Mercurial Neptune

Neptune lies nearly 3 billion miles from the sun, showing up in earthbound telescopes as a small, faint blue disk. Voyager 2 gave astronomers a closer look, but only for a few days in August 1989. So our understanding of Neptune had been shaped by a few snapshots. Researchers trying to fill in the details with the Hubble Space Telescope this past year confirmed that Neptune is no serene giant. It’s a very dynamic planet, says Heidi Hammel, an astronomer at MIT. These images, released last spring, show how Neptune’s weather changed between October 10 (top) and November 2, 1994. The pink areas are high-altitude methane-ice clouds. Voyager 2 had photographed an immense storm in the southern hemisphere, dubbed the Great Dark Spot--but by 1994 it had vanished, and a new dark spot, not visible here, had appeared in the north. (In contrast, Jupiter’s Great Red Spot has persisted since at least 1840.) No one knows what drives the weather changes on Neptune, but the feeble sunlight out there may not be enough. Neptune’s own internal heat source--heat generated by the gravitational compression of gases--may be the weather engine.