For decades, scientists have been puzzled about the giant lightning storms in Jupiter's atmosphere, but new data from NASA's Juno orbiter indicates that Jovian lightning has more in common with the terrestrial variety than previously thought. An analysis of radio signals received by the unmanned deep space probe suggest that Jupiter's lighting storms are very similar to those on Earth, but are inside out compared to ours.
The question of lightning on Jupiter has vexed space scientists ever since American astronomers Kenneth Linn Franklin and B F Burke identified the giant planet as a source of strong natural radio transmissions in 1955. Analysis of the radio spectrum indicated that one source of these transmissions were lightning bolts arcing through the Jovian atmosphere, a billion times more powerful than those on Earth.
However, when NASA's Voyager 1 probe made a flyby of Jupiter in March 1979, there was a bit of a surprise. Yes, there were radio signals that matched those of lightning in the atmosphere, but their characteristics didn't match those of terrestrial lightning. The signals were showing up in the kilohertz band of the electromagnetic spectrum, but not the megahertz range as is the case with Earth lightning.
The same was true of the other probes that followed Voyager 1. Voyager 2, Galileo, and Cassini all found visual evidence of lightning as well as kilohertz signals, but none in the megahertz. The question was, is this due to Jovian lightning having a different mechanism or was it due to the instruments on the spacecraft?
When the latest NASA Jupiter probe, Juno, entered orbit on July 2, 2016, it brought two new tools to bear. First, it was in a trajectory that brought it very close to the top of the Jovian clouds and the poles in particular. Second, it had onboard the Microwave Radiometer Instrument (MWR) – a much more sophisticated radio receiver, that operates over a wide band of the radio spectrum.
"In the data from our first eight flybys, Juno's MWR detected 377 lightning discharges," says Shannon Brown of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "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."
So far, that seemed like a reassurance that both planets have similar lightning, but another puzzle emerged. Why are so many of Jupiter's lightning storms clustered around the poles when those on Earth are more common near the equator?
According to NASA, the answer seems to be that Jupiter and Earth have atmospheres that act like massive heat engines that create lightning storms. The difference is that Earth's atmosphere gets its heat mainly from the Sun, which allows huge thunderheads to form thanks to the warm, moist air in the tropics as it rises to create thunderheads. But Jupiter gets most of its energy from inside itself, with the Sun contributing one 25th as much energy than on Earth due to the giant planet's great distance. But that tiny amount of sunshine has a big effect.
What this means is that as the hot, moist air rises from deep in the Jovian atmosphere, it meets a cap of warm air in the tropical regions of Jupiter. This stabilizes the upper atmosphere, so the convection currents needed to form thunderheads can't form. However, at the poles, this effect is much less, so the air can rise and BOOM. Yet there are still questions to be answered.
"These findings could help to improve our understanding of the composition, circulation and energy flows on Jupiter," says Brown. ""Even though we see lightning near both poles, why is it mostly recorded at Jupiter's north pole?".
The research was published in the journal Nature.
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