Extraterrestrial Superstorms

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Earth has its share of monster storms. But even our most powerful hurricanes are a breeze compared to the great planet-sized tempests of the gas giants.

The great vortices of the Jovian planets are true storms, analogous in many ways to Earth's hurricanes. But there are, of course, some differences. For example, these storms are as big as entire planets. The largest and oldest storm in the solar system is Jupiter's Great Red Spot, stretching an incredible 2 to 3 times the diameter of the planet Earth. Meanwhile, the fastest winds ever measured, clocking 1,500 miles per hour, once raged in Neptune's Great Dark Spot. Saturn's Polar Vortex is a 20,000-mile-wide monster shaped like a hexagon. Even plain-looking Uranus hides USA-sized hurricanes below its methane haze. There are many unsolved mysteries surrounding these epic storms. We may be close to finding some answers following the Juno spacecraft's recent flyby of Jupiter's Great Red Spot.

Before we get to the unknown, let's talk about what we do know about these storms. In fact, let's start with those found on Earth. Hurricane, cyclones and typhoons are all the same weather pattern. Their name just depends on where they form.

As with the gas giant storms, these monsters are powered by convection. Warm water, light and air rises from below, while cool air and precipitation sink from above, creating a vertical convection cell.

In the case of Earth storms, the energy powering that convection comes from the sun-warmed ocean. This convection sustains a central low-pressure region which sucks in the surrounding moist ocean air. These incoming winds travel such large distances across the Earth's surface that they are subject to the Coriolis force. Earth is, of course, rotating on its axis. Any object moving over a spinning surface will appear to follow a curved path relative to an observer moving with that surface. On a clockwise spinning surface there is a pool, a Coriolis force, to the left, while on a counterclockwise surface the Coriolis force is to the right. So as air rushes in all directions into a low-pressure storm cell, the Coriolis effect causes it to curl around in a circle. The result is a raging vortex in the same direction as the Earth's rotation, clockwise in the southern hemisphere and counterclockwise in the north. These storms can persist as long as the warm ocean provides energy to drive the convection cycle. Over cold water or land, this power source is cut off and the storm dies. Another key to hurricane's longevity is moisture supply. Wet ocean air is pulled in by the low-pressure core, and it rises in the convection cell. As it cools, it condenses into clouds which are driven outwards. Eventually, the moisture rains down, completing the cycle and maintaining the storm's core. This is another reason why storms weaken near or on land. Too little water and convection ceases. It's also why the most powerful storms are driven by low pressure rather than high-pressure cores. See, a low-pressure core draws air inwards, replenishing the water supply. A high-pressure core pushes air outwards, depleting the supply of moisture and shortening the storm's life. The outgoing winds of a high-pressure cell still do respond to the Coriolis force, so they form a vortex. However, that vortex rotates in the opposite direction, clockwise in the north and counterclockwise in the south. Whereas low-pressure vortices are called cyclonic, high-pressure vortices are called anti-cyclonic. So, there we have it. The most powerful Earth storms tend to be low-pressure cyclonic systems powered by sun-warmed water.

Gas giant storms are a little different. They are most often anti-cyclonic, high-pressure systems and are powered not by the sun, but by the collapse of the planet itself. See, the outer planets never quite finished forming. They originally collapsed from the vast gas disk left over after the sun's birth, but 5 billion years later, that collapse continues, albeit very slowly. Jupiter, for example, shrinks by about 2 centimeters every year.