Four years after arriving at the planet Jupiter, NASA’s Juno spacecraft is still making fresh discoveries and sending us breathtaking pictures of the gas giant and its entourage of at least 79 moons.

The most recent finding is a bizarre meteorological phenomenon, something not seen on Earth: “shallow” lightning, accompanied by slushy hailstones made of an antifreeze-like mixture of water and ammonia, dubbed “mushballs” by NASA’s Juno science team.

An illustration of NASA’s Juno spacecraft, which orbits Jupiter in an elongated, looping path that carries it as close as 2,600 miles of the gas giant’s cloud tops. (NASA/JPL-Caltech)

These mysterious weather phenomena have helped us better understand the distribution of ammonia in the Jovian atmosphere. And, they can help improve our overall understanding of distant planets orbiting stars in other solar systems, too far away for us to study in detail.

What Makes Lightning ‘Shallow’?

Since 1979, observations made by spacecraft before Juno — Voyagers 1 and 2, and Galileo — detected powerful flashes of lightning through the cloud layers of Jupiter’s turbulent atmosphere. Unlike Earth, the gas giant planet has no solid surface, and is made up of ever deeper and thicker layers of gas, mostly hydrogen and helium. The potent electrical discharges — detected by earlier missions — are believed to occur as far as 40 miles below the visible cloud tops, where temperatures and atmospheric pressure are right for the formation of lightning as we understand it on Earth.

An artist illustration of the distribution of powerful, “deep” lightning in Jupiter’s polar regions, detected decades ago by the Voyager and Galileo spacecraft. On Earth, solar heating drives most lightning activity in the warm equatorial region, but on Jupiter, where the sun’s light is 25 times weaker, the tropical areas are more stable, and lightning driven by Jupiter’s own internal heat appears to reside in the more turbulent polar regions. (NASA/JPL-Caltech/SwRI/JunoCam)

On Earth, lightning is generated where water is found in all its states — gas, liquid droplets and solid particles of ice. Water vapor feeds the growth of liquid droplets in a cloud, and strong updrafts carry the droplets to altitudes where freezing temperatures turn them to ice particles. The ice particles fall downward, colliding with the upwelling liquid droplets, and the friction of their interaction knocks electrons from water molecules. Static electric charge builds up until it’s too strong to remain static, then discharges into the air, another cloud, or the ground. The same thing happens on a much smaller scale when you drag your shoes across a carpet and build up static electricity from the friction, until you touch another electrical conductor (metal, or another person) and discharge the electrons in a tiny, sometimes painful zap of mini-lightning.

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To scientists’ surprise, Juno, passing within a few thousand miles of Jupiter’s cloud tops on the night side, detected flashes of lightning much smaller than the powerful strikes earlier missions had seen coming from beneath the clouds. Estimates place the number of these lightning strokes at about 3.75 billion per year, across Jupiter’s entire surface — that’s about 119 per second on average! These fainter flashes appear to come from much higher in the atmosphere, where it is too cold — below negative 126 degrees Fahrenheit — for droplets of liquid water to exist.

Color-enhanced image showing one of Jupiter’s turbulent storm systems. The bumpy white texture highlighting the strokes of the storm’s swirls are where power updrafts of storm cloud cells rise high above Jupiter’s general cloud tops. It is in these thunderhead towers that NASA’s Juno discovery of “shallow lighting” is thought to occur. (NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstaedt/Sean Doran)

This was baffling at first, until one Juno scientist had an idea that could not only explain the high-altitude lightning in Jupiter’s atmosphere, but also another mystery that has puzzled scientists for years: much lower than predicted amounts of ammonia in Jupiter’s upper atmosphere.

The Mushball Connection

This explanation for the shallow lightning and Jupiter’s “missing” ammonia in the upper atmosphere goes like this: Jupiter’s powerful thunderstorms and the strong updrafts of air and liquid water droplets eject plumes of water as high as 16 miles above the tops of the thunderheads, which freeze into ice crystals in the extreme cold above. There, the ice particles encounter a layer of ammonia gas, which melts the ice and blends with the water to form a liquid water-ammonia antifreeze mixture.

Diagram shows how Juno’s newly discovered “shallow lightning” may be generated by the growth of semi-slushy “mushballs” of water-ammonia that fall like hail onto updrafts of frozen water-ice particles. On Earth, it is falling solid-ice hail interacting with rising liquid water droplets that generate static electricity that drives lightning. On Jupiter, due to the involvement of ammonia, the process is turned somewhat upside down. (NASA/JPL-Caltech/SwRI/CNRS)

As the droplets of water-ammonia rise and fall, they collide with the water-ice crystals flung upward by the thunderhead far below. As with Earthly lightning, the friction of collision between the liquid “antifreeze” and solid ice particles generates static electricity, and high-altitude lightning is born.

Why there is less ammonia in some parts of Jupiter’s upper atmosphere than previously thought may be explained by what happens next.

In the cold, high-altitude layers where the lightning is generated, a crust of water ice forms around the liquid water-ammonia core of a droplet, growing thicker and enlarging the so-called “mushball” until the atmosphere can no longer support it. It falls like a hailstone, deep into Jupiter’s atmosphere, below the visible surface of its cloud tops where it cannot be detected by spacecraft like Juno. Only then, far below the clouds, does the mushball’s icy crust melt and its water-ammonia core evaporate, potentially forming a layer of ammonia beneath the clouds.

Why Are Shallow Lightning and Mushy Ammonia Hailstones Important?

There is a great diversity in planets and moons within our own solar system, each with very different compositions and environments. As we explore Jupiter’s stormy weather, or Mars’ global dust storms, or Pluto’s nitrogen-methane glacial flows, we are learning how different planets work.

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As we begin to study more closely the thousands of extrasolar planets discovered in the last 30 years, we can use what we’ve learned in our solar system as a framework to understand what lies beyond.

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