Will Jupiter's large moons save us
from extinction? - Le grandi lune di Giove ci salveranno dall'estinzione?
For Galileo Galilei, who saw them first on the night of January 7,
1610, those four lights accompanying Jupiter along its orbit represented just another great
mystery for the astronomy of his time to solve. And that is the past. For us, in our present,
they are an inexhaustible source of information about the evolution of the solar system... But
for those to come, in the very distant future, Jupiter's moons will be four
wonderfully habitable worlds, where they will take refuge to escape the grip of the dying Sun.
Don't believe it? Well...I can't argue with that. No one can know
how things will turn out in a billion years... But good fairy tales help us to live, don't they?
So let's take a tour of the most extraordinary moons in the solar system. In space, and in time.
“roll intro”
Even with just a pair of binoculars, you can see the four main satellites
of Jupiter. They are not planets, because they orbit their parent planet rather than the Sun,
but they are certainly planet-like. They are all roughly 3,000 to 5,000
kilometers in diameter, the smallest being slightly smaller than the Moon,
the largest slightly larger than Mercury, and they are all similar to the terrestrial planets.
Three sisters and a brother, form a motley crew. As siblings, the satellites have a family
resemblance but are all different. Together with Jupiter, they make a miniature planetary system,
and, it is thought, were formed at the same time as Jupiter in much
the same way that our solar system was formed. Fundamentally, they are rocky, but their position
far out in the solar system means that, with one exception, they have retained the original
ices they accrued from the solar nebula. Water and ice have played a considerable
part in their lives, although one of them now lives a dry life of fiery eruptions.
We on the Earth lie in the same orbital plane in which the four satellites revolve around Jupiter,
so to us they seem to line up in a row, moving back and forth from side to side,
sometimes passing in front of Jupiter and casting a shadow on its cloud tops and sometimes hiding,
either obscured behind it or in its shadow. Their periods lie roughly between a day and
two weeks, so they change their positions while you watch them from night to night,
or even from hour to hour through the night. When they move into Jupiter’s
shadow and are eclipsed, their light is extinguished in a few minutes,
fading progressively until what is called the ‘last speck’ of light is extinguished.
The satellites remain, as seen from Earth, as points of light. But we know their structure
from the visits of four space probes. The Voyager spacecraft, 1 and 2,
flew by Jupiter in 1979. Galileo was the first spacecraft to enter into orbit around
Jupiter in 1995 and was able to make extensive observations over eight years. A second probe,
Juno, entered orbit around Jupiter in 2016. These space probes have revealed
the landscapes of Jupiter’s satellites as, in one case, a volcanic desert,
and, in others, an Antarctic continent of rocks, icebergs, and cold, cold oceans.
The “fabulous four'' are called the Galilean satellites, because they were
discovered by Galileo in the first two weeks of 1610, with his new telescope.
On the first night, he saw only three stars, two on one side of Jupiter and one on the other. He
saw three again on the second night, but all of them were on the same side of Jupiter. He
thought at first that they were a chance line of three stars and that the change was due to
the motion of Jupiter through the three. A couple of nights later there were only two
and then a few nights later there were four. They became known as Io, Europa, Ganymede,
and Callisto, all of them in mythology Jupiter’s lovers (of both sexes).
Io, a hellish world
The closest of Jupiter's moons to the planet is Io: the fourth largest satellite in the
solar system and the densest of all at 3.5 grams per cubic centimeter.
The entire surface of Io is coated with black rocks mixed with sulfur – yellow,
orange, and red in its different forms – like a medieval picture of Hell.
There are almost no meteor craters, showing that the surface is young and that geological
processes have erased craters that had formed earlier. However, the surface of
Io is pitted, like an acne-scarred face. The pits are mostly not meteor craters.
They are calderas, among mountains taller than any on Earth, and lava flows, some of
them cold and solid, others hot and flowing. It is a volcanic landscape. In fact, with
more than 300 active volcanoes, Io is the most geologically active object in the solar system.
