Professor Dave again, I wanna tell you about Jupiter.
Now that we’ve wrapped things up with the terrestrial planets, it’s time to move on
to the big boys, the gas giants.
These planets are so large because they formed in colder regions very far away from the sun.
Because of the low temperatures, they were able to accumulate large quantities of ice
and gas, which were far more abundant in the protoplanetary disk than all the rock and
iron, so much more material was available for accumulating.
Images of these planets taken by a variety of spacecraft over the past few decades are
some of the most incredible photographs ever taken by mankind.
At around five astronomical units, let’s start with the one closest to the sun, Jupiter.
Jupiter is named after the Roman king of the gods.
The ancients did not know precisely how big this planet was, but it turned to be aptly
named, because it is the largest planet in the solar system, with more mass than all
of the other planets combined.
It has a diameter ten times that of earth, and more than thirteen hundred earths could
fit inside.
Jupiter, like the other gas giants, doesn’t really have a surface.
You couldn’t land on it like you can land on the terrestrial planets, because most of
the planet is simply gas.
There is no clear distinction between the atmosphere and the planet itself.
Jupiter is made of lots and lots of hydrogen and helium, as well as lots of hydrogen-rich
gases like ammonia, methane, and water.
As we travel towards the center, the immense gravity presses inwards, generating a lot
of pressure, and the gas actually liquifies, which means there is a huge layer of liquid
hydrogen below the outer gaseous layer.
Then, below all of this liquid there is a solid core, made of iron, rock, and water,
though Jupiter overall is much less dense than the terrestrial planets.
Heat emanating from Jupiter’s core generates convection currents in the outer layers.
In addition, Jupiter rotates quite rapidly, once around its axis every ten hours.
All of this means that the gas on its surface is swirling around at high speeds of up to
two hundred kilometers an hour, creating incredible storms, like the great red spot, a swirling
vortex bigger than the entire earth.
There is also a lot of activity in the interior, and because of the huge layer of liquid metallic
hydrogen, Jupiter has the largest magnetic field of any planet in the solar system, a
full twenty thousand times greater than earth’s.
Jupiter also has a thin ring system, which was not seen until the 1970s.
It’s made of particles of dust that are held in place by Jupiter’s gravity.
But the most impressive feature of Jupiter is its collection of moons.
There are four very large ones, all around the size of earth’s moon or larger, and
these were discovered centuries ago by Galileo with his telescope.
But beyond these major ones, there are an incredible amount of smaller moons, giving
us a grand total of at least 69 that we have found so far.
Many of these are utterly tiny and inconsequential, probably just captured asteroids, but all
four of the Galilean moons, the big ones discovered by Galileo, are well worth discussing, as
they are among the most fascinating objects in the solar system.
Ganymede is the largest of these moons, and the largest moon in the solar system.
In fact, it’s actually greater in volume than the smallest planet, Mercury, so if it
was orbiting the sun instead of Jupiter, it would be a fully-fledged planet.
Ganymede is a grayish color, rocky and icy, with an iron-rich liquid core.
Its surface is covered with craters from impacts shortly after its formation.
It is the only moon known to have a magnetic field, and it is even thought to have a subsurface
ocean of liquid water.
The name Ganymede comes from Greek mythology rather than the Romans, who really just stole
everything from the Greeks anyway.
Jupiter’s Greek name was Zeus, and Ganymede was abducted by Zeus to serve as cup-bearer
in Olympus, with allegedly amorous intentions.
Next largest of the Jovian moons is Callisto, another one of Zeus’s lovers.
This one is very similar to Ganymede, composed of rock and ice, and also likely possessing
a subsurface ocean.
Its surface is even more heavily cratered than Ganymede, being the most heavily cratered
object in the solar system.
It orbits a bit further from Jupiter than the other large moons, receiving very little
radiation, and is thus thought to be an excellent option for establishing a manned base some
time in the future, for further exploration of the Jovian system.
Next up is Io.
Not surprisingly, Io got its name from a young woman that Zeus fell in love with, and who
he turned into a cow, so that his wife, Hera, would not suspect any funny business between them.
Quite poetically, Io is the moon that is closest to Jupiter, orbiting so closely that tidal
forces are immense, its orbit constantly changing and its shape distorting due to gravitational
effects.
All of this internal friction generates a lot of heat, just the way that rubbing your
hands together vigorously warms them up.
Because of this, Io is covered with active volcanoes, erupting magma from the hot molten core.
In fact, Io is the most volcanically active object in the solar system, so much so that
there are no visible craters on Io, with the material ejected from eruptions snowing down
and covering the surface with great frequency.
Lastly, there is Europa, which, not to break the pattern, is named after another one of
Zeus’s romantic interests.
The smallest of the Galilean moons, Europa is quite possibly the most interesting place
in the solar system, because it may be the best candidate for potential extra-terrestrial
life that we are aware of.
It is an icy world, with the smoothest surface of any known solid object in the solar system,
but it experiences tidal forces similar to Io, which heat the core.
For this reason, there is a vast ocean of liquid water under the frozen crust.
There are also red cracks in the ice, which indicate the presence of minerals in the water
that gush up through cracks and freeze at the surface.
Liquid water plus minerals equals prime conditions for life, which even without sunlight, or
the ability to perform photosynthesis, may be able to perform chemosynthesis, just like
certain forms of life on earth that can be found near hydrothermal vents at the bottom
of the ocean, where sunlight does not penetrate.
We will talk more about the search for alien life later in this series.
For now, let’s just look at these worlds and appreciate their beauty.
So that’s Jupiter and the Jovian system in a nutshell.
We can now understand that Jupiter, which is rather large for a planet, consists mainly
of hydrogen and helium, just like a star.
In fact, if it were not even a hundred times more massive, it would have been a star, it
simply didn’t accumulate enough matter for gravity to become strong enough to trigger fusion.
Despite never achieving star status, it still formed a mini solar system of its own, with
a marvelous system of moons orbiting in its equatorial plane.
Now let’s move on to the next gas giant, Saturn, which is famous for its incredible
ring system.