Summer’s coming. For the western hemisphere it starts bright and early Thursday June 21 at 5:07 a.m. (Central Time). The heat of the season fuels the formation of thunderstorms. Last night, I drove home through one, with sudden downpours and lightning that lit up the sky like popping flash bulbs.
Earth’s not the only planet with thunderstorms. NASA’s Voyager 1 spacecraft flew past Jupiter in March, 1979 and confirmed the existence of Jovian lightning, which astronomers had theorized must be there given the planet’s heated interior, powerful winds and eddy-like storms. The probe detected lightning via radio. We see lightning by the light it creates, but every bolt also emits a burst of radio waves that our car radios pick up as crackles of static.
Let’s go for a ride on Juno. On May 24, 2018, NASA’s Juno probe made its 13th close flyby of Jupiter. The time-lapse movie uses real images and covers two hours of the flyby. Every 53 days the spacecraft loops only a few thousand miles over the planet’s clouds.
That’s not all. Lightning also gives off energy across a much broader range, from the AM radio dial (kilohertz frequencies) across FM and TV bands (megahertz) and even cable and satellite TV (gigahertz). And if you didn’t know already the temperature of a lightning bolts can reach up to 53,540° F (29,725° C). That’s five times hotter than the surface of the sun. No wonder each stroke has energy to spread around!
But when the Voyager hurtled by, the data showed that the lightning-produced radio signals at Jupiter didn’t quite match the details of the radio signals produced by lightning here at Earth. In a recent paper published in Nature, scientists from NASA’s Juno mission describe the ways in which lightning on Jupiter is like Earth’s in some ways and not like it in others.
“No matter what planet you’re on, lightning bolts act like radio transmitters — sending out radio waves when they flash across a sky,” said Shannon Brown, a Juno scientist and lead author of the paper. “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 (AM radio) of the radio spectrum, despite a search for signals in the megahertz range.”
Juno’s been orbiting Jupiter since July 4, 2016 and carries a super-sensitive instrument called a Microwave Radiometer Instrument (MWR), which can pick up radio emissions from the gas giant across a wide range of frequencies. During its first eight close flybys of Jupiter, Juno detected 377 lightning discharges.
“In the data from our first eight flybys, Juno’s MWR detected 377 lightning discharges,” said Brown. “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 …”
Time-lapse of thunderstorms photographed from the International Space Station as it orbits the Earth
Cool. Jupiter’s lightning is very similar to Earth’s after all, but Juno also revealed something unexpected: the giant planet has lots more lightning in its polar regions compared to the equator. That seems totally counterintuitive. Anyone who lives in the tropics knows that thunderstorms are a common, sometimes daily, occurrence. Earth gets most of its heat from the sun, and the equator gets hotter than most places because the sun is always high in the sky there. Sun-heated ground causes warm, moist air to rise and trigger towering thunderstorms that produce abundant lightning.
Lightning at Jupiter produces “whistlers” in the low-end of the radio spectrum just as lightning does here on Earth. They sound like slowly descending whistle tones. Click to listnen.
Jupiter’s orbit is five times farther from the sun than Earth’s orbit, which means that the giant planet receives 25 times less sunlight than Earth. Still, the sun’s rays do provide some warmth, heating up Jupiter’s equator more than the poles just like Earth. Scientists think that the heat is just enough to stabilize the atmosphere there, inhibiting the rise of warm air from within the planet. Warmth from the sun at in Jupiter’s equatorial region lessens the difference in temperature between the upper and lower atmosphere. Temperature differences are key factor in driving winds. When temperatures are closer to equal, winds are less.
The poles don’t have this extra warmth because the sun is always low in the sky at Jupiter’s poles just like it is at Earth’s poles. Without the extra heat to stabilize the air, warm gases from the planet’s interior rise and give birth to more intense and frequent storms. All well and good, but every time we get a closer look at some aspect of nature, new data reveals new puzzles. Juno recorded at both poles but mostly at Jupiter’s north pole. Why is that? No one has the answer … yet.
NASA’s Juno spacecraft will make its 14th science flyby over Jupiter’s swirling cloud tops on July 16.