With storms constantly raging all over its surface and a Great Red Spot that’s been churning for centuries, Jupiter could be known as the storm capital of the solar system. Now NASA’s Juno mission has discovered two new quirks of these storms – shallow lightning and “mushball” hail.
For decades, flashes of lightning have been regularly spotted in the colorful clouds that swirl around Jupiter, with much of it seeming relatively similar to the kind we get here on Earth. If so, it should be produced in clouds where water can be simultaneously found in its solid, liquid and gaseous forms.
On Jupiter, that region sits between 45 and 65 km (28 and 40 mi) below the visible clouds, which results in bright patches projected onto the cloud tops. But now Juno has spotted a different phenomenon on the dark side of the planet.
Here, the probe saw small and shallow lightning flashes in clouds at much higher altitudes than should be expected, if the lightning followed Earth rules. These flashes were seen around 25 km (16 mi) higher than the water clouds in Jupiter’s atmosphere, where temperatures plunge to around -88 °C (-126 °F) – far too cold for liquid water.
The researchers suggest a new mechanism that could explain this shallow lightning. The powerful storms fling ice crystals high into the atmosphere, where they meet ammonia vapor. This in turn melts the ice, creating a cloud made of ammonia and water vapor, mixed together.
"At these altitudes, the ammonia acts like an antifreeze, lowering the melting point of water ice and allowing the formation of a cloud with ammonia-water liquid," says Heidi Becker, lead author of one of the new studies. "In this new state, falling droplets of ammonia-water liquid can collide with the upgoing water-ice crystals and electrify the clouds. This was a big surprise, as ammonia-water clouds do not exist on Earth.”
This mechanism has another weird weather side effect – slushy hail that the team calls “mushballs.” Essentially, at those heights an ice crust can form around droplets of ammonia-water, and as they grow heavier they fall back down like hail. As the temperature climbs at those lower altitudes, the mushballs evaporate back into ammonia and water vapor, to start the process over again.
The team says that this mechanism also helps explain another long-standing Jovian mystery – where all the ammonia is. The amount of the stuff seems to change at different points in the atmosphere, and astronomers weren’t sure why. Now it turns out that these mushballs may be carrying it deeper into Jupiter’s atmosphere.
"Combining these two results was critical to solving the mystery of Jupiter's missing ammonia," says Scott Bolton, an author of one of the new studies. "As it turned out, the ammonia isn't actually missing; it is just transported down while in disguise, having cloaked itself by mixing with water. The solution is very simple and elegant with this theory: When the water and ammonia are in a liquid state, they are invisible to us until they reach a depth where they evaporate - and that is quite deep.”
The research was published in two studies – one on shallow lightning was published in the journal Nature, while one on mushballs appeared in the Journal of Geophysical Research: Planets.
An animation of shallow lightning can be seen in the video below.
Source: NASA