Extraterrestrial Superstorms (Script) (Patreon)

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. 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 two to three times the diameter of the planet Earth. Meanwhile the fastest winds ever measured, clocking fifteen hundred 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. Hurricanes, cyclones, and typhoons are all the same weather phenomenon; their name just depends on where they form. As with the gas-giant storms, these monsters are powered by convection. Warm water-laden 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 a large distances across the Earth’s surface that they are subject to the Coriolis effect.

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’s a pull – a Coriolis “force” – to the left, while on a counter-clockwise surface the Coriolis force is to the right. So as air rushes 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 anticlockwise in the north. These storms can persist as long as a 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 hurricanes’ longevity is moisture supply. As wet ocean air is pulled in by the low-pressure core and 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 do still respond to the Coriolis force and so form a vortex. However that vortex rotates in the opposite direction; clockwise in the north and counter-clockwise in the south. Whereas low-pressure vortices are called cyclonic, high-pressure vortices are 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 2cm every year. As they contract, gas giants convert gravitational potential energy into heat, which in turn powers the largest storms in the solar system.

As contraction-heated gas rises, it cools. On Earth, the only source of condensation is water. But the gas giants’ atmospheres span such a wide range of temperature and pressure that all sorts of molecular species condense. Most of Jupiter’s visible clouds are ammonia ice, tainted with colorful impurities, but there are also clouds of hydrogen sulphide and regular H2O. The phase changes from gas to liquid to solid release latent heat that lifts the storm still higher. All this happens in the troposphere, the densest and innermost layer of the atmosphere. On Earth the troposphere is about 10 miles thick, but on the gas giants it can extend a hundred miles into the planets’ murky depths, where pressure forces the gas into a metallic liquid state.

A powerful storm can breach the tropopause into the stratosphere. For example, the Great Red Spot towers 8 km above Jupiter’s iconic bright and dark belts. The Spot itself is quite cold, in fact below freezing, but all its activity warms the upper atmosphere to 1600 Kelvin. With such a wide range of molecular species driving an incredibly deep convection cycle, there’s no danger of a Jovian storm system running out of molecules to condense. Even high-pressure, outward-blowing storms don’t “dry up” like they do on Earth. In fact, all of the famous storms, like the Great Red Spot, Neptune’s Great Dark Spot, and Saturn’s polar vortex are or were anti-cyclonic.

Gas giant storms can last for many years. Neptune’s Dark Spot lasted for several years before dissipating by 1994. However Jupiter’s Great Red Spot has raged for at least 350 years. We’re still trying to unlock the secrets of these storms’ longevity. A combination of a near-inexhaustible supply of heat and an abundant supply of condensable molecules helps. In the case of the Great Red Spot, it also helps that the beast frequently cannibalizes smaller storms, absorbing their energy, and that it's sandwiched between a pair of 3-400 mile an hour jet streams that are moving in opposite directions to each other. These help keep the Spot spun up.

But even with all of this, the Great Red Spot’s centuries-long lifespan is mysterious. And, in fact, it may finally be dying out. The Spot is showing signs of shrinking. In the 1800s, the it spanned 37,000 kilometers, or about three Earths in width. However, as of April this year, the Spot spans just 16,350 kilometers, or 1.3 Earths. The storm is also circularizing, and although it’s smaller its wind speeds haven’t diminished. Is the Great Red Spot fading away? What’s causing this shrinking? The answers to the storm’s longevity AND fate may be found in the details of its structure. For that we need to get close. For that, we need Juno.

NASA launched the $1.1 billion Juno mission in 2011. The probe will swing within 3400 kilometers of the Jovian cloud tops and ultimately crash into them to avoid contaminating potentially life-bearing moons like Europa—and to peek under the gas giant’s cloud cover. Juno carries eight instruments, including a radiometer for probing the atmosphere’s high-pressure depths, an imaging spectrograph for studying cloud chemistry, a magnetometer for measuring Jupiter’s intense magnetic field, and the four-color, wide-field Junocam. Juno spends most of its orbit millions of miles away from Jupiter’s harsh radiation, but every 53 days, it ducks under this radiation belt and turns on its instruments for a two hour transit from pole to pole. During that time, Juno snaps a picture every 60 seconds, fast enough to catch at least six angles of a given feature. This is perfect for measuring cloud heights, and also to record horizontal cloud drift, which gives us wind speeds.

Juno’s flyby images of the Great Red Spot are really incredible. These are taken by Junocam, and, incredibly, processed by volunteer citizen scientists. Here are some examples. Look at the difference between these images from Hubble, Cassini, and Juno. The level of detail is incredible. All these images were taken on July 10th during Juno’s seventh flyby, from just 5600 km above the clouds. Look at all the little plumes and eddies. It’s the first time we’ve seen so many storms within storms, especially in the Great Red Spot. They whip around the edges at top speed, but in the center, everything is perfectly calm, like the eye of a mega-hurricane. 

Even accounting for the resolution limits of older photos Juno is revealing real changes. For example, there’s a transition from long streaks to smaller eddies. This represents a shift from streamline to turbulent flow. Turbulent flow acts like friction, sapping energy away from the bulk rotation. This may be connected to the Spot's shrinking. As well as getting smaller, the Great Red Spot is becoming more circular. In fact it may be completely circular in the next couple of decades. But that doesn’t necessarily mean it’s going to vanish. Besides size, velocity is a good measure of storm strength. Over the last few decades, the storm has gotten smaller but it now appears that its winds are not any slower.

The pictures from Junocam are just the beginning. Juno’s 7 other science instruments have provided an enormous amount of data, and more will come in the upcoming 30 fly-bys. Scientists are pouring over this information right now, and with help from citizen scientists, perhaps like you, we’re sure to unravel some of the secrets of the Solar System's most powerful storms. You can stay updated on Juno's discoveries on its interactive website, and by staying tuned to Space Time.