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Flagstaff, Arizona

PICTURE four strange worlds, siblings in space. On one, a giant volcano has
just spattered a region the size of England with bright yellow debris. This
world glows with a ghostly light, given off by its shroud of ionised gases. The
second world is covered in a shell of ice, but hiding beneath could be a warm
ocean that could shelter exotic life forms. The third, a seemingly calmer globe,
conceals a turbulent interior that whips up huge magnetic fields. But the fourth
is an unchanging land, which has been dead and cold for billions of years.

These disparate visions of the four largest moons of Jupiter—Io,
Europa, Ganymede and Callisto—are the latest to come from the Galileo
spacecraft, which reached Jupiter in December 1995 after a six-year odyssey
through the Solar System. Six months before its arrival, the NASA spacecraft
dispatched the Galileo atmospheric probe, which entered Jupiter’s atmosphere and
gave us our first direct view of conditions there. The very same day the Galileo
orbiter went into orbit round Jupiter, becoming the first spacecraft to enter a
giant planet’s orbit.

The orbiter has now been circling Jupiter for more than a year, dividing its
attention between the planet, its magnetic field and its moons. The spacecraft
is making close flybys of the four largest moons at altitudes as low as 250
kilometres so naturally scientists hope that the information being sent back to
Earth will help to answer an intriguing puzzle: how did these moons turn out be
so different?

The four worlds were discovered by the astronomer and mathematician Galileo
in 1610 and are known as Jupiter’s Galilean moons. They were born 4.5 billion
years ago from the cloud of gas and dust that surrounded the young Jupiter.
Until this year, we owed most of our knowledge about them to the Voyager
spacecraft flybys of 1979, which beamed back magnificent close-up pictures.

But those Voyager spacecraft had their limitations. They did not pass close
enough to the satellites to measure details of their gravity or how they respond
to Jupiter’s powerful magnetic field. Nor could they tell us much about the
composition of the moons. Lastly, the Voyager visits couldn’t throw light on any
long-term changes produced by Io’s active volcanoes.

Thanks to the Galileo mission, however, we can now get to know Jupiter’s
realm better. The orbiter carries an impressive array of instruments, including
a high-resolution camera and an infrared spectrometer to map surface
composition. Other instruments measure the dust, charged particles and magnetic
fields around Jupiter and its moons. A close watch on how Galileo is buffeted by
the gravity of Jupiter and its satellites had also provided clues about what
lies deep inside these worlds.

However, Galileo’s passage to Jupiter was not smooth. In 1991, Galileo’s
umbrella-like main antenna failed to open. The antenna was to have beamed
Galileo’s findings home at a rate of 130 000 bits per second. The antenna’s
malfunction means making do with the spacecraft’s backup antenna, which
transmits data at the excruciatingly slow rate of 100 bits per second. As the
amount of information that can be sent back to Earth is limited, Galileo
transmits only the highest priority observations.

After a year in orbit, Galileo has returned close-up data on all the Galilean
satellites. And some of the pictures we have received are truly
spectacular—none more so than images of volcanic activity on Io. We
already knew from the Voyager images that the moon is wracked by intense
volcanism. This perpetual activity is driven by tidal heating, generated by
Jupiter’s gravity. As Io travels round Jupiter in its slightly elliptical orbit,
the planet squeezes and stretches the moon, and this heats its interior.

Scientists know from Earth-based observations that every second, Io sheds
about a tonne of material—mostly oxygen and sulphur—into space. Most
of the material is ionised and forms a dense plasma that lies in Io’s orbit. Io
ploughs into this plasma trail at about 70 kilometres per second, and this
strips away even more material.

In December 1995, the Galileo orbiter flew within 890 kilometres of Io. This
was the first time that a spacecraft had flown through this plasma maelstrom.
Its instruments showed that Jupiter’s magnetic field is strongly disturbed close
to Io. We do not yet know if this means that Io has its own magnetic field, or
if the ionised gases in Io’s path are distorting the field. Galileo also
measured intense beams of electrons flowing north and south along Jupiter’s
magnetic field lines.

Galileo has also helped to confirm theories about Io’s core. Voyager
measurements of Io’s size and gravity had already told us that Io had a slightly
higher density than our own Moon, suggesting a high iron content. The intense
heat inside Io would make the rock fluid, making it easy for the iron to sink to
the centre. The existence of this core was confirmed by using precise gravity
measurements to work out how much Io is distorted by its own rotation and by
Jupiter’s gravity.

