Stardate:08:24:98
Four Strange Worlds
It wasn't supposed to happen that way. Could 2000
years of science be wrong, or was he? If he were right, it would be news
which would shock the world, and possibly land him in jail. But, he knew
he was right. He knew the world had to know, and he let it be known.
The year was 1610, and this brave soul who spoke
the truth in the face of overwhelming opposition was Galileo. He would
pay the price, but not yet.
The secret of the universe which
he had uncovered was the discovery of the four largest moons of Jupiter.
At first thinking they were stars which just happened to be behind Jupiter
at the time, Galileo faithfully recorded their apparent positions in his
log. On subsequent nights of observing, he discovered that these four moons
were orbiting around Jupiter. This simple fact, which we take for granted
today, shocked the world. For at that time, it was believed that every
object in the universe circled around the Earth, but these objects were
clearly circling around Jupiter, and not the Earth. For a short time some
people argued that these four moons may be orbiting Jupiter, but that Jupiter
itself was orbiting the Earth. It did not take long for the geocentric
(Earth centered) view of the universe to fall in disfavor. To this day,
these four moons, lo, Europa, Ganymede and Callisto are known as the Galilean
satellites in honor of this brave, careful researcher.
Another shock to the world came nearly 400 years
later, in 1979, when the Voyager spacecraft one and two flew past the king
of the planets. Looking
at Jupiter's innermost large moon Io (pronounced ee-oo), a strange bulge
seemed to be present on one side of this world. At first thought to be
a computer malfunction, the infrared cameras aboard the spacecraft showed
the bulge to be much hotter than the surrounding area. After all other
possibilities were eliminated, the conclusion was clear. What Voyager had
seen was the first volcanic eruption ever seen on a world other than the
Earth. Today we know that Io is geologically active, with volcanoes being
commonplace on this strange world. this discovery was not entirely without
warning, however.
Before the arrival of Voyager at Jupiter, a few
brave theorists (Stanton Peale, Patrick Cassen and Ray Reynolds) had the
idea that Io may have volcanoes, and their concepts were correct. The tides
on Earth raise the water level by perhaps one meter at the most, and raise
the land up to 30 cm (one foot). The huge mass of Jupiter, however, raises
the surface of Io by an astonishing 10 meters (32 feet). This constant
flexing of the surface of this enigmatic moon heats the interior of Io
to tremendous temperatures. This heat is what keeps this otherwise frozen
world geoIogically active, and the only world in the solar system other
than the Earth known to have active volcanoes. These extraterrestrial volcanoes
exhume a gIow in the dark compound of sulfur, forming a ring of gas around
this mighty planet. Io is over 3600 km (2250 miles) across. 
Ganymede, on the other hand has shown itself to
be a much less active world, but with exciting, unexpected fractures in
the ice. Ganymede is the second largest satellite in the solar system (behind
only Saturn's moon Titan), and it is the most massive. It is larger than
the planets Mercury and Pluto.
Callisto is another frozen world, with an old
crust, and some fracturing, but not as much as Ganymede. It is the outermost
of Jupiter's four large moons.
Perhaps the greatest shock of all for the human
race has come, and may yet come, from the smallest of Jupiter's large moons,
Europa. About as large as our own Moon (3120 km or 1950 miles), Europa
was the setting for Arthur C. Clarke's novels 2001. and 2010. We haven't,
as yet, found any evidence for a large alien monolith on this moon But
we have found evidence for something nearly as strange. First suggested
by the Voyager spacecrafts, and later confirmed and measured by the Galileo
spacecraft just last year, Europa is awash with an ocean of liquid water.
This is not a tiny amount of water which we are speaking of either. The
latest measurements from the Galileo spacecraft suggest that the oceans
of Europa may extend to a depth of nearly 100 kilometers (60 miles). This
is ten times the depth of Earth's deepest ocean, and is 50 times deeper
than the Titanic's watery grave. The most likely reason for the fact that
these oceans are still liquid so far from the Sun is that Europa too is
subject to great tidal forces from the great mass of Jupiter. Being further
away from Jupiter than Io is, the tidal forces are not as great there as
they are on Io, but they are enough to keep water in it's liquid state.
Also, the oceans of Europa likely have a large quantity of dissolved material
in them. Any liquid which has any other material (known as a solute) dissolved
in it has a lower freezing point than it would otherwise.
This is also the reason that salt and sand are
used on icy roadways, The freezing point of the liquid lowers, and so it
remains a liquid down to lower temperatures. Europa has also been shown
to have a substantial atmosphere high in oxygen. The oceans of liquid water,
plus the warmth and the oxygen atmosphere suggest that Europa may be compatible
with life as we know it. Whether or not life has yet evolved on Europa
is a question which should be answered in the near future. NASA has extended
the mission of the Galileo spacecraft in order to concentrate on studying
Europa and Io.
Most astronomy magazines and astronomy software
will list the positions of Jupiter's Galilean moons whenever the planet
is visible. When looking at Jupiter in the southeast this month, see if
you can identify these four strange moons.
Clear skies, and good viewing.
Jim Maynard is the head of the astronomy department
at Earth Treasures and has been an amateur astronomer for more than 20
years. He is a physics student at Keene State College and leads star parties
at Wheelock Park in Keene, New Hampshire. If you have any questions about
astronomy or star gazing, call him at 603-352-7192.
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