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.
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| 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. Credit: 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.
Reference:
Kaya Mori et al, Observation and origin of non-thermal hard X-rays from
Jupiter, Nature Astronomy (2022).
DOI: 10.1038/s41550-021-01594-8
Tags:
Space & Astrophysics