Astronomers first spotted—and drew pictures of—Jupiter's "Great Red Spot" some 300 years ago. It's that "eye of Sauron" spot on the Southern Hemisphere. In reality, it's a very large, very ancient storm, and as the weather on Jupiter changes, so does the Spot. The Spot really is properly called "Great"; it is so far the largest known storm in our Solar System, with a diameter of 15,400 miles. In other words, just the area covered by the storm on Jupiter is almost twice the size of Earth (and about one-sixth of Jupiter’s diameter). We've been watching the spot long enough now that we can see it is definitely shrinking. What's more, as currents and weather patterns shift, we've witnessed, albeit at a very great distance, new storms and smaller spots appearing in Jupiter's images.
In May, astronomers reported that not only is the Little Red Spot, as it was named when astronomers initially noticed it in 2006, competing in terms of speed with its older sibling, there are other new spots. And the Little Red Spot is already matching the maximum wind speed of its larger sibling, at 384 mph. That's more than the winds stirred up by a Category 5 hurricane on Earth, which doesn't seem quite so startling when you realize that the Little Red Spot is almost as big as Earth. In fact, new infrared data suggests that it's actually part of a larger storm system, only parts of which are visible by optical telescopes.
You might remember that when the Little Red Spot was first noticed in 2005, it was white; it merged with three smaller storms and shortly thereafter, in late 2005, it too was the deep red we associate with the Great Red Spot. The new spot also began as a white oval shaped storm. This suggests that scientists are correct in hypothesizing that the more powerful storms are actually scooping up material from below Jupiter's cloud level, exposing it to ultraviolet radiation, which changes the color of the material. If you look at the first image above, you'll notice younger, smaller storms below the two larger deep red spots, storms that are still white.
Now, with ten years of velocity data about the storms on Jupiter, collected from 1991 to 20006, researchers Xylar Asay-Davis, Phil Marcus, Mike Wong and Imke de Pader at the University of California at Berkeley, have determined that the Great Red Spot's underlying storm is shrinking, but it's not at all slowing down. It's still managing to spawn wind storms that average over 300 mph. Scientists suspect that the extremely violent storms from 2005 to 2007 may have signaled an extreme climate change on Jupiter.
The image above, taken by Cassini in 2000, gives an idea of just how turbulent weather patterns on Jupiter are, and how much they affect the cloud In this collage of Hubble images taken between 1992 and 1999 you can see the changesin the Great Spot, and in the weather systems near it. Galileo observed the Spot in 1610. Later, English astronomer Robert Hooke, observed the Spot in 1664. Giovanni Cassini observed it again in 1615. Voyager imaged the Great Red Spot on February 25, 1979; and in the image embedded above from the Cassini flyby in 2000, you can see signs of just how very turbulent the storms can be.