The Voyager spacecraft getting closer and closer to Saturn has discovered a previously invisible cloud of glowing gas circling the planet outside the boundaries of its brilliant rings.
The gas was identified today by Voyager scientists as hydrogen, which is being lit up by the ultraviolet light from the sun after escaping from the atmosphere that surrounds the giant moon of Saturn called Titan. Girdling the planet like an enormous doughnut, the hydrogen cloud extends inward from Titan, which is 746,000 miles distant from Saturn, to the moon called Rhea, which is 327,000 miles from the planet.
"We think that as much as one kilogram [2.2 pounds] of hydrogen escapes from Titan every second of time to keep this cloud formed," Dr. William R. Sandel of the University of Southern California said at the Jet Propulsion Laboratory where the flight of Voyager is controlled. "We believe this gas cloud contains as much as 20,000 tons of hydrogen."
Invisible through conventional telescopes, the hydrogen cloud around Saturn may be the only one of its kind because there is no other moon in the solar system like Titan. The largest moon of the solar system's nine planets, Titan is also the only one with an appreciable atmosphere.
Though they can't see it happening, scientists speculate that Titan's methane atmosphere is broken up by the sun's ultraviolet light into hydrogen, ethane and acetylene. The hydrogen is so light that it escapes, leaving the other two gases behind.
"Since the hydrogen's velocity is less than the orbital velocity of Titan, it will fall inward towards Saturn," Sandel said. "It goes into orbit around Saturn because it can't escape the planet's gravitation pull." Why does the cloud stop at Rhea outside the planet's rings? Said Sandel: "We don't know the answer to that one."
Now less than 2 million miles from Saturn and approaching the planet at more than 35,000 miles an hour, Voyager sent a photograph back to Earth of the outermost of Saturn's dazzling rings that helped to explain why Saturn is the only planet with such brightly lit rings. Scientists have long speculated that all the planets including the Earth were once encircled by rings that were broken up and sucked back into the planet by gravity over time.
The picture sent back today shows two new Saturn moons that were not known to exist before Voyager detected them last week. One of them is just inside the outer ring and a second new moon is immediately outside the same ring. Scientists theorize that it is the positioning of these two moons that keeps the outer ring intact and has kept it intact for almost five billion years. The two moons literally compress the outer ring as they circle the ring at different speeds.
"The question you have to ask yourself about Saturn is why are the rings still around," the University of Arizona's Dr. Bradford Smith said, "I think we can now answer this question, at least with regard to the outer ring.
"The outside moon is moving slower than the ring and the inside moon faster than the ring. Picture a particle in the ring trying to move outward and along comes this moon to put it back into place. The same thing happens with the other moon. The two moons keep everything inside the ring."
As it approached Saturn, the Voyager photographed for the first time the moon Rhea whose frost-covered surface was streaked with dark patches of blue that made it look like a sapphire glowing in the heavens. Pictures of the giant moon Titan gave it a golden hue that scientists said was due to the photochemical breakup of its methane atmosphere by the ultraviolet light from the sun.
"When we pass within 2,500 miles of Titan early Wednesday our pictures will be 80 times better than they are right now," Smith said. "However, there is no guarantee that our camera will be able to peer through the thick haze that we think is always over the moon. We may never know what it looks like on the surface."