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.
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. But even though
Jupiter's atmosphere derives the majority of its heat from within the planet
itself, this doesn't render the Sun's rays irrelevant. 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. "Even though
we see lightning near both poles, why is it mostly recorded at Jupiter's north
pole?"
In a second Juno lightning paper published
today in Nature Astronomy,
Ivana Kolmašová of the Czech Academy of Sciences, Prague, and colleagues,
present the largest database of lightning-generated low-frequency radio
emissions around Jupiter (whistlers) to date. 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.
More information on Juno can be found at: https://www.nasa.gov/juno - https://www.missionjuno.swri.edu
More information about Jupiter can be found at: https://www.nasa.gov/jupiter
The public can follow the mission on Facebook and Twitter at: https://www.facebook.com/NASAJuno - https://www.twitter.com/NASAJuno
News Media Contact
DC Agle
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-9011
agle@jpl.nasa.gov
JoAnna Wendel
NASA Headquarters, Washington
202-358-1003
joanna.r.wendel@nasa.gov
Richard Lewis
University of Iowa, Iowa City
319-384-0012
richard-c-lewis@uiowa.edu
Deb Schmid
Southwest Research Institute, San Antonio
210-522-2254
dschmid@swri.org
Source: JPL-Caltech/News
