“There was snow and hail and rain and lightning and thunder; and there were
rolling mists, travelling with incredible velocity. It was dark, awful, and
solitary to the degree; there were mountains above mountains, veiled in angry
clouds; and there was such a wrathful, rapid, violent tumultuous hurry
everywhere as rendered the scene unspeakably exciting and grand.” Travels in
Italy by Charles Dickens
WHILE EARTH has no shortage of vivid scenes like these, ease of travel is
gradually turning the exotic into the mundane. Soon there’ll be nowhere intrepid
left to go. Unless, that is, you are prepared to hike a little farther than most
sightseers.
Travel 400 000 kilometres, for instance, and you could watch Earth rise above
the Moon’s grey horizon. This distant view of our planet was enough to drive
some astronauts to religion. “That beautiful, warm, living object looked so
fragile, so delicate, that if you touched it with a finger it would crumble and
fall apart,” wrote James Irwin after Apollo 15.
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But that’s just a taster. If you travelled much, much farther, to Jupiter’s
moon Io, you could witness one of the most extraordinary sights in the Solar
System. The pictures beamed back from Jupiter over the past few years by NASA’s
Galileo spacecraft show that Io is truly spectacular. “Io is a fabulous place,”
says Lou Frank, a plasma physicist at the University of Iowa. “A beautiful,
violent environment.”
For a start, it is the most volcanically active place in the Solar System, as
Voyager 1 and 2 revealed when they swung by in 1979. The energy that feeds this
volcanic fervour comes from the gravitational fields of Jupiter and two of its
other large moons —Ganymede and Europa—which knead Io as if it were
a giant ball of dough. On Earth, the Sun and Moon together create ocean tides up
to 18 metres in height. But Io’s solid crust regularly rises and falls by almost
100 metres—about the height of a 30-storey building.
Wracked by these convulsions, much of Io’s core is white hot and under
fantastic pressure. Wherever there’s a weakness in the crust, lava erupts,
squirting jets of dust and sulphur dioxide gas high into space.
But since the heady days of Voyager, it has been impossible to catch more
than a glimpse of these sights. Io is more than 700 million kilometres away, and
even Earth’s most powerful telescopes are dazzled by the bright sunlight
reflected from Jupiter, making their images blurry and poor.
All that changed when Galileo arrived at Jupiter in 1995. Since then, the
space probe has circled the system, sensors bristling, sometimes pointing them
at Europa or Ganymede, at other times swooping low over the surface of Io
(91av, 5 April 1997, p 42).
“It has taken some truly fantastic
photos,” says Frank, who is the principal investigator on Galileo’s plasma
science experiments.
These images revealed that Io is even more tormented than anyone had
realised. The moon has more than 100 active volcanoes. Huge mountains tower over
a landscape splattered with rocks and ice of every hue. Clusters of
vents—some the size of California—belch out gas and white-hot lava,
and the jets stretch as much as 400 kilometres up into the sky.
It’s a world of bizarre extremes. The largest of Io’s volcanoes, Loki, is the
most powerful in the Solar System, generating more heat than all Earth’s active
volcanoes combined. Yet while Loki and the other volcanoes bubble with molten
rock and sulphur, most of the moon’s surface is frozen. The sulphur dioxide gas
pumped out by these volcanic vents condenses as it hits the freezing atmosphere.
All round the active volcanoes, pale blue snow drifts down onto the rocky
landscape.
Dancing lights
Though spectacular, these scenes are just a prelude. As the moon drifts
behind Jupiter into darkness, a light show begins that would inspire even the
most jaded space tourist. “The first thing you’d notice,” says Frank, “is that
the atmosphere around you is glowing—a haze of lights dancing above your
.”
You’d be bathed in the ghostly red and green glow of Io’s aurora. Across the
moon’s equator, the sky would be lit by massive columns of electric-blue light
that resemble giant fluorescent tubes—one end on the surface, the other
stretching away into the blackness of space.
