‘Shallow Lightning’ and ‘Mushballs’ Reveal Ammonia to NASA’s Juno Scientists

The spacecraft may have found where the colorless gas has been hiding on the solar system’s biggest planetary inhabitant.

New results from NASA’s Juno mission at Jupiter suggest
our solar system’s largest planet is home to what’s called “shallow lightning.”
An unexpected form of electrical discharge, shallow lightning originates from
clouds containing an ammonia-water solution, whereas lightning on Earth
originates from water clouds.

Other new findings suggest the violent thunderstorms
for which the gas giant is known may form slushy ammonia-rich hailstones Juno’s
science team calls “mushballs”; they theorize that mushballs essentially
kidnap ammonia and water in the upper atmosphere and carry them into the depths
of Jupiter’s atmosphere.

The shallow-lightning
findings will be published Thursday,
Aug. 6, in the journal Nature, while the mushballs research is currently
available online in the Journal of Geophysical Research: Planets.

Since NASA’s Voyager mission first saw Jovian lightning flashes in 1979, it has been thought that the planet’s lightning is similar to Earth’s, occurring only in thunderstorms where water exists in all its phases – ice, liquid, and gas. At Jupiter this would place the storms around 28 to 40 miles (45 to 65 kilometers) below the visible clouds, with temperatures that hover around 32 degrees Fahrenheit (0 degrees Celsius, the temperature at which water freezes). Voyager, and all other missions to the gas giant prior to Juno, saw lightning as bright spots on Jupiter’s cloud tops, suggesting that the flashes originated in deep water clouds. But lightning flashes observed on Jupiter’s dark side by Juno’s Stellar Reference Unit tell a different story.

“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 in Southern California and the lead author of the Nature

and her team suggest that Jupiter’s powerful thunderstorms fling water-ice crystals
high up into the planet’s atmosphere, over 16 miles (25 kilometers) above Jupiter’s
water clouds, where they encounter atmospheric ammonia vapor that melts the ice,
forming a new ammonia-water solution. At such lofty altitude, temperatures are below
minus 126 degrees Fahrenheit (minus 88 degrees Celsius) – too cold for pure liquid
water to exist.

This animation takes the viewer on a simulated journey into Jupiter’s exotic high-altitude electrical storms. Get an up-close view of Mission Juno’s newly discovered “shallow lighting” flashes and dive into the violent atmospheric jet of the Nautilus cloud. Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill

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.”

The shallow lightning factors into another puzzle about
the inner workings of Jupiter’s atmosphere: Juno’s Microwave Radiometer instrument
discovered that ammonia was depleted – which is
to say, missing – from most of Jupiter’s atmosphere. Even more puzzling was
that the amount of ammonia changes as one moves within Jupiter’s atmosphere.

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.”

This graphic depicts the evolutionary process of “shallow lightning” and “mushballs” on Jupiter. Image Credit: NASA/JPL-Caltech/SwRI/CNRS

› Full image and caption

Jovian Mushballs

second paper, released yesterday in the Journal of Geophysical Research: Planets,envisions the strange brew of 2/3 water
and 1/3 ammonia gas that becomes the seed for Jovian hailstones, known as mushballs.
Consisting of layers of water-ammonia slush and ice covered by a thicker water-ice
crust, mushballs are generated in a similar manner as hail is on Earth – by growing
larger as they move up and down through the atmosphere.

“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.”

“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.”

Understanding the meteorology of Jupiter enables us to
develop theories of atmospheric dynamics for all the planets in our solar system
as well as for the exoplanets being discovered outside our solar system. Comparing
how violent storms and atmospheric physics work across the solar system allows planetary
scientists to test theories under different conditions.

More About the Mission

The solar-powered Jupiter explorer launched nine years ago today,
on Aug. 5, 2011
. And last month marked the fourth
anniversary of its arrival at Jupiter
. Since entering
the gas giant’s orbit, Juno has performed 27 science flybys and logged over 300
million miles (483 million kilometers).

a division of Caltech in Pasadena, California, manages the Juno mission for the
principal investigator, Scott Bolton, of the Southwest Research Institute in San
Antonio. Juno is part of NASA’s New Frontiers Program, which is managed at NASA’s
Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission
Directorate in Washington. Lockheed Martin Space in Denver built and operates the

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Source: Jet Propulsion Laboratory

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