Springtime on Neptune:

By Alan Boyles

The clouds are getting brighter down south on Neptune — and that could signal a change of seasons on the eighth rock from the sun, astronomers say. The observations, made over the course of six years by the Hubble Space Telescope, serve as the best evidence yet that the big blue planet goes through a cycle of weather variations like those seen on Earth, but on a much longer time scale. The research is published in the May issue of the journal Icarus. “We’re at the end of southern spring, and the end of northern fall,” said Lawrence Sromovsky, a senior scientist at the Space Science and Engineering Center at the University of Wisconsin at Madison and a leading authority on Neptune’s atmosphere. That would be analogous to, say, November on Earth. In Neptune’s case, each of the four seasons would last not just three Earth months, but 40 Earth years. That’s why it’s been much more difficult to track Neptune’s seasonal variations. “Its year is a little less than 165 of our years, and that’s quite a bit longer than the careers of most of our observers,” Sromovsky said. Using the Hubble data, Sromovsky and his colleagues developed a climate model that correlated variations in cloud brightness with how the atmosphere should respond to changes in sunlight exposure, due to the tilt of Neptune’s axis. It’s the tilt that determines the seasons, on Neptune as on Earth. The researchers found that the weather variations tended to match the model — with Neptune’s thermal response lagging behind the peak solar exposure by about 30 years. In comparison, Earth’s seasonal lag at midlatitudes is roughly one month, which explains why Northern Hemisphere summers are hotter in July and August even though the summer solstice is in June. It’s only natural that Neptune would have seasons, Sromovsky said. “Any planet that has a tilted spin axis will have some sort of seasonal input of solar energy and some sort of response,” he noted. Mars, for example, has clearly defined seasons that last twice as long as Earth’s, while Jupiter has very little seasonal variation because its axis doesn’t have much of a tilt. There are many more mysteries surrounding planetary weather, Sromovsky said. For example, what’s the weather like on southern Neptune? “We clearly see that there is more cloudiness,” he said, “but what that means in terms of the usual attributes of storminess — high wind shear, precipitation, turbulence — is not so clear.” Some astronomers say Neptune’s weather could be violent, with winds gusting up to 900 mph. On the other hand, passing through the cloud bank “could be a fairly mild event if you were riding along in an airplane in Neptune’s atmosphere — it might be like a gentle fog,” Sromovsky said. Confirming and fleshing out Neptune’s weather patterns will require more work with the climate models as well as more data, Sromovsky said. “The most convincing result would be a longer timespan of observations,” he said. “It’s pretty easy to get fooled into thinking you’ve got an explanation when you don’t have the full time period of the observation.” Until recently, Hubble and the Voyager spacecraft’s flyby in 1989 provided the only glimpses at Neptune’s cloud cover. But now astronomers can draw upon a wider set of observational tools. “Now, large telescopes on the ground, working in the near-infrared with adaptive optics, are giving us images of Neptune as well,” Sromovsky said. So stay tuned for updated weather reports from the edges of the solar system. In addition to Sromovsky, the authors of the Icarus paper include Patrick Fry and Sanjay Limaye, both of the UW-Madison’s Space Science and Engineering Center; and Kevin Baines of NASA’s Jet Propulsion Laboratory. You can find more about the Neptune research, including some cool videos, from the Hubble Site Web portal as well as the Space Science and Engineering Center
Author:Alan Boyles Cosmic Log