The hailstones are very different from those on earth, they consist largely of ammonia.

This is evident from new observations by Juno. The probe has been circling Jupiter since 2016, where it studies the turbulent atmosphere of the gas giant. Everything is going on in that atmosphere, as it turns out. Not only do very remarkable hailstones fall down, but also small lightning bolts are created high in the atmosphere.

Lightning Bolts
It has been known for some time that it can lightning on Jupiter. In 1979 space probe Voyager already saw flashes of light on the gas giant. They were believed to originate in a manner similar to the thunderbolts we know here on Earth. According to that assumption, the flashes arose in thunderstorms that harbored liquid water, water ice and water vapor. It means very concretely that they would arise at about 45 to 65 kilometers below the cloud tops visible to us, where the temperature is around 0 degrees Celsius.

But observations from space probe Juno now reveal that there is also a completely different type of lightning on the gas giant: Juno has spotted superficial lightning bolts. “Flys past the tops of the clouds allowed us to observe something very surprising,” says researcher Heidi Becker. “Smaller, more superficial flashes of light, which originated at much higher altitudes in Jupiter’s atmosphere than was thought possible until recently.”

Originate
These superficial lightning bolts are created by virtue of deeper thunderstorms outside, Becker and colleagues suspect. Those deeper, powerful thunderstorms would whip up water ice crystals that then move higher into the atmosphere – about 15 miles above Jupiter’s water clouds – where they encounter ammonia vapor that melts the ice, creating a mixture of ammonia and water. “At this altitude (where the temperature is around -88 degrees Celsius, ed.) The ammonia acts as an antifreeze and lowers the melting point of water ice,” Becker explains. “This can create clouds that consist of liquid water and ammonia.” Droplets that fall from these clouds – and therefore also consist of ammonia and water – collide with rising ice crystals. “And thus electrify the clouds.

Hailstones
This theory not only explains the formation of the superficial lightning bolts, but may also help explain why there is surprisingly little ammonia in some parts of Jupiter’s atmosphere. “When Heidi discovered superficial lightning bolts, we realized that we now had evidence that ammonia mixed with water high up in the atmosphere,” said researcher Scott Bolton. Drops of that mixture fall down and collide with ice crystals, causing not only lightning bolts, but also growing into exotic hailstones, the core of which consists of that mixture of liquid water and ammonia and around which an ice crust is formed. The result is a mushy ammonia-rich hailstone that the researchers refer to as a ‘mushball’. “Eventually the mushballs grow so big that (…) they fall deeper into the atmosphere, where they encounter higher temperatures and eventually evaporate completely,” says researcher Tristan Guillot. For example, the hailstones transport ammonia from the upper layers of the atmosphere. “Combining these two results was instrumental in solving the mystery of Jupiter’s missing ammonia,” said Bolton. “It turns out that the ammonia has not really been lost, but because it mixes with water it is transported downwards, as it were, in disguise (…) When the water and ammonia are liquid, they are invisible to us.” “Combining these two results has been instrumental in solving the mystery of Jupiter’s missing ammonia,” said Bolton. “It turns out that the ammonia has not really been lost, but because it mixes with water it is transported downwards, as it were, in disguise (…) When the water and ammonia are liquid, they are invisible to us.” “Combining these two results was instrumental in solving the mystery of Jupiter’s missing ammonia,” said Bolton. “It turns out that the ammonia has not really been lost, but because it mixes with water it is transported downwards, as it were, in disguise (…) When the water and ammonia are liquid, they are invisible to us.”

Image: NASA / JPL-Caltech / SwRI / CNRS.

It has been nine years this week since Juno was launched. After a journey of about five years, the probe arrived at the gas giant in 2016. Juno initially settled in a wide orbit, taking about 53 days to complete a lap around the gas giant. The plan was to shorten the turnaround time to 14 days a few weeks after arrival. But technical problems prevented Juno from carrying out the necessary maneuver. Because of this, it is still stuck in its 53-day job. Fortunately, the probe can also achieve many mission objectives from this orbit. The only drawback is that due to the longer turnaround time, it also takes a bit more time to collect the necessary data. The good news, however, is that Juno is exposed to less radiation from this orbit and is therefore likely to last longer. In 2018, NASA therefore decided to extend the mission for another three years, until July 2021 . The probe has already completed 27 orbits around Jupiter and made several interesting discoveries . Juno recently discovered a new cyclone on Jupiter . Juno has also taken many beautiful photos of the gas giant.

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