Definition
The Great Red Spot is a giant red high-pressure storm that has been swirling in Jupiter's southern hemisphere for nearly 200 years. It is bigger than Earth. Jupiter has no solid ground, and its strong jet streams help too, so the storm has kept its shape for a very long time.
A Giant Red Eye on Jupiter's Surface
Think of the strong whirlpool that forms when you pull the plug on a full bathtub. On Jupiter, a whirlpool like that stretches out as wide as a planet. Look at Jupiter through a telescope, and you'll see a big red spot set sharply near the southern half. That is the Great Red Spot.
This whirlpool is almost too big to imagine. At its peak, it was wide enough to fit three Earths side by side. Even after shrinking, it is still large enough to swallow a whole Earth with room to spare. Along its edge, fierce storm winds of 400 to 600 kilometers per hour (about 250 to 370 mph) blow without a break.
Even the strongest hurricanes on Earth lose power and fall apart after a few days or weeks. The Great Red Spot, though, has kept spinning for nearly 200 years without stopping, counting from around the 1830s, when its present form began to be recorded. Scientists disagree about whether the red spot seen on Jupiter in the 17th century is the same storm.
Why Hasn't It Disappeared in Nearly 200 Years?
A hurricane on Earth grows by drawing huge amounts of energy from water vapor over the ocean. But the moment it hits land, it weakens fast. Its fuel, the water vapor from the sea, gets cut off. And friction with the bumpy ground steals its spinning energy.
Jupiter, though, is a gas planet with no ground to stand on. There are no continents or rocks at all at the bottom of the whirlpool to create friction and stop the spin. The storm never trips over a floor, so its energy is preserved.
On top of that, Jupiter has fast-moving air currents (jet streams) that flow in opposite directions along its stripes. These currents pass above and below the Great Red Spot. They push the storm from both sides, like rubbing a spinning top between your two palms, and keep feeding it spinning energy.
A Little More Precisely
To be a little more precise, the Great Red Spot is not a low-pressure system where air gathers toward the center. It is a high-pressure vortex, with higher pressure than its surroundings. Air rises in the center and gets pushed outward, and the whole thing turns in a huge counterclockwise spin in Jupiter's southern hemisphere.
As for why it looks red, chemical reactions in the atmosphere are the leading explanation. Ammonia and hydrogen sulfide compounds from deep in the atmosphere ride strong updrafts up to high altitudes. There, sunlight's ultraviolet rays break them apart and turn them into reddish and orange-tinted material.
Recent observations by space probes and telescopes show that the Great Red Spot has shrunk since a hundred years ago, and it is slowly changing from an oval toward a circle. It has gotten narrower from side to side, but it is still thick from top to bottom. How this legendary giant storm will evolve is an exciting question for modern planetary science.
Common misconceptions
The Great Red Spot is a giant red rocky mountain or patch of land rising from Jupiter's surface.
Jupiter is a gas planet with no solid surface at all. The Great Red Spot is not land. It is a huge whirlpool of gas that has been spinning in the atmosphere for nearly 200 years.
Where you meet it
The Juno spacecraft came within 9,000 km above the Great Red Spot and took images that revealed the deep 3D structure of its swirling red clouds.
Amateur astronomers can look at Jupiter through a small telescope and spot the Great Red Spot set among the stripes for themselves.
In one sentence
The Great Red Spot is a giant high-pressure storm on Jupiter that has spun for nearly 200 years because there is no hard ground to slow it with friction and nearby jet streams keep refilling its energy.