Fierce Storms in Neptune’s Atmosphere Baffle Scientists
Scientists have discovered a “Rogue’s Gallery” of storm centers in the atmosphere of Neptune, including hurricane-like storms that are so intense they defy explanation.
Packing winds of up to 400 m.p.h., the fierce storms appear as dark spots on photos that are being sent back to Earth from the Voyager spacecraft, which is zipping toward a close encounter with the distant planet Thursday night. Scientists had expected Neptune’s atmosphere to be more like that of Uranus, a bland, gray ball in the dark sky. Instead, it has mysterious storms and high level clouds that whip across the planet’s upper atmosphere so fast that scientists have named them “scooters.”
“It’s surprising to see all this weather activity,” said Caltech atmospheric scientist Andrew Ingersoll, a member of the Voyager’s imaging team.
Scientists are at a loss to explain it because Neptune is so far out in the solar system that it receives only about 1/1,000th as much energy from the sun as does Earth. Because solar radiation drives the Earth’s atmospheric system, it was reasonable to expect Neptune’s atmosphere to be at least as bland as that of Uranus.
“The sun is, after all, what powers the weather,” Ingersoll said Sunday as scientists and reporters from around the world converged on the Jet Propulsion Laboratory in Pasadena for this week’s astronomical extravaganza.
Neptune also has an interior source of heat that is about equal to that which it gets from the sun. But even when added together, Ingersoll said, the two energy sources would not seem to be enough to explain the phenomenon.
“Any way you slice it up it’s very little energy to drive the kind of weather we are seeing at Neptune,” he said.
Scientists have been keeping tabs on four “dark spots” for several weeks, ever since the images from Voyager became sharp enough to detect them. The largest is called the Great Dark Spot because it is similar to the Great Red Spot on Jupiter, although less dynamic.
Jupiter’s “Great Red Spot eats other spots,” Ingersoll said, providing the fuel to keep the storm system going. If Neptune’s Great Dark Spot is also dining on its lessers, that has not been observed so far--in fact, Voyager images show the giant spot losing material in the form of long streamers.
“We haven’t seen anything sucked into it yet,” Ingersoll said, raising the question of where the spot is getting the fuel to keep it running.
No one, at this point, knows.
“We have a Rogue’s Gallery” of atmospheric features, Ingersoll said, citing also a “bright polar feature” near the south pole, and broad bands of different shades of blue extending around the planet below the equator. The color is caused by methane in Neptune’s atmosphere.
“We don’t understand the weather well enough to explain the bands,” Ingersoll said. “Maybe they are related to wind currents. It’s just beyond us right now.”
Ingersoll expects at least some of those questions to be answered in the days ahead as Voyager closes in on Neptune, which for the moment is the most distant known planet in the solar system. Pluto, which normally holds that rank, travels in an oblong orbit and for the next few years will remain inside of Neptune’s orbit.
This morning, engineers at the Jet Propulsion Laboratory will make the last major course change planned for Voyager’s trajectory. If successful, it will send Voyager to within 3,000 miles of Neptune’s cloud tops, the closest approach it has made to any planet.
Hopes for a Clear View
Five hours later, the 12-year-old Voyager will pass Neptune’s largest moon, Triton, which is also generating a little excitement these days. Scientists are hoping that Triton’s thin atmosphere will be clear enough for them to see its surface, and there was some reason Sunday to believe that they will be in luck.
Voyager images of Triton already show some features, but it has not been possible to say for certain whether they are on the surface or in the satellite’s atmosphere. One way to determine that is to track their movements on consecutive images and see if they correspond to what would be expected if the features are fixed on the surface as opposed to clouds in the atmosphere.
Some scientists were reasonably convinced Sunday that at least some of the features are indeed on the surface, holding out the promise of a truly spectacular meeting with Triton Friday morning.
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