Boulders, Not Dust, Detected in Uranus Ring
The brightest of the rings around Uranus is not made up of dust after all, as had been presumed, scientists at the Jet Propulsion Laboratory said Monday. Instead, it seems to be made up of large, icy boulders whipping around the massive planet every eight hours, they said.
And that makes the rings of Uranus quite different from those of Saturn, the most prominent rings in the solar system.
The finding was just one more puzzle in what has become a veritable carnival of surprises that has left many involved with the Voyager project “happily bewildered,” in the words of chief scientist Ed Stone.
Aid to Learning
“We learn the most when we see things we can’t readily explain,” said Stone, who is chairman of the physics department at Caltech.
That has been the tone of the weeklong gathering of about 200 scientists from across the nation who have been trying to sort through mountains of data sent back by Voyager 2, the 8-year-old robot on a grand tour of the outer reaches of the solar system.
Uranus also turns out to have a bizarre weather system with winds in excess of 200 m.p.h. and an atmosphere that varies in temperature by no more than a few degrees.
The scientists will wrap up their activities at the laboratory today and head back for their home bases, where they will spend the next few years trying to figure out exactly what they have learned.
So far, they see Uranus as a planet covered by an extremely hot, electrically charged ocean beneath gaseous clouds where the temperature plunges to minus-350 degrees. While much of the information from Voyager 2 has confirmed what had previously been believed, Uranus also has been found to play by its own set of rules, challenging many ideas about such things as global weather.
For example, the high-level atmosphere above the pole of the planet that is pointed away from the sun is a little warmer than the pole that has been constantly bathed in sunlight for the past several years, and no one knows why.
Another Puzzle
And scientists who have been studying the remarkably clear photographs of Uranus’ moons were wrestling Monday with another curious puzzle.
The most recent photos show the moon Umbriel to be a dull, inactive satellite that has not experienced the kind of geological turmoil that has twisted and scarred the surface of its smaller cousin, Miranda. Nor does Umbriel show the kind of surface disruptions that have left the moon Titania looking like a refugee from the San Andreas Fault.
Instead, Umbriel, the least luminous of all the Uranian moons, reveals an old surface that appears to be virtually unscathed since its infancy.
“Sandwiched between extremely active bodies is one that is very old and very inactive,” said Laurence A. Soderblom of the U.S. Geological Survey, deputy leader of the Voyager imaging team. “Why? We don’t know why.”
Object of Curiosity
Scientists have long been curious about why the rings of Uranus are so dark, and now they may be close to an answer. The planet has nine rings that are detectable from Earth, and Voyager 2 now has found evidence of at least 11 more rings or partial rings.
Scientists had theorized--correctly, as it turns out--that the lack of luminosity could be blamed on a shortage of small particles in the rings, thus reducing the amount of reflected sunlight. But until Voyager 2 passed behind the rings Friday morning they had no way of learning whether they were right.
As the small spacecraft sped around the backside of the rings, it used two radio frequencies to transmit a tone back to Earth. As the rings passed between the spacecraft and Earth, the ring particles reflected some of the radio waves and absorbed others. Since the greatest interaction occurs between particles that are of the same diameter as the length of the radio waves, scientists were able to measure the size of the particles by determining which radio frequencies are most affected by the “dilution and scattering of the radio energy” by the particles, said Leonard Tyler of Stanford University.
Tyler’s team has spent most of its time studying the brightest of the rings, known as the epsilon ring, and has tentatively concluded that it is made up chiefly of large particles at least several feet in diameter.
Little Dust Found
The particles, which he described as “icy boulders,” are “much larger than we would have anticipated,” he said.
And although a backlighted photograph of the rings released Monday shows bands of light caused by fine dust, Tyler said the total amount of dust is very little relative to the mass of the rings.
“If you were to sweep it all up and put it in a box, it would not be a very big box,” he said.
Oddly enough, something is indeed “sweeping” the Uranian system, removing whatever dust it finds. Voyager scientists are reasonably convinced that such sweeping is performed by the planet’s magnetic field that moves wildly about Uranus as it rotates at 55 degrees from the axis of the planet’s rotation.
The planet is now believed to rotate once about every 16.8 hours, and its magnetic field rotates at about the same speed. Small particles are likely to carry an electrical charge, and thus could be picked up by the magnetic field and swept clear of the system.
Strange ‘Cold Collar’
Another curious Uranian feature is a “cold collar” that stretches around the planet about 20 to 40 degrees from the equator. Barney Conrath of the Goddard Space Flight Center said some sort of internal heat source that warms only certain areas of the planet may account for the “cold collar.”
Scientists reported earlier that they found no evidence of auroral activity similar to the Earth’s “northern lights” while Voyager 2 was closing in on the planet’s south pole. What they found instead was a form of internal electromagnetic radiation, which they have dubbed “electroglow.”
Yet when Voyager passed behind the planet, it “did detect auroral activity,” Lyle Broadfoot of the University of Arizona reported Monday.
It is not yet clear why the aurora was found on the dark side and not the light side which is exposed to the solar winds.
