Smeared Photo Helps Scientists : Icy Boulders Give Clue to Odd Neptune Ring

Scientists studying photographs relayed by the Voyager 2 spacecraft have discovered huge icy boulders in a mysterious ring around Neptune, possibly providing a clue as to why the planet’s outermost ring is so bizarre.

The icy “moonlets” measure up to about 10 miles in diameter, far larger than the dust-like particles believed to make up the rest of the ring.

More importantly, they were found in an area that is much brighter and wider than the rest of the ring. Ground-based astronomers discovered that segment of the ring several years ago and concluded erroneously that they had found only partial rings, called ring arcs.

Clue in Photograph

No one could explain then why any planet would have ring arcs because scientists could see no reason why ring particles would not be distributed fairly uniformly around the entire planet.

Voyager scientists determined last week that Neptune’s rings are complete, although the segments discovered by astronomers earlier were lumpy and highly irregular. That too has eluded explanation.

That changed Saturday when scientists at NASA’s Jet Propulsion Laboratory announced that a smeared photograph may provide the answer. For the Voyager, even when something goes wrong it seems to turn out right.

A smeared photograph is the nemesis of any shutterbug, but one sent back by the intrepid 12-year-old robot caught the interest of the scientists, said Bradford Smith of the University of Arizona, leader of the Voyager’s imaging team.

Smith said that when scientists looked at the image, they detected about six or eight bright spots that had been moving along the ring during the exposure and thus appeared as long lines. Camera movement, which smeared the image, also caused the lines to twist, resulting in what appeared to be a braided ring.

The “braids” were in fact just bright spots that had to be clumps of matter large enough to show up as streaks, just like a distant star that also appeared in the image, Smith said.

That told the scientists that the ring contained “some embedded rocks,” Smith said.

The “rocks,” which he said probably contain a lot of ice and might best be described as “moonlets,” dominate the fatter segments of the ring. The narrow band of smaller particles extending the rest of the way around Neptune was discovered just before Voyager’s closest approach last Thursday night.

“We’ve been looking to see if there’s something that is different about this particular ring” that could explain why it is so inconsistent, Smith said. “This may be it.”

More Than Two Rings

Voyager data also revealed Saturday that Neptune is more than just a two-ring circus. Voyager images now show that the distant planet has up to five rings.

Smith said the most recently discovered rings are so broad that their small particles are spread so thin they can just barely be seen, and all lie closer to the planet than the outermost ring that was confirmed last week.

The broadest of the rings is many hundreds of miles in width and lies closest to Neptune, he added.

The discovery of the moonlets was only one of many developments Saturday at JPL, where scientists continue to revel in the immense amount of data that Voyager sent back to Earth in its grand finale as the unchallenged leader among planetary explorers.

Gravity Slows Voyager

The Voyager is now nearly a million and a half miles beyond Neptune, speeding toward space at nearly 38,000 m.p.h. Its speed is less now because it is fighting Neptune’s gravity instead of being pulled by it as it heads off toward its first encounter with another star.

It is due to pass within about 25 trillion miles of Bernard’s Star in the year 8571.

Meanwhile, JPL scientists were entering the home stretch Saturday, comparing notes as they prepared to wrap up the “close phase” of the Neptune encounter this Tuesday.

There are 11 scientific teams on the Voyager project, each concentrating on different disciplines ranging from the study of the “interplanetary medium” to the composition of Neptune’s backward traveling moon, Triton.

The planetary encounter provides a unique opportunity for experts in one field to compare their results with findings by colleagues in other disciplines. That cross-fertilization makes it possible for experts to engage in instant analysis, although not always with a great deal of confidence.

For example, geologists on the imaging team were fascinated by features on the surface of Triton that appeared to have been shaped by winds, just as sand dunes in the desert are molded by the wind on Earth.

For that to work, however, Triton would need a fairly dense atmosphere, and the geologists asked the project’s atmospheric scientists if their data showed that Triton’s atmosphere could be heavy enough.

“We need (a dense atmosphere),” Smith said, “but they won’t give it to us.”

Nitrogen in Atmosphere

So if some of the features on Triton’s surface were in fact shaped by the wind, it must have been at some other time, in some other season, when the conditions on the moon’s surface were different than they are today.

Scientists have also determined that Triton’s atmosphere has one of the components of Southern California’s smog, nitrogen.

That discovery also required the assistance of a distant star.

Bill Sandel of the University of Arizona said it had not been possible for astronomers on Earth to determine all the constituents of Triton’s atmosphere because they cannot separate the moon from its atmosphere when viewed from the Earth. So researchers have debated among themselves over the nature of Triton’s atmosphere.

“This has been a controversy that has been with us for years,” Sandel said.

An ultraviolet spectrometer aboard the Voyager solved that problem by studying the light from a star as it was eclipsed by first the atmosphere and then the icy surface of Triton. Since different light frequencies are absorbed by different elements, the spectrometer could define the atmosphere’s composition by detecting which frequencies were absorbed.

Data from that experiment shows that nitrogen dominates Triton’s atmosphere, although there is some methane (natural gas) close to the surface.

Scientists also reported Saturday that Neptune’s magnetic field is inclined at least 30 degrees from the planet’s axis of rotation. On most planets, including Earth, the inclination is about 10 degrees, so Neptune’s is odd but not as odd as Uranus’, which is inclined 60 degrees.

‘Humble’ Magnetosphere

The magnetic field also creates an envelope around the planet, called the magnetosphere, which plays an important role in such things as the auroral activity that creates the northern and southern lights on Earth.

Physicist Tom Krimigis of Johns Hopkins University reported Saturday that “Neptune’s magnetosphere is rather humble.”

That may be one reason Voyager scientists have not found conclusive evidence that the spacecraft detected northern lights when it passed within 3,000 miles of the clouds over Neptune’s north pole.