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.
