Images hint at seasonal changes on distant planet. (Springtime on Neptune).

Belying its location in the deep freeze of the outer solar system, Neptune is anything but dormant. It sports giant storms and near-supersonic winds. Now, images taken by the Hubble Space Telescope indicate that this frigid ball of gas, which receives only 0.1 percent as much sunlight as Earth does, even undergoes a change of seasons. If Larry A. Sromovsky of the University of Wisconsin-Madison and his colleagues are correct, it's now spring on Neptune's southern hemisphere.

Visible-light images taken by the team with Hubble in 1996, 1998, and 2002 show that a band of clouds encircling the planet's southern hemisphere has grown larger and brighter, the researchers report in the May Icarus. The findings are consistent with ground-based pictures taken since 1972 by G. Wesley Lockwood of the Lowell Observatory in Flagstaff, Ariz. Recent near-infrared observations also hint at increased cloud cover on Neptune, Sromovsky's team notes.

Seasonal changes on Neptune would occur for the same reason that they do on Earth. Because these planets' rotation axes are tilted, their northern and southern hemispheres alternately tip toward or away from the sun. The hemisphere tipped toward the sun receives more heat, which can induce increased cloud cover, Sromovsky notes.

He and his colleagues find that only Neptune's high-latitude regions, which endure the largest changes in sunlight from season to season, show brightness variations. This pattern supports the seasonal scenario for Neptune.

However, data collected from 1950 to 1966 at Lowell Observatory contradict the seasonal model, Sromovsky's team acknowledges. Those data do not show the small decrease in brightness predicted for that period by the seasonal model.

Because it takes 165 years for Neptune to circle the sun, each of four seasons on the distant planet would last for more than 40 years. The puzzle, however, is how Neptune can have seasons at all, given the feeble amount of solar energy that reaches the planet, which lies some 4.5 billion kilometers from the sun.

"Should seasonal variations be verified by future observations or by reanalysis of past observations, that will send the atmospheric dynamicists into a tizzy trying to explain how it could possibly be," notes Heidi B. Hammel of the Space Science Institute in Ridgefield, Conn. "Right now, though, this interpretation is tantalizing but not ironclad."

Sromovsky, Lockwood, and Hammel all agree that seasonal variations may only be part of the explanation for Neptune's remarkably dynamic atmosphere. Variations in the intensity of the sun during its 11-year cycle may also play a role, for example.

Recent studies by Hammel's team show hints of a seasonal variation in Uranus' brightness (SN: 1/27/01, p. 56), which may peak in 2007.

"Since Larry [Sromovsky]'s seasonal model predicts a peak in Neptune's brightness in 2020, the next few decades could be banner years for revealing the true nature of the radiation balance in these distant giant planets," Hammel says.