze, however, says Bradford A. Smith, leader of the imaging-science team, which analyzes the photographs sent back by the space probe.

"This strongly suggests that the lack of contrast is something intrinsic to the planet, so the question remains," Dr. Smith says.

Saturn's cloudtops sport a few sports which the scientists feel are quite similar to some on Jupiter. But Saturn lacks the swirling storms that are prominent features on its sister planet, and it has one unique cloud feature which has the meteorologists entranced. This has been dubbed the "No. 6 Cloud," because in one picture it took on a shape vaguely like a "6." This was given joking significance because Saturn is the sixth planet from the Sun."It gives the impression of a corkscrew, spiraling up and filling out," Dr. Smith explains.

Despite its placid appearance, Saturn has one of the most frenetically moving jet streams in the solar system. At the equator the clouds have been clocked at 1,100 miles an hour, four times the speed of the fastest winds on Jupiter.

several of Saturn's 17 moons are also the subjects of extreme interest. Its largest, Titan, is the size of Mercury and is the only satellite in the solar system with a dense atmosphere. Actually Voyager 1 passed by Titan much closer than will Voyager 2. Its pictures showed Titan's atmosphere to be so thick that it was impossible to get even a glimpse of the surface. It strengthened the idea even a glimpse of the surface. It strengthened the idea that Titan's present atmosphere may be similar to that on Earth before life evolved. The flyby confirmed that the moon has a nitrogen-based atmosphere like Earth, with small amounts of methane and hydrogen. No oxygen was detected, as scientists also think was the case early in Earth's history. Activities of the second spacecraft will be limited to trying to detect the strong winds that scientists believe prevail there.

Saturn's other moons were thought to be made mainly of ice, with crater-pocked surfaces. Because they were supposed to contain only small amounts of rocky stuff, experts believed that these satellites had too little radioactive material to melt their interiors -- a process considered necessary to cause geological faults, large rift canyons, and other, similar features. Contrary to the theory, however, several of these moons display faults and canyons. So the second spacecraft will be looking at these features more closely to find any clues to the process by which they were formed.

Saturn also has two unusual satellites which Voyager 2 will photograph closely. One, enceladus, is unusually bright. Scientists expect to find its surface to be extremely smooth and icy.

The other, Iapetus, has a bright and dark face. It has the greatest contrast of any object in the solar system. Roughly one-half of the moon reflects light, about like "dirty snow," while the other is extremely dark, about like asphalt.

"The dirty snow is probably just that," Dr. Smith says. "We don't know what the dark material is." One of the most acceptable theories right now holds that the dark material comes from a dark satellite, Phoebe, which orbits slightly farther out.

A final Saturn mystery involves bursts of radio signals that come regularly from the planet. From Voyager 1 measurements, the scientists have determined that these radio signals originate from one area on the planet and are emitted only when that area is facing the sun.The problem is that the researchers can find nothing exceptional about this region, nothing that would give any indication why it should be an active radio source.

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