The distant, blue-green planet Neptune has again surprised astronomers with the emergence of a new great dark spot in the cloudy planet's Northern Hemisphere, discovered by the Hubble telescope.

Only last June, Hubble images revealed that a great dark spot in the Southern Hemisphere discovered by the Voyager 2 spacecraft during its 1989 flyby had mysteriously disappeared. The new dark spot is a near mirror image of the one found in the Southern Hemisphere. Bright, high-altitude clouds accompany the new northern dark spot. Atmospheric gases that flow up over the spot cool to form the methane-ice crystal clouds. The new spot might be a hole in Neptune's methane cloud tops, giving astronomers a peek at lower levels of the atmosphere.