Jupiter's Great Red Spot: A Storm Bigger Than Earth

What you would actually see

Through even a modest telescope, Jupiter shows a series of tan, cream, and rust-colored bands running parallel to its equator. Tucked into the South Tropical Zone, bordering the southern edge of the darker South Equatorial Belt around 22° south latitude, sits an oval the color of salmon, brick, or pale orange depending on the year. That is the Great Red Spot. It is not a mark on a surface — Jupiter has no solid surface — but a coherent, spinning column of atmosphere that has held together for generations.

The Spot is an anticyclone: a high-pressure region where gas piles up and spins counterclockwise (as seen from the north), completing one full rotation about every six Earth days. This is the opposite of a terrestrial hurricane, which is a low-pressure cyclone. Within the Spot, gas upwells at the cold central cloud tops and diverges outward at altitude — the reverse of a hurricane's central inflow — and the whole feature slowly drifts in longitude relative to the clouds around it, its drift rate varying over the decades while Jupiter itself spins beneath it every 9 hours and 56 minutes — the fastest rotation of any planet in the Solar System.

Color varies noticeably over the years. Sometimes the Spot is a vivid orange-red; other times it fades to a washed-out salmon and nearly blends into the belt. It has never been the fire-engine red of popular illustration. What stays constant is the shape: a wide oval, roughly as tall as Earth in the north-south direction and, until recently, considerably wider than Earth east-to-west.

Bigger than a planet — the numbers

Scale is the Great Red Spot's headline feature. Measurements by the Hubble Space Telescope between December 2023 and March 2024 put its width at about 14,750 km. Earth's mean diameter is 12,742 km, so our whole planet would fit inside the Spot's east-west span with room to spare. A century ago it was far larger still — wide enough, astronomers estimated, that two or three Earths could have lined up across it.

The winds are equally staggering. The high-speed ring encircling the Spot blows at more than 430 km/h (about 120 m/s). For comparison, the strongest tornado winds ever measured on Earth peaked near 480 km/h, but those last for seconds over a few hundred meters; the Great Red Spot sustains its gale continuously around a rim tens of thousands of kilometers long.

• Width (2024): ≈ 14,750 km — larger than Earth
• Height (N–S): ≈ 12,000–13,000 km
• Rim winds: > 430 km/h
• Rotation period: ≈ 6 Earth days
• Depth: ≈ 300–500 km below the cloud tops

That depth figure is one of the most striking modern discoveries. When NASA's Juno spacecraft flew directly over the Spot in 2017, its microwave radiometer peered beneath the clouds, and later gravity measurements — published in Science in 2021 — showed the storm's roots reach roughly 300 to 500 km deep. That is shallow compared to the Spot's enormous horizontal width, yet far deeper than Earth's entire weather layer, which is confined to the lowest ~15 km of atmosphere.

What powers a 190-year storm

On Earth, hurricanes die within days because they run on warm ocean water; once a storm moves over land or cool seas, its fuel is cut off. Jupiter offers no such off-switch. There is no land to create friction and no cold surface to starve the storm, so a well-organized vortex can persist almost indefinitely if it can keep gathering energy.

Several factors conspire to keep the Great Red Spot alive:

• Internal heat. Jupiter radiates about 1.6 times more energy than it receives from the Sun, leftover heat from its formation and slow gravitational contraction. This heat drives convection from below, feeding the atmosphere with energy independent of sunlight.
• Jet-stream shear. The Spot is wedged between two powerful, oppositely-directed jet streams — an eastward jet to its south and a westward jet to its north. This velocity shear spins the vortex up like a ball bearing caught between two moving surfaces.
• Vortex mergers. Over its lifetime the Spot has swallowed smaller storms that drifted into it, absorbing their energy and momentum. Astronomers have watched it consume lesser spots in real time.

A frozen or solid boundary would drain a storm's energy through friction. Jupiter's fluid, boundary-free atmosphere lets the vortex conserve its spin remarkably well. The result is a storm whose lifetime is measured not in days but in centuries.

Why it is red — and why nobody is certain

Here is a genuine, unresolved puzzle: we do not fully know why the Great Red Spot is red. Jupiter's clouds are made mostly of ammonia, ammonium hydrosulfide, and water — none of which are colored. Something else is producing the ruddy tint, and the leading explanation is a class of chemical reactions rather than a single pigment.

