Why lightning to Jupiter is an unresolved planetary mystery.

Jupiter is a world of extraordinary beauty that is deeply chaotic. His time is running out of subtlety the best way.

NASA's Juno probe has been studying the planet for nearly five years, unveiling new depths to the drama and splendor of this profoundly strange place. It also illuminates some of the planet's greatest atmospheric mysteries, including the unusual nature of Jupiter's thunderbolt.

As Juno prepares for his extended mission starting in August 2021, it is worth considering why Jupiter's lightning remains one of his most fascinating enigmas.

Hold on, there's lightning on Jupiter?

Yes, but for a long time, no one knew with certainty. In fact, until 1979, lightning on Jupiter was merely a hypothesis; its existence was not confirmed by NASA's Voyager 1 probe which captured radio signals typically indicative of lightning. There was only one catch: the radio waves were in a completely different range of lightning encountered on Earth.

Over the following decades, NASA's Voyager 2, Galileo, and Cassini spacecraft also uncovered radio evidence of lightning on Jupiter. But the instruments used in these missions were not sufficiently sensitive to provide a complete image of Jovian lightning. After Juno arrived in the Jupiter system in July 2016, however, the data started to trickle in: the spacecraft microwave radiometer instrument (MWR) detected 377 lightning discharges in its first eight overflights.

The data helped scientists understand where lightning is distributed on the gas giant and how often it happens. Information has also shown how many jovian jigsaw puzzles they were missing. The new data and details in Juno have given rise to more questions than ever before.

Shocking developments in Jupiter's storms.

Even on Earth, lightning is a kind of mystery that cannot be solved. We know that in general, for cloud-to-ground lightning, negative charges in a storm cloud collide with positive charges on the ground. Just like a battery, contraries attract, and an electric shock is released in the form of a lightning flash.

Nevertheless, there are gaps in our understanding that could be bridged by further research into Jovian lightning, and vice versa.

With the information we have, there seem to be parallels in the basic mechanics of lightning on the two planets. But for many reasons, such as Jupiter's atmospheric composition, there are striking differences. On Earth, most of our lightning strikes around the equator, but on Jupiter, it seems to be more widespread in the hemispheres, particularly the north. This is certainly a landmark for Jupiter since its north pole is already pandemonium; it seems to be the focus of a group of cyclones.

It remains unclear why lightning does not appear to hit the equator of Jupiter. Evidence suggests that it could be influenced by how time forms on Jupiter in general, which is very different from how time operates on our planet. Earth's time, for example, is primarily formed in the troposphere, which is the layer of atmosphere closest to Earth. So many of our natural occurrences are affected by the Sun and the fact that our planet has strong earth.

As for Jupiter, there is no «real» surface. Its climate appears to be driven by powerful forces in the depths of the planet, which can lead to spectacular storms of the gas giant.

Although the amount of sunlight Jupiter receives appears to play a part in its climate, the connection is not fully understood. According to NASA, since the planet is 757 million kilometers (approximately 470 million miles) from the Sun, it receives approximately 25 times less sunlight than Earth. Sunlight may create more balanced upper atmospheric conditions near Jupiter’s equator, where the planet receives more sunlight, and greater instability toward the poles, since warm air from deep within the planet can rise freely. This instability is ideal for the formation of lightning.

Of course, as is usually the case with Jupiter, there is seldom an answer that makes everything clear. The magnetic field of Jupiter may be related to the climatic processes of the planet, and theoretically, to lightning. In Juno’s extended mission, scientists hope to get more high-resolution data from the spacecraft’s suite of instruments that will give further evidence to support theories – or offer up new ones.

Shallow Lightning & Mushballs

Here is where the mystery of Jupiter multiplies once again: recent discoveries suggest that the gas giant is home to more than one type of lightning.

The lightning detected by previous spaceships appears to come from deep water clouds in the atmosphere of Jupiter. This type of lightning seems to be generated in conditions quite similar to those of the Earth, ie from a water cloud. But more recently, Juno has found evidence of a ‘superficial flash' that seems to come from a very high place in Jupiter's atmosphere.

Juno's low-light star tracker, the Stellar Reference Unit (SRU), initially spotted shallow flashes while investigating the dark side of Jupiter. The SRU found lightning that was unusually small and "shallow" in comparison to the previously discovered lightning strike.

In Jupiter's high-altitude atmosphere, temperatures may fall to less than -88 degrees Celsius (-126 degrees Fahrenheit), which is much too cold to support liquid water. But there is, hypothetically, a kind of ammoniacal hail in this area. The scientists at Juno realized that there might be a connection between that and the superficial lightning they observed.

Enter the pièce de résistance of shallow lightning: “mushball.'' According to researchers, it all starts when Jupiter’s violent storms catapult water high up into Jupiter’s atmosphere, where it freezes into ice crystals. Ice crystals are then combined with ammonia gas, which melts as antifreeze. Over time, these ammonia-water raindrops collide with ice crystals from jovian upward currents, creating friction and thus, the conditions to form lightning.

The idea of this ammonia-water hail could not only be a clue to the formation of shallow lightning, but it may also explain why parts of Jupiter’s atmosphere seem to be missing ammonia. In a way, the existence of such anti-freeze mud would kill two proverbial birds with a single mushball.

The unsettled mysteries remain.

Jupiter is arguably the oldest planet in our solar system and yet it has hardly started to share its secrets. By deepening our understanding of Jovian weather, including lightning, we can learn more about the origins of the gas giant and, by extension, our entire cosmic court.

The protracted Juno mission will play a crucial role. Getting closer and closer to the planet will give us a better understanding of Jupiter's lightning and its massive storm systems and polar cyclones. Mission scientists say they're hoping to get more mushball evidence, too.

If there's anything Jupiter can do, it's anarchy and surprises. Although no one knows what Juno's next assignment will reveal, it will certainly not be boring.

Original Publication on The Planetary Society.