The ground is made up of solidified mounds of smooth, black lava and loose,
jagged rocks. Where the land has been sectioned by the explosion of a volcano that had excavated
a crater, drifts of yellow or orange ash are exposed. In the still-active areas,
there are vents of steam and sulfurous gas, heat radiating from the red, molten lava oozing from
below. As the closest of the satellites to its parent planet, Io is squeezed and relaxed
by strong tidal forces originating in Jupiter. The heat generated by this repeated working of
Io’s interior has melted its rocks and created about four hundred volcanoes, some so active
they shoot lava 400 kilometers into the sky. The volcanoes on Io were discovered in the
images of the Voyager 1 space probe during its flight through Jupiter’s system of satellites,
As the encounter was ending and the spacecraft was tracking away from Jupiter and its satellites,
behind them, was a particular star that was dimmed by a kind of cloud,
very large, just above Io’s surface. The ‘cloud’, positioned over a heart-shaped
feature on Io, was an ash cloud from a volcano, now named Pele (after the Hawaiian goddess of
volcanoes), and the heart-shaped feature was the volcano itself,
with its slopes, ejecta, and lava flows. And this was the first volcanic activity
outside the Earth that anyone had ever witnessed. Io is only a little larger than our own Moon. It
is slightly ellipsoidal (the shape of a rugby football, or an American football);
Jupiter’s tidal forces have locked on to the long axis, which points towards the parent planet,
so that, like all the Galilean satellites, Io gazes at Jupiter with the same face all
the time. Its rocky surface is all but ice-free (presumably the volcanic heat has evaporated all
the water) and is coated with sulfur: its colors are the colors of different forms of sulfur.
Material ejected by the volcanoes forms a thin atmosphere and feeds into Jupiter’s magnetosphere.
The volcanoes generate lava flows hundreds of kilometers long and hundreds of times the
volumes of recent flows from volcanoes on Earth, bulldozing earlier deposits into deep channels.
The extensive volcanic activity has built around hundred and fifty mountains on Io, the
tallest of which exceed Mount Everest in height. Io’s life is one of stress. Although imprisoned in
the tight grip of Jupiter’s gravitational field, its body is never at rest. It is always in fits,
always fevered, bleeding, and twisting! Perhaps the least sympathetic of the Galilean satellites,
and the one that will give the most trouble for possible future colonization.
Its average diameter is 3640 km, and it orbits Jupiter with a period of 1.77 days at a distance
of 421800 km from the planet's center and 350000 km from the top of its clouds. Its
average temperature is -143°C, with very faint traces of an atmosphere.
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Europa, beautiful and glacial Europa is the second-nearest Galilean
satellite to Jupiter. By contrast to fiery Io, Europa is a world covered with cold ice,
as smooth and completely spherical as a billiard ball. It is almost featureless. Only the nearly
sunken traces of a few recent meteor craters break the monotony of a white, flat landscape.
The ice is cracked into floes, with mineralized water splashing and oozing up through the gaps,
overflowing onto the surface, staining it in a spider’s web of red threads. The red stains are
traces of deposits left by the evaporating water. Europa looks like a world in stasis,
but there is activity below its icy surface. The ice is a kilometer thick, floating on an
ocean of salty water perhaps 5 kilometers deep. The water is warmed from below by geothermal
energy. When the ice floes thrust together, they make hills of ice on the surface, but they are
only a couple of hundred meters high. The landscape here is similar to the
Arctic sea ice around the coasts of northern Canada or Siberia.
Altogether, there is more water on Europa than there is on Earth.
In some future space mission, a lander might settle on the ice and try to penetrate through the
ice layer, perhaps by using a radioactive probe to melt the ice and work its way down through
the meltwater. It would be a suicide mission for the probe, as the water would refreeze above it,
sealing it in. But what would it find? The still waters of Europa run deep,
and it is tempting to imagine that, as it breaks through the lower surface of the ice,
the penetrating probe might shine its lamps on alien oceanic creatures, taking pictures
of them swimming below the maze of ice floes. Okay, these are just dreams for the time being.
