For centuries, scientists had predicted and written about lightning on the surface of Jupiter, until even a few decades ago, when we had our own spacecraft fly past the planet. On 5 March 1979, NASA’s Voyager 1 spacecraft flew past Jupiter, confirming the existence of Jovian lightning for the first time. However, the data showed that the lightning-associated radio signals did not match the details of the radio signals produced by lightning on Earth.
Since then, the questions shifted to the origin of the peculiar lightning on the gas giant's surface, but without another probe flying past the planet and hence collecting data from really close quarters, none of these questions could be conclusively answered.
Enter Juno, NASA's space probe orbiting Jupiter which entered a polar orbit of the planet in July 2016, and has been sending back spectacular images ever since. Juno's primary missions include measuring the planet's composition, gravity field, magnetic field and polar magnetosphere. Among its array of highly sensitive instruments is the Microwave Radiometer Instrument (MWR), which records emissions from the gas giant across a wide spectrum of frequencies.
In a paper published in Nature, scientists associated with the Juno mission describe the ways in which lightning on Jupiter is actually analogous to that on Earth.
Lightning bolts act like radio transmitters, sending out radio waves when they flash across a sky, and that is true for any planet, a NASA official explained.
"But until Juno, all the lightning signals recorded by spacecraft (Voyagers 1 and 2, Galileo, Cassini) were limited to either visual detections or from the kilohertz range of the radio spectrum, despite a search for signals in the megahertz range. Many theories were offered up to explain it, but no one theory could ever get traction as the answer," Shannon Brown of NASA’s Jet Propulsion Laboratory in Pasadena (also the lead author of the paper) said.
In the first eight flybys, Juno’s MWR detected 377 lightning discharges, Brown said. "They were recorded in the megahertz as well as gigahertz range, which is what you can find with terrestrial lightning emissions. We think the reason we are the only ones who can see it is because Juno is flying closer to the lighting than ever before, and we are searching at a radio frequency that passes easily through Jupiter’s ionosphere."
What Brown essentially means is that Juno is studying the signals from the lightnings on Jupiter at a different scale when compared to earlier probes, also aided by it's close physical proximity to the planet.
"These discoveries could only happen with Juno," Scott Bolton, principal investigator of Juno from the Southwest Research Institute, said. "Our unique orbit allows our spacecraft to fly closer to Jupiter than any other spacecraft in history, so the signal strength of what the planet is radiating out is a thousand times stronger. Also, our microwave and plasma wave instruments are state-of-the-art, allowing us to pick out even weak lightning signals from the cacophony of radio emissions from Jupiter."
While the new paper showed how Jupiter lightning is similar to Earth’s, it also noted that these lightning bolts flashed at very different positions on each planet. "Jupiter lightning distribution is inside out relative to Earth," Brown said. "There is a lot of activity near Jupiter’s poles but none near the equator. You can ask anybody who lives in the tropics, this doesn’t hold true for our planet."
Basically, while lightning bolts congregate near the equator on Earth, they are observed more near the poles on Jupiter. Why, you ask?
Earth is only the third planet from the Sun in the solar system, whereas Jupiter is two positions behind with an asteroid belt in between. On Earth, the vast majority of its heat is derived externally from the Sun. Because our equator bears the brunt of this direct sunlight, warm moist air rises (through convection) more freely in that region, which fuels towering thunderstorms that produce lightning.
On the other hand, Jupiter’s atmosphere derives the majority of its heat from within the planet itself, although this doesn’t render the Sun irrelevant. The Sun's rays heat up Jupiter's equator just as they heat up Earth's, but scientists believe that this heating at Jupiter’s equator is just enough to create a stability in the upper atmosphere, inhibiting the rise of warm air from within.
The poles, which do not have this upper-level warmth and therefore no atmospheric stability, allow warm gases from Jupiter’s interior to rise, therefore creating the ingredients for lightning.
“These findings could help to improve our understanding of the composition, circulation and energy flows on Jupiter,” said Brown. But another question looms, she said. “Even though we see lightning near both poles, why is it mostly recorded at Jupiter’s north pole?” For now, that question will have to wait.
NASA's Juno will make its 13th flyby over Jupiter's mysterious cloud tops on 16 July.