Video summary
This lecture introduces the study of the six largest moons located in the outer solar system, distinguishing them from the single large moon orbiting Earth within the inner system. While there are over 200 known moons in total, most are small captured objects from the asteroid or Kuiper belts that lack detailed discussion here. The focus remains on these major satellites, which exhibit complex geological histories driven primarily by tidal interactions with their parent planets. These gravitational forces generate internal heat, preventing the moons from becoming geologically dead and instead fostering dynamic surfaces involving ice volcanism, subsurface oceans, and tectonic activity, effectively making them active worlds despite the freezing temperatures of the outer solar system.
Jupiter hosts four of these large moons, each with distinct characteristics revealed through their surface features and internal structures. Io is the most volcanically active body in the solar system, with its surface constantly renewed by lava flows that erase impact craters within a short timeframe; this extreme activity is caused by intense tidal heating from Jupiter as the moon's elliptical orbit twists it like clay. Europa follows as an icy world hiding a vast global ocean beneath its cracked crust, containing more water than all of Earth's oceans combined and potentially harboring life in its dark depths. Ganymede stands out as the largest moon in the system, even exceeding Mercury in size, and possesses its own magnetic field generated by a molten interior, while Callisto presents an ancient, heavily cratered surface that has not fully differentiated into layers like its neighbors.
Moving outward to Saturn and Neptune, two more unique large moons are examined: Titan and Triton. Titan is the only moon with a significant atmosphere composed mostly of nitrogen, which obscures the surface but allows for radar imaging revealing methane lakes and rivers that mimic Earth's water cycle. This moon hosts cryovolcanoes that erupt mixtures of water and ammonia instead of molten rock, sustaining geological activity through tidal forces. Triton, Neptune's largest moon, is an unusual retrograde orbiter with a young, active surface featuring ice volcanoes that spew slushy mixtures of water and ammonia; its lack of impact craters suggests ongoing resurfacing processes, though much remains unknown due to the limited data from a single flyby in 1989.
In conclusion, these large moons demonstrate that geological activity is not exclusive to rocky planets or Earth-like environments but can also occur on icy bodies far from the Sun. The primary driver for this activity across all six outer moons is tidal heating, which keeps their interiors warm enough to drive processes such as ice volcanism and tectonic shifting. By studying these worlds, astronomers gain insights into the potential for life beyond Earth, particularly in subsurface oceans shielded by thick ice shells, and understand how gravitational interactions shape the evolution of celestial bodies throughout the solar system.
Read the full video transcript
Greetings and welcome to the
introduction to astronomy.
In this lecture, we are going to talk
about the large moons in the solar
system.
Now, there are not a lot of large moons,
but they are among the best studied
moons in the solar system. So, we'll
have a little bit more detail to talk
about these than the smaller moons. So,
let's look at the moons of the outer
solar system and what we see, these are
selected moons. We'll come and talk
about some of these here,
but there are more now more than 200
moons known in the solar system. Three
of those are in the inner solar system.
That is Earth,
Earth's moon, and the two that small
ones that orbit Mars.
There are six
large moons in the outer solar system.
So, there's seven total, one in the
inner solar system, that's our moon, and
then six in the outer solar system. Four
around Jupiter,
one around Saturn, and one around
Neptune.
Now, we can also look at some of the
medium moons, and I will discuss those
separately, and the small moons, which I
really won't discuss in any detail here.
Many of the small moons are captured
objects. So, captured objects from the
Kuiper Belt or from the asteroid belt,
and I will not be going into great
detail in these. Many of them are very
small and a very tiny bits of rock and
ice that have been captured.
So, let's start out looking at the four
moons around Jupiter.
So, these were first seen by Galileo and
recorded. If you recall, this is one of
the first instances of seeing something
where objects did not orbit either the
Earth or Sun. So, it was different. We
were seeing that other objects could be
orbited and that they didn't have to be
one center of motion in the universe.
We will look at Io, the first one here,
which is volcanically active and is the
most volcanically active object in the
solar system.
We will look at Europa,
which is a which has a very watery and
then a great ocean below its surface.
And we will look at Ganymede and
Callisto here.
Ganymede and Callisto, which have much
older, heavily cratered surfaces. So, we
will look at each of these in turn in a
little bit more detail.
