Voyager Finds an Unexpected Neptune Ring
The Voyager spacecraft has detected the first complete ring around Neptune, a camera-shy band of tiny dark particles that is barely detectable by the spacecraft’s sensitive instruments.
It is a “very tenuous ring with very little material” compared to the brilliant bands around Saturn, but “it is, indeed, a complete new ring,” said Bradford Smith, head of the Voyager imaging team.
Ground-based observations had suggested that Neptune’s rings were broken into pieces, called ring arcs, but an instrument aboard Voyager that measures light reflected by the ring detected its surprising presence early Tuesday morning.
Mystery Deepened
The discovery came on a day when the mystery of the ring arcs deepened. Voyager scientists also learned from new photos Tuesday that they can successfully forecast the weather on Neptune from nearly 3 billion miles away, enabling them to learn more about the planet’s giant dark spot.
There are no photos of the complete ring yet because the ring is so faint that it can barely be detected in the dark sky.
Later this week, after Voyager zips over the north pole and around the backside of Neptune, the photo opportunity should improve significantly because the ring will be backlighted by the sun. Just as a fashion photographer uses a spotlight to highlight a model’s hair, the ring should stand out because sunlight will bounce off the tiny particles in a process called “light scattering.”
Scientists gathered at the Jet Propulsion Laboratory in Pasadena hope that will allow them to say more about the newly discovered ring and the mysterious ring arcs that lie farther out from the planet. It is not clear why the arcs formed as partial instead of complete rings.
The presence of the arcs has been suspected for more than a decade because of evidence collected by ground-based telescopes.
Stars have been eclipsed sometimes, but not always, when passing behind Neptune’s immediate neighborhood. That indicated that sometimes the stars were hidden by rings, but other times they were not, suggesting that the rings were broken into arcs.
Arcs Confirmed
Voyager confirmed the presence of the broken rings earlier this month as it sped toward Neptune. Tuesday morning it revealed the complete ring lying about 33,000 miles from the planet’s center.
Smith said ground-based telescopes did not pick up even a hint of the ring because it is far too thin to block out the light from stars that pass behind it. He estimated that 98% of the light from a passing star would filter through the ring.
Although ground-based astronomers are credited with discovering the arcs, Voyager has revealed that the arcs are rather peculiar.
Rings are formed because the gravity from small nearby moons “shepherds” material into long streamers around the planet. The rings of Saturn, Jupiter and Uranus are of uniform structure and composition, varying only slightly from one area to the next because of the influence of other satellites.
By comparison, Voyager has revealed that the Neptunian arcs are a bit sloppy.
Smith said the arcs are “lumpy” and inconsistent--wider in some areas than in others, ending abruptly in some areas while slowly dying out in others.
‘Messy’ Arc
“The Neptune arc is messy,” Smith said. “It doesn’t behave in some nice, systematic way.”
Scientists are at a loss to understand the dynamics of a system that would create such inconsistent arcs.
“It’s got to be telling us a story, and we hope we can figure it out,” Smith said.
One theory appears to have been eliminated by Voyager. If Neptune had a small moon in a nearly polar orbit, its gravity could break up rings in the planet’s equatorial plain when it passed through.
“We haven’t found that inclined moon,” said Caltech physicist Edward Stone, Voyager’s chief scientist. That leaves scientists still looking for an explanation for the ring arcs.
Smith said additional data from Voyager over the next few days may explain why Neptune’s arcs seem relatively chaotic. It is likely, he added, that other small satellites are perturbing the arcs, and they are just waiting to be found.
Undiscovered moons, for example, could stretch or compress parts of the arcs, or even cause gravitational turbulence like waves splashing on a beach.
“Almost certainly there are more satellites” to be discovered by Voyager, Smith said. Voyager closed to within 2 million miles of Neptune on Tuesday night at a speed of nearly 38,000 miles per hour. Its instruments continued to send back data on Neptune and its moons, including increasingly sharp photos of the planet’s mysteriously turbulent atmosphere.
Sunlight, which drives atmospheric weather patterns, is too faint at Neptune to explain the 400-m.p.h. winds and a giant hurricane-like storm center south of the planet’s equator.
The Voyager continued to record images of the planet, using light of various wavelengths to probe deeper into the atmosphere. Since different wavelengths are absorbed at different altitudes, the multispectrum approach has allowed scientists to determine the height of various atmospheric structures.
It turns out that the Great Dark Spot--an enormous storm center the size of the Earth--is much deeper in the atmosphere than Jupiter’s famed Great Red Spot. Wispy clouds that appear as white areas in the Voyager photos are apparently just that--wispy clouds high above the Great Dark Spot.
Predicting Weather
Scientists hope to concentrate on the Great Dark Spot today, aided in part by some daring bit of weather forecasting. Meteorology is an inexact science, even on Earth, but accurate predictions are essential for Voyager.
It takes more than four hours for a command to travel from Earth to the spacecraft, so mission controllers have to know exactly where to point their camera. Neptune rotates in a 16-hour “day,” and they cannot photograph targets on its surface unless they know precisely when and where they will be.
That has put Caltech atmospheric scientist Andrew Ingersoll on the spot. He was charged with predicting Neptune’s weather so accurately that Voyager’s cameras could be trained on atmospheric structures as they rotated past the spacecraft’s “picture window.”
On Tuesday, Voyager’s computer was told where Ingersoll had predicted the Great Dark Spot would be. He hit it on the mark.
“He’s still on the payroll,” Smith quipped.
As Voyager sped toward a Friday morning encounter with Neptune’s largest moon, Triton, it sent back more photos of the satellite’s mottled surface. Much of the surface appears pinkish, apparently because of “radiation damage to the methane ice” on the moon’s surface, Smith said.
The most recent images suggest that Triton will have an interesting surface, but Smith cautioned that little can be said about Triton just yet.
“We won’t really understand Triton’s surface until geological structures can be resolved,” he said. “And that won’t happen until we are right on top of it.”
That will come Friday, five hours after Voyager passes 3,000 miles over Neptune’s cloud tops.