And it is perhaps best to play it safe, as safe as these measurements are: trivial but established...
With a diameter of just over 3100 km, Europa is slightly smaller than the Moon and is the
sixth largest satellite and the 15th largest object in the solar system. Its density of 3
grams per cubic centimeter suggests that it is similar in composition to terrestrial planets,
being predominantly composed of silicates. Europa orbits Jupiter with a period of about
three and a half days at a distance of 670900 km. It too with a hemisphere constantly facing
Jupiter. The average temperature on the surface is -170°C. Observations using
the Hubble Space Telescope's onboard spectrograph revealed the presence
of a tenuous atmosphere composed of oxygen. Perhaps the most promising Galilean...who knows!
Ganymede the Big and Callisto the loner
And then, further outward, we find Ganymede, which with a diameter of 5262 km is the
largest of the Galilean moons, indeed the largest moon in the solar system.
And even further away is Callisto, which is almost as big, the third-largest moon in the
solar system, as big as Mercury but a third as massive. This means that they are both much less
dense than planets made of rock and iron. The two moons must be mixed with something
much lighter. That something is water – liquid water and ice. Both Ganymede and
Callisto have rocky surfaces that are cratered, like the Moon and Mercury.
They look like the Moon, particularly Ganymede, which has two sorts of surface. A third of it is
dark in color, with lots of craters (thus, very old). The other two-thirds is lighter in color,
not so cratered (so younger): its peculiarity is that it is laced with grooves and ridges.
The lighter terrain on Ganymede is like the Moon’s maria, caused by the upwelling of molten
material from the interior that flooded lower areas of the surface. The difference is that
the upwelling was not lava, but water melted by an asteroid impact. Callisto is similar,
with parts of its surface lying in ripples, waves frozen by the intense cold. Ganymede
has an iron core that produces a weak magnetic field, but not Callisto.
Their big secret is that several lines of argument point to both satellites having, under their
rocky surface, a liquid ocean of salty water. The ocean hidden in Ganymede is perhaps 1,000
kilometers deep, and like Europa’s ocean holds as much or more water than on Earth.
Callisto’s ocean is only a few hundred kilometers deep.
Like Europa’s oceans, these oceans (if they do indeed exist) may have life swimming in them.
There could well be a greater chance of finding life on the Galilean satellites than on Mars.
But as we have already said, it is better to keep our feet on the ground and stick to the facts.
Ganymede orbits Jupiter in 7.1 days at a distance of 1070400 km, too, with one hemisphere constantly
facing the gas giant. Controversially, it has a very weak oxygen atmosphere,
while the temperature on the surface is -164°C. In contrast, Callisto, the outermost and most
isolated of the four Galileans, has a diameter of 4821 km and orbits in 16.7 days at a distance
from Jupiter of 1,868,000 km. It is also the least dense of the four, with a density
of just 1.8 grams per cubic centimeter. The temperature on the surface averages -153°C,
while the almost nonexistent atmosphere is mostly made up of traces of carbon dioxide.
But a billion years from now... Questo, as we said at the beginning,
is the present… made up of exciting photographs, surveys, and data. A present that tells us about
four worlds of wild and hostile nature. But it is in the future that Io, Europa, Ganymede,
and Callisto will perhaps find a chance to serve the cause of the survival of the human species.
In a billion years, in fact, as the Sun begins to emit more heat and set on the path
to becoming a Red Giant, the habitable zone of the Solar System will shift outward by at least half a
billion kilometers, and Jupiter's satellite system will find itself right in the middle of the zone.
This will mean a rise in temperature for the four Galileans, leading to the melting of ice and the
release of large amounts of gas. Atmospheres will form. How conducive to life as we know it,
cannot be predicted, but surely those inhospitable worlds will be able to help
our species substantially to escape from the increasingly torrid Earth.
Of course, if we are still there, and if we have the technological means to cope with the enormous
logistical problems that may arise. And in any case, going to Io, Europa,
Ganymede, and Callisto will always be better and easier than searching for new Earths among the
very remote array of extrasolar planets. What do you guys think about this?