In June 1996, Galileo took its first images of Io. Between the Voyager 1 and
Voyager 2 flybys, four months apart, volcanic activity had dramatically changed
Io’s surface. The debris from three volcanoes—Pele, Surt and Aten—
had coloured vast areas of the surface, many hundreds of kilometres in diameter.
So what would 17 years of volcanism have done to the surface?

Surprisingly, relatively little has changed. The deposits from Surt and Aten
have disappeared, and the Pele deposits have changed less in the past 17 years
than in the four months between the Voyager encounters. So it seems that some of
the big volcanoes, like Surt and Aten, are spewing out volatile material that
disappears over time. Others, like the more active Pele, continually recoat the
same areas with the same stuff. So although patterns on Io’s surface may alter
in the short term, there isn’t much change in the long term.

True violence

Galileo has spotted one major exception to this, in a large area surrounding
a volcano called Ra Patera. The Hubble Space Telescope had already revealed that
a volcanic eruption in 1994 or 1995 had covered the area in bright yellow
material. Now Galileo’s images, ten times sharper than Hubble’s, have revealed
the true violence of this eruption. Bright volcanic debris has blanketed a
region the size of England, while a new dark lava flow covers an area the size
of Yorkshire.

On a smaller scale, changes produced by dozens of less violent eruptions are
apparent everywhere on Io. There are new lava flows and new haloes of debris
around existing volcanoes. In other places, new volcanoes appear to have burst
through previously featureless plains. More bizarrely, one of the most active
volcanoes on Io, Prometheus, has moved 75 kilometres west of its original site
since the Voyager flybys. For some reason, a new vent had opened, probably
tapping the same underground reservoir of lava that fuelled the original
volcano.

Galileo’s infrared detectors have also spied on Io’s volcanoes. These
detectors reveal the glow of more than a dozen hot spots, sources of lava that
are at temperatures similar to ones on Earth but with a much greater total heat
output. In the darkness of Jupiter’s shadow, Galileo’s visible and infrared
images show Io glowing. This is partly due to heat from the volcanoes, but it is
also due to the plasma round Io bombarding the moon’s tenuous atmosphere and
volcanic plumes, which creates a diffuse, aurora-like light.

If there was a prize for the strangest moon in the Solar System, Europa would
be on the shortlist. Spectra taken from Earth show that the moon has a bright
surface made of water ice. We also know that Europa has a high density, so most
of its interior must be made of something heavier than ice. With the Voyager
flybys, pictures revealed that though the icy surface was smooth, it was cracked
like an eggshell on every scale. Most intriguing, Europa appeared to be entirely
free of impact craters, suggesting that movement of the ice has wiped them out
over time. So was Europa geologically alive beneath the ice? And might the ice
be molten a few kilometres down, forming a warm ocean that could perhaps even
support life?

Galileo’s images have fuelled further speculation. They show that some of the
cracks on the moon’s surface have soft, dark edges, while others are punctuated
by diffuse, dark dots. These dark smudges might be eruptions of dirty water,
seeping up through the cracks from an underlying ocean. In other areas, there
are smooth patches that may be frozen flows of water. Elsewhere, parts of
Europa’s surface have clearly fractured, with darker ice filling the gaps
between them. As much of the surface is free of impact craters, some of this
activity must be very recent. The mobility of the surface might be evidence that
the ice floats on a layer of water.

However, it could be that many of these features form if the ice just below
the surface is simply a little warmer and softer and undergoes only occasional
local melting. Since Galileo’s images show several impact craters in some areas,
some of the surface has been geologically inactive for long enough—at
least 10 million years, but perhaps much longer—to accumulate these scars.
Such long-term inactivity may be hard to reconcile with the idea of water
lurking not far below the surface.

Ocean or no, Europa has a surface unlike any other in the Solar System.
Galileo reveals that most of the “cracks” turn out to be low, very regular,
ridges. For some reason, these ridges are often double, like railway tracks
crisscrossing the surface. Ridges overlap earlier ridges in tangled confusion.
Elsewhere, the neat ridges have collapsed into a chaos of jumbled blocks.

