Puzzling X-rays from Jupiter
Astronomers using the Chandra X-ray Observatory have spotted a
mysterious pulsing x-ray beacon near the north pole of the giant planet.
March 7, 2002: Every 45 minutes a gigawatt pulse of x-rays courses
through the solar system.
Astronomers are accustomed to such things. Distant pulsars and black
holes often bathe the galaxy with blasts of x-radiation. But this time
the source isn't exotic and far away. It's right here in our own solar
system.
"The pulses are coming from the north pole of Jupiter," says Randy
Gladstone, a scientist at the Southwest Research Institute and leader of
the team that made the discovery using NASA's orbiting Chandra X-ray
Observatory.
"We weren't surprised to find x-rays coming from Jupiter," he continued.
Other observatories had done that years ago. The surprise is what
Chandra has revealed for the very first time: the location of the beacon
-- surprisingly close the planet's pole -- and the regular way it
pulses.
NASA's Einstein x-ray satellite first spotted Jupiter's x-ray glow in
1979. No one looked again for many years until researchers (Gladstone
among them) pointed the German x-ray observatory ROSAT toward Jupiter in
1992. The glow was still there.
Scientists wondered ... what was it?
The x-rays came mostly from Jupiter's northern hemisphere, but the
Einstein and ROSAT maps weren't crisp enough to reveal exactly where.
Some researchers figured they were seeing x-ray emissions from powerful
auroras.
Above: Every 45 minutes an x-ray source blinks near Jupiter's north
magnetic pole. This animation, based on data from the Chandra X-ray
Observatory, shows the hot spot pulsing 15 times during one complete
10-hour rotation of the giant planet.
Above: NASA's Voyager 1 spacecraft snapped this photo of an active
volcano on Io. Ionized sulfur and oxygen from such volcanoes feed
Jupiter's auroras.
(click to
enlarge)
Indeed, says Gladstone, Jupiter has "Northern Lights" just as Earth does
-- only on a different scale. Jupiter's auroras are hundreds to
thousands of times more powerful than our planet's. Furthermore, the
glowing rings around Jupiter's magnetic poles are twice the diameter of
Earth itself!
Auroras happen on both worlds when electrons and ions rain down on the
upper atmosphere. Such particles are guided by lines of magnetic force
toward the poles where they slam into air molecules and cause them to
glow.
An important difference between auroras on Earth and Jupiter concerns
the source of the charged particles. On our planet, most of the raining
electrons and ions come from the solar wind or from our planet's
ionosphere. On Jupiter, many of them come from volcanoes: hot belching
vents on Jupiter's moon Io fill the giant planet's magnetosphere with
ionized sulfur and oxygen. Ions from Io are accelerated by local
electric fields toward Jupiter's auroral zone.
When Gladstone and colleagues trained the Chandra X-ray Observatory on
Jupiter they expected to find the planet's northern x-rays coming from
its giant auroral ring. After all, the auroral ring of our own planet is
an x-ray source -- Jupiter's would likely be the same.
"We used Chandra's High Resolution Camera to image the planet during a
10 hour period on Dec. 18, 2000," says Ron Elsner, an x-ray astronomer
at the NASA Marshall Space Flight Center who worked with Gladstone. "We
hoped it would pinpoint the x-ray source better than earlier satellites
had managed."
(click on image to
enlarge)
And indeed it did. But the new image was a surprise. Chandra revealed
that most of the x-rays came from a hot spot located very close to
Jupiter's north magnetic pole -- not from the auroral ring itself.
Furthermore, it pulsed!
Above: A composite image of Jupiter, its glowing auroral ring (blue),
and a north-polar x-ray pulse (pink). Gladstone notes: "The x-rays we
detected were 'soft' -- less energetic than 1 keV and less penetrating
than the mildest of medical x-rays." It poses no danger to astronauts or
Earthlings.
Above: Jupiter's magnetosphere is vastly bigger than the giant planet
itself. Jupiter is the black dot in the middle of this false-color
image, which shows the distribution of charged particles trapped inside
the magnetic field.
(click to
enlarge)
"The 45-minute pulsations are very mysterious," adds Elsner. They're not
perfectly regular like a signal from E.T. might be; the period drifts
back and forth by a few percent. "This is a natural process," he adds,
"we just don't know what it is...."
While the researchers were using Chandra to observe Jupiter, two NASA
spacecraft -- Cassini and Galileo -- were near the giant planet. Galileo
was deep inside Jupiter's magnetic field, while Cassini was outside
sampling the solar wind. Neither craft detected 45-minute variations in
their surroundings, such as plasma waves or surges of energetic
particles, "although such variations have been detected by Galileo at
other times," notes Gladstone. Galileo has also picked up 1 to 200
kHz-frequency radio bursts that come and go with a 45-minute period, as
did the Ulysses spacecraft when it flew by Jupiter in 1992.
"Maybe Jupiter's magnetic field, when it gets hit by a solar wind gust,
rings like a bell with a 45-minute period," speculates Gladstone. It
would make an impressive bell indeed: Jupiter's magnetic field is the
biggest thing in the solar system -- larger even than the Sun.
Or perhaps, he continued, "x-ray producing ions might be bouncing back
and forth between Jupiter's north and south poles." The poles are
connected by magnetic field lines that take some fast-moving particles
about 45 minutes to traverse. It's possible that Jupiter's south pole is
also an x-ray hot spot, blinking at the same rate as the north -- but no
one knows because the south pole is not as easy to see from Earth.
One thing seems clear: The fact that the known hot spot is so close to
Jupiter's north magnetic pole means that Io does not feed it. "There are
no magnetic field lines that connect Io to the north pole," adds
Gladstone, "so we have to consider other sources of ions -- like the
solar wind."
Solving the puzzle will require more data. "The next step is to gather
some x-ray spectra," he says. "If we see spectral lines from volcanic
elements like sulfur and oxygen, then we'll know that Io is involved --
even if we don't understand how." On the other hand, spectral lines from
carbon and nitrogen would indicate the solar wind as a source of ions.
Until then Jupiter's x-ray beacon -- relentlessly pulsing, and not where
it ought to be -- will likely remain a mystery.
Above: Magnetic field lines connect Jupiter's satellite Io to the giant
planet's auroral zone -- but not to the north polar hot spot.
