Neptune features the solar system’s fastest winds, exceeding 1,200 miles per hour, despite receiving 900 times less sunlight than Earth. Internal heat escaping from its deep interior provides roughly 162 percent of its absorbed solar power, driving atmospheric circulation that scientists have studied since NASA’s 1989 Voyager 2 flyby.
Supersonic Winds and Dim Twilight on Neptune
At Neptune’s average distance of 4.5 billion kilometres from the Sun, noon resembles a dim terrestrial twilight. Sunlight is about 900 times weaker than at Earth, yet bright methane-ice clouds can race around the planet in winds exceeding 2,000 kilometres per hour. The apparent contradiction makes Neptune one of planetary science’s most compelling weather laboratories. On Earth, sunlight powers most weather. Neptune receives so little solar energy that another source must matter, and the planet supplies one from below: it radiates substantially more energy than it absorbs from the Sun. Internal heat is the leading part of the answer, but it is not a complete explanation. Rapid rotation, convection, atmospheric waves and low friction appear to organise the available energy into powerful east-west jets. Exactly how the pieces work together remains unsettled. The sunlight is weak, but not moonlight-weak Neptune orbits roughly 30 times farther from the Sun than Earth does. Because light intensity falls with the square of distance, the planet receives about one nine-hundredth as much sunlight. NASA describes high noon there as comparable to dim twilight on Earth. That is much brighter than a full-moon night, despite a common comparison. Full moonlight on Earth typically provides only a fraction of a lux, while dividing terrestrial daylight by 900 still leaves illumination of roughly a hundred lux, depending on atmospheric conditions and the Sun’s position. The accurate comparison is that Neptunian daylight is hundreds of times brighter than full moonlight, but around 900 times dimmer than daylight near Earth. The distance is nevertheless extreme. According to NASA’s Neptune facts, sunlight takes about four hours to reach the planet. Neptune completes one orbit in 165 Earth years, so each season lasts more than four decades. Voyager found a world moving at 2,000 kilometres per hour Neptune has no solid surface where an anemometer could stand.
By comparison, the strongest winds ever recorded on Earth top out around 250 miles per hour. What makes the number strange is where it happens. Neptune sits about 30 times farther from the Sun than Earth does, and the sunlight that reaches it is roughly 900 times fainter than the light warming our own planet. High noon on Neptune would look to us like dim twilight. There is almost no solar energy out there to stir the air, and yet Neptune, in NASA’s words, is our solar system’s windiest world
. The one time anyone got close Nearly everything we know about Neptune’s weather comes from a single encounter. In 1989, NASA’s Voyager 2 became the first and only spacecraft to fly past Neptune, and its cameras caught bright white clouds of frozen methane racing around the planet, along with two enormous storms. One of them, an oval large enough to hold the entire Earth, was named the Great Dark Spot. It has since vanished, and new storms have appeared elsewhere, so the spot was not a permanent fixture like Jupiter’s Great Red Spot but a passing system. The clouds Voyager tracked were the markers scientists used to time the winds. Watch a bright patch of methane ice move from one image to the next, measure how far it traveled and how long it took, and you have a wind speed. Those measurements are what put Neptune at the top of the solar system’s wind chart. Neptune has never been easy to study. It is the only planet in the solar system invisible to the naked eye, and it was the first found through calculation rather than a telescope search. Astronomers in 1846 used the unexplained tug on Uranus’s orbit to predict where an eighth planet must lie, and located Neptune close to that spot on the first night they looked. A single spacecraft pass, more than three decades ago, is still the closest anyone has come. Faster than sound, on almost no fuel NASA describes Neptune’s winds as supersonic. At high altitudes the speeds can exceed 1,100 miles per hour, about 1.5 times the speed of sound. The winds do not all blow the same way; they move in broad east-west bands at different speeds, which is why the planet’s cloud features seem to slide past one another. The winds are visible only because of what they carry. The bright streaks Voyager photographed are clouds of methane ice crystals, and it is those features, blown along a
The Legacy of Voyager 2 and Atmospheric Tracking
Wind speeds are inferred by following clouds and storm features as they move relative to the planet’s underlying rotation. During its 1989 flyby, Voyager 2 revealed bands, bright clouds and the Earth-sized Great Dark Spot. Near that storm, the spacecraft measured winds approaching 2,000 kilometres per hour, according to NASA’s Voyager fact sheet. Later observations by the Hubble Space Telescope and ground-based observatories showed that dark vortices can appear and disappear while the major jet pattern persists. The strongest measured flow is a broad westward equatorial jet, not a single gust like one inside a terrestrial tornado. Neptune rotates once in about 16 hours, and th
Sources: Spacedaily.
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