Neptune: Cold Planet, Hot South Pole

Sept. 21, 2007 — Though it receives only a tiny fraction of sunlight, the planet Neptune puts what it gets to good use, creating a seasonal hot spot at its polar region that is venting methane gas into space.

The observations, reported in this week's issue of Astronomy and Astrophysics, may explain why Neptune — 30 times further from the sun than Earth — has the most wicked winds in the solar system. At that distance, the planet receives about one-thousandth the amount of sunlight as Earth.

"The wind blows there at more than 2,000 kilometers per hour (240 mph)" said lead author Glenn Orton, with NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The stiff winds are caused by a seemingly slight temperature variation between Neptune's south pole, which is tilted slightly toward the sun in summer, and the rest of the planet. Orton and his colleagues determined the south pole is 10 degrees Celsius (18 degrees Fahrenheit) warmer than the planet's average temperature of minus 200 degrees C (minus 392 degrees F).

That's enough to turn frozen methane in Neptune's atmosphere into a gas, which is leaking into space, the researchers said.

The temperature difference builds up over the planet's prolonged seasons. It takes Neptune 165 Earth-years to orbit the sun, so it has been summertime on Neptune's southern hemisphere for about 40 years now.

"A lot of solar energy input during that time can make big temperature differences between the regions in continual sunlight and those with day-night variations," Orton said. "This is a likely factor in Neptune having the strongest winds of any planet in the solar system."

The methane also gives Neptune its distinctive blue hue as the gas absorbs red light from the sun and reflects blue light from the sun back into space.

Astronomers figure that in another 80 years or so the methane leak will be stemming from Neptune's north pole.

The findings were made with infrared-sensitive cameras, filters and other instruments on Europe's Very Large Telescope in Chile.

The team also discovered high-latitude hot spots in Neptune's atmosphere that are unlike any feature found at any other planets. Scientists theorize that pockets of gas are welling up from deeper in the atmosphere.

Unlike Jupiter and Saturn, which have been studied for years by dedicated probes in orbit, Neptune has been visited only once during a 1989 flyby of NASA's Voyager 2 spacecraft.

Nevertheless, researchers believe Neptune may have the most active atmosphere of all the gas giant planets, despite getting less energy from the sun.

NASA's Jet Propulsion Laboratory

The Very large Telescope Project

NASA's Neptune page

A Neptune photo gallery