Predicting Neptune : Experts Play Guessing Game on What Voyager Finds
Who says scientists are cautious folk?
A couple of years ago Prof. Alexander Dessler of Rice University suggested that his fellow planetologists lay it on the line and predict what the spacecraft Voyager would find when it offered them their first close look at Neptune. That encounter is coming next week.
Dessler, then editor of Geophysical Review Letters and normally a congenial fellow not given to mischievousness, thought it would be “fun” to put his colleagues on the spot.
It was a bold suggestion because if there is one thing that the Voyager has revealed during its 12-year odyssey through the solar system, it is that the outer planets are full of surprises.
The response to Dessler’s request has been rather surprising. Scientists across the country have laid out their predictions in Geophysical Review Letters, which is the official journal of the American Geophysical Union.
By next Thursday evening, the one-ton robot will reveal who was right and who was wrong when it zips over the cloud tops of Neptune, makes a 45-degree turn with the help of the planet’s gravity, and then slips past Neptune’s oddball moon, Triton. It will be Voyager’s fourth and final encounter with a planet.
The close encounter promises to provide enough information to rewrite the textbooks, because many of the theories about the distant planet will be disproved. That conclusion is based on the track record Voyager compiled at Jupiter, Saturn and Uranus, where many expectations turned out to be wrong.
Here are some of the predictions for what Voyager will find at Neptune.
Prediction: Neptune has a polar ring.
The answer to this one may dictate Voyager’s very survival. Anthony R. Dobrovolskis and Thomas Y. Steiman of NASA’s Ames Research Center and Nichole J. Borderies of the Jet Propulsion Laboratory speculated in a report in the August issue of Geophysical Research Letters that Neptune may have a ring of debris orbiting its polar regions. At other planets, most notably Saturn, rings orbit around the equator.
But Neptune is odd in that its largest moon, Triton, travels in a highly unusual orbit that carries it far above and below Neptune’s equator.
That peculiar orbit, the scientists have concluded, might cause rings to form far from the planet’s equator because Triton’s gravity would tend to “shepherd” material into polar rings.
“Voyager 2 stands a significant chance of encountering a polar ring,” the three scientists wrote. In non-technical terms, that means hold onto your seats.
Traveling at 17 miles per second, if Voyager smashes into even a few dust-sized particles, the impact could wipe out the venerable spacecraft. The picture show would end even before Voyager reaches Triton five hours after the Neptune flyby.
Dessler considers that possibility “scary.”
“This was the most spectacular prediction,” Dessler said. Most scientists seem to think the prediction is not likely to come true, and by the end of next week, three bold souls will be regarded as either prophets or--fortunately for Voyager--mistaken.
Prediction: Triton has a very thin atmosphere.
The journal has published more than 20 papers over the last few months in response to Dessler’s challenge, and the results indicate there is about as much interest in Triton as there is in Neptune. Triton is the only large moon in the solar system that orbits in the opposite direction of its planet’s rotation, leading many scientists to believe that it did not form with Neptune but was instead a passing body that became trapped in the large planet’s gravitational field.
About the only thing that is known about Triton is that it has an atmosphere. S. Alan Stern of the University of Colorado speculated in a response to Dessler’s challenge that Triton’s atmosphere is extremely thin. He believes Triton’s atmospheric pressure is probably no more than 100 millibars, compared to the Earth’s sea level pressure of slightly over 1,000 millibars. But he also said it could be as low as 1/100th of a millibar.
“That’s a range of 10,000,” Dessler observed. “Everybody agrees it’s got to be in there somewhere.” Thus Stern is considered pretty safe in his prediction.
Prediction: Neptune will have lightning in its atmosphere.
Some of the snapshots from Neptune should look familiar. Scientists expect the planet’s heavy atmosphere to have lightning. That is because convection in a planet’s atmosphere, like that which occurs in boiling water, is what causes lightning.
W. J. Borucki of Ames Research Center predicts that Neptune will, indeed, have lightning.
Voyager should record approximately 60 lightning events during its flyby, Borucki speculated. That is only about 1/19th as much activity as seen at Jupiter, another gaseous giant.
Prediction: Neptune has a strong magnetic field.
Since all of this was Dessler’s idea, it is worth noting that he is a bit out on a limb. He is the co-author of two papers predicting what Voyager will find at Neptune.
Dessler’s predictions concern Neptune’s magnetosphere, the huge area around the planet that is ruled by Neptune’s magnetic field.
Neptune’s dynamic cloud activity suggests that the planet is heated from within, making it--like the Earth--a dynamo.
“So it ought to have a strong magnetic field,” Dessler said.
However, Voyager has detected no radio waves from Neptune, and “that is really surprising” because a strong magnetic field would be expected to produce radio waves, he added.
The strength of the magnetic field, and the absence of radio waves, has led Dessler to conclude that Neptune’s magnetosphere is surprisingly quite “like it’s asleep.” That would make it an extremely unusual planet.
Prediction: Neptune will have a very weak radiation belt.
If so, “there won’t be any bright auroras, nothing of any consequence.” On Earth, auroras produce northern and southern lights, so Dessler does not expect to find them when Voyager zips over Neptune’s north pole.
Prediction: Neptune will have strong “northern lights.”
Andrew F. Cheng of Johns Hopkins University couldn’t disagree with Dessler more. He has predicted that Neptune’s magnetosphere is much like the Earth’s, producing strong auroras and energetic radiation belts.
Other predictions:
John A. Stansberry of the University of Arizona believes the northern half of Triton’s surface will be covered with a “frost” of nitrogen and methane, and will be very bright. Triton’s southern equatorial region will be banded by ancient organic compounds, thus appearing dark red.
W. Reid Thompson of Cornell University believes Triton may have a hazy atmosphere a little like Southern California’s smog, and the satellite’s icy surface may be yellowish in color.
Some scientists have predicted that Triton will have lakes of liquid nitrogen; others believe the nitrogen will be frozen. Some suggest Triton has a rugged, irregular surface. Others think it is more likely that the surface is fairly smooth because Neptune’s gravitational field would have created tidal forces in Triton due to the moon’s highly irregular orbit. The friction from tidal forces would have produced enough heat to keep the surface of Triton molten during its early history, thus erasing most impact craters.
Any time now, Voyager will tell who is right.
WHAT ELSE WILL VOYAGER DISCOVER? JUPITER: Motions of atmosphere Faint rings Geology of major moons, including volcanoes on one Three new moons SATURN: Haze in atmosphere “Spokes” in rings Geology of major moons “Braided” ring Three new moons URANUS: Haze in atmosphere Tilted magnetic field two new rings Geology of moons Ten new moons NEPTUNE: To date: Large spot in atmosphere Four new moons Sources: Jet Propulsion Laboratory, Sky and Telescope magazine; The Encyclopedia of U.S. Spacecraft.