Now, let's start off with Io. Io is, as
I've already said, the most volcanically
active object in the solar system. About
25% of its surface, 1/4 of its surface
is warm lava.
And there are no impact craters. So,
everything we see here is volcanic.
And there are some volcanoes The
volcanoes do not last a long time. The
planet is essentially turning itself
inside out and resurfacing resurfacing
itself in a very short time scale. So,
any craters that do form Now, remember,
that mean it does get impacted by
craters, but in the impact craters
simply do not last long enough. The
erosional and volcanic processes here
are much greater than they are on Earth.
We do see the large volcano known as
Pele, which is again comparable You can
see comparable to Olympus Mons. And you
can see again how they compare to the
Hawaiian Islands, not just the
volcanoes, but the entire islands
themselves.
So, um
very large volcano there, but much much
of the surface is constantly changed.
Now, the question comes,
Why is Io, which is about the size of
our own moon, so volcanically active?
And it all comes down to tidal heating.
Tidal effects of Jupiter that will heat
up the interior.
Now, when you look at the interior as we
believe Io looks, let's take a look at
that here.
And Io has a rocky
metallic core, and then a very, very hot
rocky mantle. It has very little ice and
no ice of any kind.
So, what it is is it is constantly being
pulled by Jupiter's the force of
Jupiter. Now, remember how a tidal force
works. Jupiter pulls stronger on one
side
than it does on the other because one
side is closer to Jupiter. And that
causes it to be deformed. Now, in the
case of the moon pulling on Earth, it's
not enough to really distort the solid
rock that Earth is composed of. However,
Jupiter is 300 times more massive than
Earth, and Io is about the size of our
moon and about the same distance our
moon is from Earth. So, the tidal forces
are much, much larger, and it can
actually distort that significantly.
Let's take a little look at that here in
this animation, and we can see how as Io
moves around Jupiter,
it is constantly being stretched and
pulled. Now, note that it is locked to
Jupiter just as our moon is to Earth.
One side always keeps facing Jupiter.
Now, if it just did that, it would just
be an elongated object pointing towards
Jupiter, and there would be no
significant tidal changes because it
would just be stretched out, and that
bulge would always point toward Jupiter.
However, Io is in an elliptical orbit,
and there are interactions with Europa
and Ganymede, the next two moons out,
that will constantly twist its orbit a
little bit, and that gives it this
uh
this uh tidal effect. Now, it's ends up
being something like kneading a lump of
clay. So, if you take a lump of clay,
it's cold and hard and doesn't want to
move much, but if you sit there and hold
it and work it for a little while, it
will slowly become warm and pliable. And
that's what's happened to Io over
billions of years. The interior has
warmed up by this constant heating of
the tidal forces from Jupiter, making it
incredibly at volcanically active.
Now, that's Io. Let's look a little
further out as we work our way out from
Jupiter, and we see Europa, which is a
water world. It has a very icy surface,
so while
the surface of Io was rock, the surface
of Europa is ice, and it does have an
ocean of salt water down below.
Now, interestingly, this is actually
smaller than our moon, but it has more
water than the entire Earth. Now, we
think of ourselves as a water world, but
remember, the water in the oceans is
only
goes down a few miles, and it goes
thousands of miles down to the core.
So, the vast majority of the Earth is
dry.
Here on Europa, we do have that metallic
interior, that metallic core, and then a
rocky mantle,
and then above that, we have a ocean of
water, many uh tens or a hundred
kilometers thick. So, it's a very thick,
deep ocean there, and that is where we
may think, you know, could life possibly
exist there?
And then it has a very thick icy crust,
so much thicker than Earth's crust, but
it still does get cracked by tidal
forces from Jupiter. Now, Europa is
further away, so the tidal forces are
far less, but they still exist, and that
will heat up things interior of Europa
as well.
Like Io, it has an extremely young
surface with only a handful of impact
craters seen.
We see cracks on the surface and icy
flows from material. So, if the surface
cracks a little bit, you can have
material flowing up like an ice volcano
that will flow out and fill in the
lowing lower-lying areas.
In a way, Europa is much like the Arctic
Ocean here on Earth. The Arctic Ocean is
all below a big layer of ice.