The favored idea is that chromophores — light-absorbing compounds — form when sunlight breaks apart molecules high in the storm. Ultraviolet radiation is thought to split ammonia and acetylene, and the fragments recombine into complex, reddish organic-like compounds and possibly sulfur- or phosphorus-bearing molecules. Because the Great Red Spot towers higher than the surrounding clouds, its top layers are exposed to more direct sunlight, so this photochemical "sunburn" is strongest there — which would explain why the Spot is redder than the pale bands around it.

The evidence is suggestive but not conclusive. The exact chemistry is still debated, and the fact that the Spot's color fades and intensifies over the years shows the process is dynamic, not a fixed dye. A common misconception is that the color reflects temperature, like a red-hot coal — it does not. The Spot's cloud tops are actually cold, around −160 °C, though Juno and ground-based data show the atmosphere above it is unusually warm, hinting the storm pumps energy upward in ways still being untangled.

A shrinking giant

For all its permanence, the Great Red Spot is changing before our eyes. When astronomers began measuring it carefully in the late 19th century, it stretched roughly 40,000 km east-to-west. Today it is barely a third of that. It has been shrinking for more than a hundred years, and the contraction sped up notably after about 2012, at which point it was losing on the order of hundreds of kilometers of width per year. As it shrinks, its shape has grown rounder and more nearly circular, and its color has, at times, deepened.

The story got stranger in 2024. A team led by Amy Simon at NASA Goddard, using Hubble, found that the Spot does not simply shrink steadily — it also oscillates in width on roughly a 90-day cycle, breathing in and out like a squeezed stress ball. The cause of this rhythmic pulsing is not yet understood, and it was invisible until Hubble's cadence of high-resolution imaging revealed it.

What happens next is genuinely uncertain. Some researchers expect the shrinkage to continue until the Spot stabilizes at a smaller size or even breaks apart; others think it may persist for a long time yet. It is worth stating plainly: nobody knows whether the Great Red Spot will keep shrinking, settle down, or eventually disappear. We are watching, in real time, a storm older than the light bulb possibly approach the end of its life — or simply enter a new phase.

Four centuries of watching

The Great Red Spot's history is tangled with a real observational mystery. In 1665, the Italian-French astronomer Giovanni Domenico Cassini recorded a dark oval on Jupiter he called the "Permanent Spot," and he and others tracked it until about 1713. Robert Hooke may have seen a spot even earlier, in 1664, though his was likely a different, more northern feature. Then the record goes quiet: for roughly 118 years, no clear observations of a comparable spot survive.

The first drawing that unambiguously matches today's Great Red Spot was made by the German astronomer Samuel Heinrich Schwabe on 5 September 1831. The Spot became prominent and famous around 1879, and it has been under essentially continuous observation ever since. Recent studies suggest Cassini's Permanent Spot most likely faded away and the current Spot formed later, meaning the storm we watch today may not be the same one Cassini saw — though whether the earlier spot dissipated, reformed, or was simply lost to poor records remains debated.

The modern era of understanding came from spacecraft. NASA's Voyager 1 and 2 returned the first close-up portraits in 1979, revealing the Spot's turbulent structure and the way it rolls between jet streams. The Galileo orbiter studied it through the 1990s, and Juno, in Jupiter orbit since 2016, delivered the depth and gravity data that finally probed beneath the clouds. Today, Hubble images Jupiter annually under the Outer Planet Atmospheres Legacy program, and the James Webb Space Telescope adds infrared views — so the oldest known storm in the Solar System is also one of the most closely monitored.

The Great Red Spot vs. Earth's most powerful storms

Property	Great Red Spot	Earth hurricane (Cat 5)
Diameter	≈ 14,750 km	≈ 500–1,500 km
Peak winds	> 430 km/h	≈ 250–320 km/h
Lifetime	≥ 190 years (still active)	1–4 weeks
Rotation sense	Anticyclonic (high pressure)	Cyclonic (low pressure)
Vertical depth	≈ 300–500 km	≈ 15 km (troposphere)
Energy source	Internal heat + jet-stream shear	Warm ocean surface