While Europa’s surface intrigues researchers, it is Ganymede’s interior that
confounds them. The satellite is immersed in Jupiter’s immense magnetic field,
which sweeps past at 170 kilometres per second. Now Galileo tells us that the
moon has a magnetic field of its own making. This is the first known example in
the Solar System of a magnetic field within a field. At Ganymede’s surface, the
field is pretty substantial—about 10 per cent as strong as that at the
Earth’s surface.

More clues about Ganymede’s interior came from analysing how its gravity
influenced the route of Galileo’s close flybys. Ganymede has a relatively low
density, and its insides must be half rock and half ice. Galileo showed that the
rock was concentrated at the centre of the moon. So Ganymede must once have been
hot enough for the ice to soften or melt, allowing the rock to sink to the
centre. The gravity data also hinted that there may be a small iron core beneath
the rock, which could be the source of the magnetic field.

What is remarkable about the magnetic field is that it probably requires
vigorous movement of electrically conducting material in Ganymede’s interior,
either in a layer of water within the ice, or in the possible iron core. It’s
not clear where the heat to produce this movement could come from. Ganymede lies
farther away from Jupiter than Io or Europa, so the planet’s gravitational pull
is weaker and the moon experiences far less tidal heating. And, unlike Europa,
Ganymede is quite heavily cratered, suggesting that its surface has not changed
for several billion years. If nothing has happened to the surface for that long,
how can the interior still be active? One intriguing possibility is that we have
been wrong about the age of Ganymede’s surface. The impacts may be occurring
more rapidly than we thought, in which case the observed craters may have
appeared in the last billion years.

Some theories about how the orbits of Jupiter’s moons have changed and
influenced each other suggest that about a billion years ago, the gravitational
pull of Io and Europa may have temporarily forced Ganymede’s orbit to become
elliptical. Its distance from Jupiter would have varied considerably as it
orbited, and the resulting changes in gravitational pull would have created
strong tidal heating. Ganymede could then have had a rejuvenating burst of
activity whose afterglow—a deep interior, warm and active enough to
produce a magnetic field—could still be felt now.

But what about Ganymede’s surface? Voyager showed a relatively heavily
cratered surface composed of dark ice cut by broad swaths of grooved
terrain—younger, cleaner ice sculpted into myriad parallel ridges and
grooves. Galileo’s images show that the ridges and grooves are made up of still
finer parallel fractures. The precise nature of this intense geological faulting
is still uncertain, however. Some of the older faults have been completely
erased by a newer, smoother surface that may be a result of water erupting from
the interior.

Finer details of Ganymede’s surface were revealed by Galileo’s spectroscopic
instruments, which found ozone near the moon’s poles. The instruments also found
possible evidence of surface deposits of dry ice, and a tenuous atmosphere of
hydrogen escaping from the moon. Ganymede is turning out to be a richer and more
complex world than we ever imagined.

Dark icy surface

While Io, Europa and Ganymede are remarkable for their energetic and varied
geological activity, Callisto has almost no geological activity. Voyager showed
that its dark icy surface is covered in impact craters, measuring from about
2000 kilometres in diameter down to just a kilometre or two. Strangely, the more
detailed Galileo images showed that there are hardly any craters less than 10
kilometres across. Old, worn-down rims of large craters protrude from a smooth,
dark coating which is itself almost uncratered. Either the dark coating is
younger than most of the impact craters or, more likely, it is continually on
the move so that it erases small craters. But we have no clear idea about what
is continually stirring up the surface.

An even better picture of Callisto and the other moons will emerge before the
Galileo mission officially ends in November 1997. The grand finale should be
another flyby of Europa. But plans are already under way to continue Galileo’s
exploration for another two years, concentrating on Europa. If all goes well,
the mission will end in spectacular fashion with a close flyby of Io in late
1999.

The data from the flybys should help researchers to understand why Jupiter’s
moons are so strangely different. Tidal heating may turn out to have been the
biggest factor. Acting most powerfully closest to Jupiter, tidal heating has
thoroughly cooked Io, leaving a sulphurous, volcano-riddled world. It gently
warms more distant Europa, jostling its icy surface and perhaps melting an
underground ocean. Farther out, it may once have shot a heat pulse at Ganymede
that reshaped its surface and energised a magnetic field. Only distant Callisto
has remained untouched.

The Galileo Web site at www.jpl.nasa.gov/galileo has more information
about the Galileo mission, including the latest pictures.