Ever since NASA's Voyager 1
spacecraft flew past Jupiter in March, 1979, scientists have wondered about the
origin of Jupiter's lightning. That encounter confirmed the existence of Jovian
lightning, which had been theorized for centuries. But when the venerable
explorer hurtled by, the data showed that the lightning-associated radio
signals didn't match the details of the radio signals produced by lightning
here at Earth.
In a new paper published in
Nature today, scientists from NASA's Juno mission describe the ways in which
lightning on Jupiter is actually analogous to Earth's lightning. Although, in
some ways, the two types of lightning are polar opposites.
"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 of NASA's Jet
Propulsion Laboratory in Pasadena, California, 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 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."
Enter Juno, which has been
orbiting Jupiter since July 4, 2016. Among its suite of highly sensitive
instruments is the Microwave Radiometer Instrument (MWR), which records
emissions from the gas giant across a wide spectrum of frequencies.
"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, and we are searching at a radio frequency that
passes easily through Jupiter's ionosphere."
While the revelation showed
how Jupiter lightning is similar to Earth's, the new paper also notes that
where these lightning bolts flash on each planet is actually quite different.
"Jupiter lightning
distribution is inside out relative to Earth," said Brown. "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."
Why do lightning bolts
congregate near the equator on Earth and near the poles on Jupiter? Follow the
heat. Earth's derives the vast majority of its heat externally from solar
radiation, courtesy of our Sun. Because our equator bears the brunt of this
sunshine, warm moist air rises (through convection) more freely there, which
fuels towering thunderstorms that produce lightning.
They do provide some warmth,
heating up Jupiter's equator more than the poles -- just as they heat up Earth.
Scientists believe that this heating at Jupiter's equator is just enough to
create 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,
driving convection and 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?"
The data set of more than
1,600 signals, collected by Juno's Waves instrument, is almost 10 times the
number recorded by Voyager 1. Juno detected peak rates of four lightning
strikes per second (similar to the rates observed in thunderstorms on Earth)
which is six times higher than the peak values detected by Voyager 1.
"These discoveries
could only happen with Juno," said Scott Bolton, principal investigator of
Juno from the Southwest Research Institute, San Antonio. "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. "
NASA's Juno spacecraft will
make its 13th science flyby over Jupiter's mysterious cloud tops on July 16.
NASA's Jet Propulsion Laboratory, 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 NASA's Science Mission Directorate.
The Microwave Radiometer
instrument (MWR) was built by JPL. The Juno Waves instrument was provided by
the University of Iowa. Lockheed Martin Space, Denver, built the spacecraft.
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