In May last year, Galileo dodged behind Io to take pictures of this light
fantastic. The gases around it glowed green, red and blue, with the red lights
spread around the moon’s edges. The green glow was concentrated on the night
side of Io while the blue lights lit up at the moon’s equator.
Where do Io’s lights come from? On Earth, the flickering glow of the northern
and southern lights is created by the solar wind, the plasma of charged
particles sprayed out by the Sun. These particles are funnelled towards the
poles by Earth’s magnetic field, and when they collide with our atmosphere, they
smash so hard into molecules of nitrogen and oxygen gas that the molecules
twinkle with light.
Io, however, is shielded from the solar wind by Jupiter’s huge magnetic
field, so the energy for its auroras must come from a different source.
Astronomers believe that Io creates its own “solar wind”, with a little help
from Jupiter. Jupiter’s massive magnetic field—its
magnetosphere—stretches almost 14 million kilometres into space. As Io
orbits, it cuts through this field. “This motion sets up a potential difference
across the surface of Io,” says Frank, “half a million volts, maybe a little
more. That has to short out somewhere.”
Io’s rocky surface is a poor conductor, so the easiest way to discharge this
massive potential is along the curving magnetic field lines that link Io to
Jupiter. A stream of electrons—somewhere between a hundred thousand and a
million amps—shoots down to Jupiter from the negatively charged side of
Io. Similarly, a return stream races up from Jupiter to the positively charged
side of Io, the side farthest from Jupiter (see Diagram).
This two-lane electrical highway, or “flux tube”, acts just like the solar
wind on Earth. As the electrons flow through the atmosphere around Io, they
crash into molecules of gas, and the energy from the collisions make these gases
light up like an enormous fluorescent lamp.
It was long believed that Io had little atmosphere to speak of. And without
much gas, the fluorescent lamp would flicker and die. But the particle detector
on Galileo told a different story. “When Galileo went close by Io, it skimmed
through a fairly dense atmosphere and ionosphere,” says Frank. “That came as a
big surprise.”
But just what is this atmosphere like? Every element emits and absorbs
specific colours, so one reason why astronomers are so keen to enjoy the moon’s
light show is that it reveals much about the clouds of gas that surround Io. “We
think the green glow is caused by sodium,” says Paul Geissler of the Lunar and
Planetary Laboratory at the University of Arizona, a member of Galileo’s imaging
team. “The red one is probably due to oxygen, and we think the bright blue is
from sulphur dioxide.”
The lights also reveal much about the dynamics of the moon’s atmosphere.
“They help us understand the atmosphere and the processes that generate the
emissions,” says Geissler. For instance, the green sodium glow is concentrated
on just one face of the moon. Astronomers believe that the motion of Jupiter’s
magnetic field relative to Io is sweeping sodium atoms away from the moon’s
atmosphere, creating a kind of luminous green wake.
The distribution of the lights also reveals that the flux tube isn’t the only
source of their power. The electrons and ions that pour out of Io’s volcanoes
have created a giant plasma cloud around Jupiter, shaped into a torus by the
planet’s magnetic field. As Io orbits, it cuts through this cloud. The red glow
of oxygen around Io’s poles, says Geissler, shows that it is being excited by
the electrons in the torus.
After a few experiments in the lab back on Earth, Geissler has even been able
to model some of these emissions. From their colour and intensity, he can
estimate and map the concentrations of gases at certain points on Io. “Its
atmosphere is really surprising,” he says. “It is patchy and changeable.” In
some places, near volcanoes for instance, the atmospheric pressure may be up to
a thousand times higher than at other, less volcanic spots.
From Geissler’s maps, it is clear that the blue emissions, concentrated in a
region along the moon’s equator, match the distribution of Io’s most active,
sulphur dioxide-spewing volcanoes. And this could explain one of the moon’s
mysteries.
