Red Spot

Four: The Great Red Spot

Jupiter is a planet of storms. Vast cyclones rage through its atmosphere. The largest of these hurricanes is so enormous that it is visible from Earth. It was first seen in the seventeenth century, when it was named the Great Red Spot. Its swirling clouds cover an oval 7500 miles long and 1500 miles wide (12,000 x 25,000 km)—an area large enough to swallow two whole Earths. It is so large that it takes six days to make one rotation. Smaller cyclones—often looking like small white ovals when seen from space—come and go constantly. Occasionally they collide with the Great Red Spot and are absorbed by it, like debris swirling down a drain.

The Great Red Spot is a vast hurricane-like storm system. But how can a storm be so large, let alone last for four hundred years? The answer to the last question is that Jupiter does not depend on the Sun to drive its weather, as Earth does. A hurricane or cyclone on our planet derives its energy from Sun-warmed oceans. When the storm moves over cooler waters or land its source of energy is gone and it disappears. The temperature changes caused by Earth’s seasons also help to create and break up storm systems. But Jupiter does not depend on the Sun for heat. It creates its own. The gravitational compression of its vast bulk generates more energy than it receives from the Sun. As long as that energy source continues to exist there is nothing to keep a storm from stopping once it has started. Jupiter does not have any land, either, to break up storms—the planet is pretty much the same all over—nor does it have any seasons.