Jupiter’s southern hemisphere is shown in this image from NASA’s Juno mission. New observations by NASA’s NuSTAR reveal that auroras near both the planet’s poles emit high-energy X-rays, which are produced when accelerated particles collide with Jupiter’s atmosphere. Credits: Enhanced image by Kevin M. Gill (CC-BY) based on images provided courtesy of NASA/JPL-Caltech/SwRI/MSSS
The planet’s auroras are known to produce low-energy X-ray light. A new
study finally reveals higher-frequency X-rays and explains why they eluded
another mission 30 years ago.
Scientists have been studying Jupiter up
close since the 1970s, but the gas giant is still full
of mysteries. New observations by NASA’s NuSTAR space observatory
have revealed the highest-energy light ever detected from Jupiter. The light,
in the form of X-rays that NuSTAR can detect, is also the highest-energy light
ever detected from a solar system planet other than Earth. A paper in the journal
Nature Astronomy reports the finding and solves a decades-old mystery: Why
the Ulysses mission saw no X-rays when
it flew past Jupiter in 1992.
X-rays are a form of light, but with much higher energies and shorter
wavelengths than the visible light human eyes can see. NASA’s Chandra X-ray
Observatory and the ESA (European Space
Agency) XMM-Newton observatory have
both studied low-energy X-rays from Jupiter’s
auroras – light shows near the planet’s north and south
poles that are produced when volcanoes on Jupiter’s moon Io shower the planet
with ions (atoms stripped of their electrons). Jupiter’s powerful magnetic
field accelerates these particles and funnels them toward the planet’s poles,
where they collide with its atmosphere and release energy in the form of light.
Electrons from Io are also accelerated by the planet’s magnetic field,
according to observations by NASA’s Juno spacecraft, which
arrived at Jupiter in 2016. Researchers suspected that those particles should
produce even higher-energy X-rays than what Chandra and XMM-Newton observed,
and NuSTAR (short for Nuclear Spectroscopic Telescope Array) is the first
observatory to confirm that hypothesis.
NuSTAR detected
high-energy X-rays from the auroras near Jupiter’s north and south poles.
NuSTAR cannot locate the source of the light with high precision, but can only
find that the light is coming from somewhere in the purple-colored regions. Credits:
NASA/JPL-Caltech
“It’s quite challenging for planets to generate X-rays in the range that
NuSTAR detects,” said Kaya Mori, an astrophysicist at Columbia University and
lead author of the new study. “But Jupiter has an enormous magnetic field, and
it’s spinning very quickly. Those two characteristics mean that the planet’s
magnetosphere acts like a giant particle accelerator, and that’s what makes
these higher-energy emissions possible.”
Researchers faced multiple hurdles to make the NuSTAR detection: For
example, the higher-energy emissions are significantly fainter than the
lower-energy ones. But none of the challenges could explain the nondetection by
Ulysses, a joint mission between NASA and ESA that was capable of sensing higher-energy X-rays than NuSTAR. The
Ulysses spacecraft launched in 1990 and, after multiple mission extensions,
operated until 2009.
The solution to that puzzle, according to the new study, lies in the
mechanism that produces the high-energy X-rays. The light comes from the
energetic electrons that Juno can detect with its Jovian Auroral Distributions
Experiment (JADE) and Jupiter Energetic-particle Detector Instrument (JEDI),
but there are multiple mechanisms that can cause particles to produce light.
Without a direct observation of the light that the particles emit, it’s almost
impossible to know which mechanism is responsible.
In this case, the culprit is something called bremsstrahlung emission. When
the fast-moving electrons encounter charged atoms in Jupiter’s atmosphere, they
are attracted to the atoms like magnets. This causes the electrons to rapidly
decelerate and lose energy in the form of high-energy X-rays. It’s like how a
fast-moving car would transfer energy to its braking system to slow down; in
fact, bremsstrahlung means “braking radiation” in German. (The ions that
produce the lower-energy X-rays emit light through a process called atomic line
emission.)
Each light-emission mechanism produces a slightly different light profile.
Using established studies of bremsstrahlung light profiles, the researchers
showed that the X-rays should get significantly fainter at higher energies,
including in Ulysses’ detection range.
“If you did a simple extrapolation of the NuSTAR data, it would show you
that Ulysses should have been able to detect X-rays at Jupiter,” said Shifra
Mandel, a Ph.D. student in astrophysics at Columbia University and a co-author
of the new study. “But we built a model that includes bremsstrahlung emission,
and that model not only matches the NuSTAR observations, it shows us that at
even higher energies, the X-rays would have been too faint for Ulysses to
detect.”
The conclusions of the paper relied on simultaneous observations of Jupiter
by NuSTAR, Juno, and XMM-Newton.
New Chapters
On Earth, scientists have detected X-rays in Earth’s auroras with even
higher energies than what NuSTAR saw at Jupiter. But those emissions are
extremely faint – much fainter than Jupiter’s – and can only be spotted by
small satellites or high-altitude balloons that get extremely close to the
locations in the atmosphere that generate those X-rays. Similarly, observing
these emissions in Jupiter’s atmosphere would require an X-ray instrument close
to the planet with greater sensitivity than those carried by Ulysses in the
1990s.
“The discovery of these emissions does not close the case; it’s opening a
new chapter,” said William Dunn, a researcher at the University College London
and a co-author of the paper. “We still have so many questions about these emissions
and their sources. We know that rotating magnetic fields can accelerate
particles, but we don’t fully understand how they reach such high speeds at
Jupiter. What fundamental processes naturally produce such energetic
particles?”
Scientists also hope that studying Jupiter’s X-ray emissions can help them
understand even more extreme objects in our universe. NuSTAR typically studies
objects outside our solar system, such as exploding stars and disks of hot gas accelerated by the
gravity of massive black holes.
The new study is the first example of scientists being able to compare
NuSTAR observations with data taken at the source of the X-rays (by Juno). This
enabled researchers to directly test their ideas about what creates these
high-energy X-rays. Jupiter also shares a number of physical similarities with
other magnetic objects in the universe – magnetars, neutron stars, and white
dwarfs – but researchers don’t fully understand how particles are accelerated
in these objects’ magnetospheres and emit high-energy radiation. By studying
Jupiter, researchers may unveil details of distant sources we cannot yet visit.
More About the Missions
NuSTAR launched on June 13, 2012. A Small Explorer mission led by Caltech
and managed by JPL for NASA's Science Mission Directorate in Washington, it was
developed in partnership with the Danish Technical University and the Italian
Space Agency (ASI). The telescope optics were built by Columbia University;
NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and DTU. The
spacecraft was built by Orbital Sciences Corp. in Dulles, Virginia. NuSTAR’s
mission operations center is at the University of California, Berkeley, and the
official data archive is at NASA’s High Energy Astrophysics Science Archive
Research Center. ASI provides the mission’s ground station and a mirror data
archive. Caltech manages JPL for NASA.
For more information on NuSTAR, go to: http://www.nasa.gov/nustar and http://www.nustar.caltech.edu/
JPL manages the Juno mission for the principal investigator, Scott J.
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 the agency’s Science Mission Directorate.
Lockheed Martin Space in Denver built and operates the spacecraft.
Follow the Juno mission on Facebook and Twitter, and get more
information about Juno online at: https://www.nasa.gov/juno
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