Even as the Cassini spacecraft reports back from up close to Saturn, astronomers continue to learn about the ringed planet from telescopes on Earth. New results from the W. M. Keck Observatory in Hawaii, for example, have revealed a hot spot near Saturn's south pole--the first warm polar cap discovered in the solar system.

Sunlight has bathed Saturn's southern hemisphere for the past 15 years without a break, so it's no surprise that the environs are slightly warm. But researchers at NASA's Jet Propulsion Laboratory were startled to see just how hot the south pole is, with a stratospheric temperature of 151 kelvins. "There is nothing like this compact warm cap in Earth's atmosphere," study co-author Glenn S. Orton says. "This phenomenon on Saturn is longer-lived because we've been seeing hints of it in our data for at least two years." In addition, there is a distinct boundary in temperature found 30 degrees north of the planet's tip at which the stratospheric temperature changes from 146 kelvins to 150 kelvins. The image above is a mosaic of 35 individual exposures taken using Keck's infrared radiation camera. (The black square results from incomplete mosaic coverage of that ring section during the observing sequence.)

What is causing Saturn's hot spot remains unclear, though particulates that absorb sunlight and are trapped in the planet's upper atmosphere could be to blame. Future observations by the infrared spectrometer on board Cassini should help scientists piece together the puzzle. "One of the obvious questions is whether Saturn's north pole is abnormally cold and whether a cold polar vortex has been established there," Orton says. "That's something we can't see from Earth, and Cassini's instruments will be in a unique position to observe it."