Today, Aug. 5, 2020, marks nine years since the launch of Juno, a spacecraft that has revolutionized our knowledge of the largest planet in the Solar System: Jupiter. Ever since Juno arrived at Jupiter and entered orbit on July 5, 2016, it has helped to answer some questions but also revealed new surprises.
Two more such surprises were announced today by NASA: new evidence for “shallow lightning” and possible Jovian hail, composed not of water ice as on Earth, but frozen ammonia, which the Juno scientists refer to as “mushballs.”
The mushballs research has been published in the Journal of Geophysical Research: Planets while the shallow lightning findings will be published tomorrow, August 6, in Nature.
Shallow lightning is a unique form of lightning on Jupiter, originating in the water-ammonia clouds, which are common on Jupiter, but don’t exist on Earth. Earth’s lightning, on the other hand, occurs in water clouds.
The findings underscore the fact that while Jupiter (and other planets) can have phenomena or processes that resemble those on Earth, their cause or composition can be quite different.
While lightning had been observed before on Jupiter by Voyager in 1979, it was thought that, like on Earth, it only occurred in water clouds, 28 to 40 miles (45 to 65 kilometers) below the top-level clouds, with temperatures taround 32 degrees Fahrenheit (0 degrees Celsius). But when Juno saw lightning on Jupiter’s dark side, using its Stellar Reference Unit, it saw smaller flashes at higher altitudes. This suggests that lightning can also occur in water-ammonia clouds.
“Juno’s close flybys of the cloud tops allowed us to see something surprising – smaller, shallower flashes – originating at much higher altitudes in Jupiter’s atmosphere than previously assumed possible,” said Heidi Becker, Juno’s Radiation Monitoring Investigation lead at NASA’s Jet Propulsion Laboratory. She is also the lead author of the new paper.
So how does this happen?
Jupiter has powerful thunderstorms, more powerful than any on Earth, and the researchers think that those storms fling water-ice crystals into the upper atmosphere, over 16 miles (25 kilometers) above the water cloud layer. There is also ammonia vapor at those altitudes, and that vapor melts the water-ice crystals. This creates an an ammonia-water solution where it is too cold for pure water to remain liquid.
“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,” said Becker. “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.”
Those electrified clouds can produce lightning just like the other water clouds can.
The shallow lightning was an unexpected discovery, but another mystery has involved the ammonia itself. According to Juno’s findings, the gas has been largely missing from Jupiter’s atmosphere, and the amount of ammonia changes in different regions in Jupiter’s atmosphere. Why?
“Previously, scientists realized there were small pockets of missing ammonia, but no one realized how deep these pockets went or that they covered most of Jupiter,” said Scott Bolton, Juno’s principal investigator at the Southwest Research Institute in San Antonio. “We were struggling to explain the ammonia depletion with ammonia-water rain alone, but the rain couldn’t go deep enough to match the observations. I realized a solid, like a hailstone, might go deeper and take up more ammonia. When Heidi discovered shallow lightning, we realized we had evidence that ammonia mixes with water high in the atmosphere, and thus the lightning was a key piece of the puzzle.”
The other finding from Juno is evidence for Jovian hailstones, which scientists are calling mushballs. They seem to form similar to how hailstones form on Earth, but are composed of layers of water-ammonia slush and ice covered by a thicker water-ice crust.
“Eventually, the mushballs get so big, even the updrafts can’t hold them, and they fall deeper into the atmosphere, encountering even warmer temperatures, where they eventually evaporate completely,” said Tristan Guillot, a Juno co-investigator from the Université Côte d’Azur in Nice, France, and lead author of the second paper. “Their action drags ammonia and water down to deep levels in the planet’s atmosphere. That explains why we don’t see much of it in these places with Juno’s Microwave Radiometer.”
Both the shallow lightning and mushballs seem to answer the question of why Jupiter’s ammonia is so depleted.
“Combining these two results was critical to solving the mystery of Jupiter’s missing ammonia,” said Bolton. “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.”
Juno also recently photographed the north pole of Jupiter’s moon Ganymede (announced on July 22, 2020), something never done before, until now.
“The JIRAM data show the ice at and surrounding Ganymede’s north pole has been modified by the precipitation of plasma,” said Alessandro Mura, a Juno co-investigator at the National Institute for Astrophysics in Rome. “It is a phenomenon that we have been able to learn about for the first time with Juno because we are able to see the north pole in its entirety.”
“These data are another example of the great science Juno is capable of when observing the moons of Jupiter,” said Giuseppe Sindoni, program manager of the JIRAM instrument for the Italian Space Agency.
To help celebrate the date of Juno’s launch, the mission team did some interviews on how they feel about this milestone and the mission so far. Some quotes include:
“Juno went from proposal to launch in about five or six years and that seems like a really long time but most of the little steps in between always feel really rushed. There’s never enough time to do something that’s never been done before.”
— Heidi Becker, Investigation Scientist & Radiation Monitoring Investigation Lead, JPL
“The idea that you can couple our scientific imaging and understanding of the planet, with artistic representations of not only what the planet means but what exploration means, has been very valuable to the mission– and to the public.”
— Paul Steffes, Investigator, Georgia Tech
“We look at view graphs and charts and analyses all the time – what blows my mind is when I go out at night, and I look up at Jupiter, and I realize, we’re there.”
— Rick Nybakken, Project Manager 2012-2017, JPL
Juno has also determined how much water exists in Jupiter’s atmosphere, viewed massive polar cyclones, watched auroras which “defy earthly laws of physics,” found that storm belts penetrate deeper into the atmosphere than thought, determined that the planet’s interior is not as uniform as previously believed and, of course, viewed the Great Red Spot.
So far, Juno has performed 27 science flybys of Jupiter and logged over 300 million miles (483 million kilometers).
Juno was launched on Aug. 5, 2011 on an Atlas V rocket from the Cape Canaveral Air Force Station in Florida.
More information about Juno is available on the mission website.