Lightning is one of the phenomena that do not only exist on Earth: space probes have also observed such discharges on Jupiter. But now images from NASA’s Juno probe reveal that Jupiter’s lightning bolts are also twitching in the cloud areas where they shouldn’t actually exist. Because with minus 70-80 degrees Celsius it is too cold on the top of the cloud cover for water droplets that are needed for lightning formation. However, ammonia could provide an explanation for this. Because this gas acts as an “anti-freeze agent” for the water ice crystals in Jupiter’s high atmosphere.
The gas giant Jupiter is known for its huge cyclones and planet-spanning cloud bands. Some of its storms even reach up to 3000 kilometers into its interior. Around one percent of the enormous mass of Jupiter is therefore constantly in motion and races in the storm bands around the planet. And the cloud layers of the gas giant also have an enormous vertical extent. They consist mostly of frozen ammonia and ammonium hydrogen sulfide, but some clouds also contain water. Images from NASA’s Voyager spacecraft showed as early as 1970 that these clouds also contain thunderstorms with lightning. The observations and physical models suggested, however, that these lightning bolts only occur in deeper cloud layers. Because the temperatures there are around freezing point, so that there could still be cloud droplets made of liquid water – the charge carriers in the electrically charged thunderclouds.
Unexpected high lightning bolts
But now measurement data from NASA’s Juno space probe on Jupiter has also detected lightning in an unusual location. “The probe’s close flyby allowed us to see something surprising – small, flatter lightning bolts that formed at much greater heights in Jupiter’s atmosphere than was previously thought possible,” reports first author Heidi Becker from NASA’s Jet Propulsion Laboratory in Pasadena. The flashes usually only last a few milliseconds and flicker in quick succession. From the extent of the lightning bolt in the clouds, sometimes only 30 kilometers, the researchers conclude that these discharges must originate from a depth that is well above the previously known lightning bolts. They locate the lightning in a zone with temperatures of less than minus 66 degrees – this is far below the values at which liquid water can still exist.
“This is unexpected and suggests that these lightning bolts are generated by a process that is still unknown,” says co-author Yuri Aglyamov of Cornell University in Ithaca. “Once again, Juno’s observations have shown that Jupiter’s atmosphere is far more complex than expected.” But how can Jupiter’s altitude flashes arise? In search of an explanation, the scientists remembered another observation: Juno and other space probes before her had shown that some deeper areas of the Jovian gas envelope contain remarkably little ammonia. However, this impoverishment could not be explained with previous models. “Rain of ammonia can wash away some of the ammonia, but it doesn’t go deep enough to explain the impoverishment of the lower layers,” says co-author Scott Bolton of the Southwest Research Institute in San Antonio.
“Mud balls” made of ammonia
This is where the study by a second research team comes in. Tristan Guillot from the University of the Cote d’Azur used a new atmospheric model of Jupiter to show that strong storms can throw water ice crystals into the upper gas layers. When these crystals encounter gaseous ammonia there, this lowers the freezing point of the water and the crystals turn into liquid water droplets. “At these altitudes, the ammonia acts like an anti-freeze agent, allowing the formation of clouds of ammonia-water droplets,” explains Becker. When these droplets collide with rising ice crystals, a charge exchange takes place and the clouds become electrically charged. If a certain threshold is reached, the voltage discharges and the altitude lightning occurs.
At the same time, however, this interaction between ammonia and water causes a kind of sleet to form in the high clouds. They consist of layers of semi-liquid ammonia-water slush, which are surrounded by a solid water-ice crust. Similar to hailstones on earth, more and more layers of ice accumulate on these “mud balls” over time, so that they get bigger and bigger. “Eventually the mud balls get so big that even the strong updrafts can no longer keep them in suspension,” explains Guillot. The ammonia-water “balls” fall out of this cloud layer and reach deep layers of the Jupiter gas envelope. “This transports ammonia and water into the deep areas of Jupiter’s atmosphere – and it explains why the ammonia seems to be missing in some places.” In these areas, the hail of ammonia has caused impoverishment. The “mud balls” explain not only the flat lightning bolts, but also the impoverishment of the Jupiter gas envelope in places.
Source: Heidi Becker (NASA Jet Propulsion Laboratory, Pasadena) et al., Nature, doi: 10.1038 / s41586-020-2532-1; Tristan Guillot (Université Cote d’Azur) et al., JGR Planets, doi: 10.1029 / 2020JE006403