Source of Neptune's Hot Spots Found

Thermal images show temperatures near the top of Neptune's troposphere (upper left image), with the hottest temperatures found at the south pole (see graphic, upper right). The lower two images show temperatures at higher altitudes in Neptune's stratosphere, or lower atmosphere. Credit: VLT/ESO/NASA/JPL/Paris Observatory

This picture of Neptune was taken by Voyager 2 and shows the Great Dark Spot, an Earth-size storm system. Credit: NASA/JPL

New measurements reveal Neptune's south pole is warmer than the rest of the planet, as expected. But it's still frigid.

The polar region's relative warmth could provide a route for methane gas to escape from the deep atmosphere, explaining mysterious hot spots that have been reported there, according to the study scientists.

The first temperature maps of the blue planet's lower atmosphere, detailed in the journal Astronomy and Astrophysics, suggest temperatures at the south pole are elevated by about 18 degrees Fahrenheit (10 degrees Celsius) compared with the rest of the planet, for which the average temperature is an icy -328 degrees F (-200 degrees C).

"The temperatures are so high that methane gas, which should be frozen out in the upper part of Neptune's atmosphere (the stratosphere), can leak out through this region," said lead author Glenn Orton of the Jet Propulsion Laboratory in California. "This solves a long-standing problem of identifying the source of Neptune's high stratospheric methane abundances."

Orton and his colleagues used ESO's Very Large Telescope to map the temperature variations.

Located about 30 times farther away from the sun than Earth is, Neptune receives about a tenth of a percent of the sunlight reaching our planet. Over time, however, the tiny bit of sunlight significantly affects Neptune's atmosphere.

The warmer climes detailed in the new report are consistent with the fact that Neptune's Southern Hemisphere, because of the planet's tilt and orbit, has been bathed for about 40 years in the sparse sunrays that can reach the farthest planet in our solar system.

A year on Neptune lasts about 165 Earth years, making summers last for 40 years rather than months. Now that the Southern Hemisphere's summer on Neptune is coming to a close, Orton and his colleagues predict that as the north pole turns sunward, an abundance of methane will leak out of that pole once it becomes warmer in about 80 years.

"Neptune's south pole is currently tilted toward the sun, just like the Earth's south pole is tilted toward the sun during summer in the Southern Hemisphere," Orton said. "But on Neptune the Antarctic summer lasts 40 years instead of a few months, and 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."

While not a major constituent of Neptune's atmosphere, methane is responsible for the planet's blue hue. When methane gas in the upper atmosphere absorbs red light from the sun, it reflects blue light back into space.