Usually, the volcanic plumes on Io are made visible by light bouncing off the
huge amounts of dust particles they contain. These particles scatter sunlight,
giving the sulphurous jets a pale sheen. But dense patches of sulphur dioxide
seem to cover a greater fraction of the surface than can be explained by the
plumes that are visible in sunlight. This prompted Torrence Johnson, an
astronomer at the Jet Propulsion Laboratory at Pasadena, to suggest in 1995 that
giant invisible “stealth plumes” could be pumping out the extra gases.
Geissler set out to simulate the blue emissions seen by Galileo using a
computer model. He discovered that he needed to include the sulphurous plumes of
just five of Io’s volcanoes to reproduce the patterns seen on Io. “One
particular volcano, Acala, was the biggest culprit,” says Geissler. “That’s
particularly interesting because we’ve never actually seen Acala’s plume in
ܲԱ.”
Out of the darkness
But what really excited Geissler and his team was a sequence of images taken
by Galileo while Io was in Jupiter’s shadow. This showed that the pattern of
light emission on Io can change dramatically.
In the first set of pictures taken about 11 minutes after Io had entered
Jupiter’s shadow, all the gases glowed fairly brightly. But in a set recorded
about 40 minutes later, the astronomers noticed that the red and green glow had
faded by almost a third. Not only that, the patterns of blue luminescence near
Io’s equator had brightened by about the same amount.
“We’re not sure why that is,” Geissler admits, “but we think it’s possible
that the conductivity of Io’s ionosphere is decreasing.” During daylight,
charged particles in the ionosphere play an important role in conducting current
between Jupiter and its moon. But without sunlight, Io’s ionosphere cools
rapidly. As temperatures plummet, Geissler believes the concentration of ions
and electrons in the ionosphere drops, perhaps because they recombine and
recondense on the surface as frost, or escape from the atmosphere altogether.
Since the conductivity of the ionosphere is reduced, the current has to find an
alternative path. And that path seems to be via the volcanic plumes.
While the rest of the ionosphere is cooling down, the volcanoes continue to
pump out hot gases high into space. So the currents are probably carried by the
ions and electrons in the plumes. “It would be absolutely spectacular to see it
from the surface of Io,” says Geissler. “The blue glow would extend all the way
down the plumes, down to the volcano.” And as the night wears on, these blue
lights would get brighter and brighter.
And there’s more. “Now face towards Jupiter,” says Frank. “From Io, Jupiter
would look enormous, it would swamp your vision.” Nevertheless, Io leaves its
mark on the giant planet. Where the flux tube from Io touches down on Jupiter,
it leaves an intense bright red glow, a spot about 2000 kilometres across that
follows Io as it orbits. Jupiter rotates faster than Io, so the spot moves
across the clouds at about 5 kilometres per second. If you could look up at this
spot from somewhere inside Jupiter’s atmosphere “you’d see it fly like a meteor
from one horizon to the other”, says John Clarke, an astronomer at the
University of Michigan at Ann Arbour.
And at the planet’s poles you’d see Jupiter’s own magnificent auroras, ovals
and spots of light that flicker like flames. Its auroras are very
bright—between a hundred and a thousand times more intense than the ones
we have on Earth. “The light is a deep ruby red colour,” says Clarke. “Like a
flame with a kind of violet tint.”
The light’s source is the myriad ions and electrons that race around the
planet out beyond the torus. They are accelerated to huge speeds by Jupiter’s
magnetic field and eventually follow the field lines back in towards Jupiter.
When they smash into its atmosphere—which is mainly hydrogen—they
create a wonderful sight.
Thanks to Galileo, we have taken our first vicarious steps towards
experiencing the most spectacular scenes in the Solar System. But it’s unlikely
humans will witness the delights of Io for themselves anytime soon. The powerful
radiation belts that surround Jupiter would roast human flesh in an instant.
“They’re very, very intense,” says Frank. “The views would be
magnificent—you just wouldn’t enjoy them for very long.”
So if you’re considering signing up for a future interplanetary tour, here’s
something to bear in mind: “See Io and die” will mean precisely that.
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Further reading:
Galileo imaging of atmospheric emissions from Io by Paul
Geissler and others, Science, vol 285, p 870