Now, we can look at some of those cracks
we see on the surface. Let's take a
closer look at those.
And here is an example of one of those
where we've seen material that has
flowed out from the interior.
And the various different structured
terrains from watery material, slushy
material that reached the surface and
was then frozen. So, you can get some
flows, but nothing liquid will last
because there is no atmosphere. So, it's
a vacuum there, and once that material
starts to to go, it will very quickly
solidify once it reaches the surface of
Europa. But, that constantly resurfaces
it, again minimizing how many impact
craters we can see on the surface.
Now, let's go a little further out to
Ganymede. Ganymede is the largest moon
in the solar system. It's actually
larger than the planet Mercury. It has
more craters than Io and Europa,
and it has an older surface. Now, when
we look at the surface here, we have to
remember when we get to the outer solar
system, this surface is made up of ice.
So, we're not looking at rock here. We
are not looking at impacts on rock. We
are looking at impacts on ice. Now, we
have to think of ice a little bit
differently than we do here on Earth.
Here, we have ice usually relatively
close to its melting point. In Ganymede,
it is way below its melting point. It's
not even close. So, it behaves much like
rock does here on Earth.
We do see uh signs of some tectonic and
maybe volcanic activity, some icy flows
of material that have occurred in the
past. Ganymede likely also has a liquid
water interior.
It's also interesting in that it has a
magnetic field. You remember a magnetic
field required some kind of liquid uh
material that could conduct electricity.
Now, on Earth, that was the molten outer
core. On Jupiter, it was metallic
hydrogen. On Uranus and Neptune, we
wondered if it might be some kind of a
slice slushy mixture that was going
there. Could Ganymede be something
similar to that, or is there something
else happening with Ganymede as well?
But, we do know that there has to be
some kind of molten interior, some kind
of material there.
Now, we can take a little bit closer
look at Ganymede. And as we see it
rotate here, we can actually look at the
whole thing. This is taken from a number
of images put together, and we can see
all the different structures, lighter
and darker regions. So, the very bright
spots we see are impact craters. So,
those are relatively fresh impact
craters that dug into the surface. It
also picks up darker material, some
generally carbon compounds that look a
little And those are the darker
regions that we see.
So, [snorts] those are some that are a
little bit been there a little bit
longer and we can see the very young
craters that have just been exposed.
Now, that doesn't mean last week, last
month, that means over the last few
million years or 10 million years. It
takes a long time for the moon to pick
up that much material.
Now, let's go ahead and look at the last
moon of Jupiter that we're going to talk
about here and that is Callisto.
Callisto has a very old, heavily
cratered surface and in fact an icy
surface much like Ganymede. Lots of
impact craters. You can see all of those
bright spots here. So, it's very similar
to the lunar highlands.
And again, to remember, ice in the outer
solar system is much like rock. So, we
get volcanoes, we get volcanoes of ice.
Now, one interesting thing we note about
Callisto that is not completely
understood, it is not fully
differentiated.
So, most of the large objects and things
of this size differentiate so that the
densest materials go down to the core.
For whatever reason, Callisto has not
done this making it different than other
large objects that we see.
Now, that's the four large moons around
Jupiter. There are two more in the outer
solar system. First of those is Titan,
the largest moon of Saturn and is the
only moon with a significant atmosphere.
And that makes it difficult to study
because we cannot see its surface.
Its atmosphere is comparable to Earth in
terms of pressure
and it is primarily made up of nitrogen.
It's actually about 50% more atmospheric
pressure than Earth. Not
Not a lot more.
It is the only object in the outer solar
system to date that has been landed on
and we have been able to study the
surface by land the lander that landed
there as part of the Cassini mission. We
had the Huygens lander that landed on
Titan and gave us some images of its
surface. Now, the rocks that we see
here, these are not made of rock. These
are chunks of water ice. So, we see lots
of water ice, but what we are seeing is
that we also will see lakes and rivers.
So, we can look at those.
In a radar image, radar is very good,
reflects very well off a rough surface
and does not reflect so well off smooth
surfaces. So, the dark regions here are
lakes of methane. So,
Titan is also unusual in that it is the
only other planet that has a cycle on
it. We have a water cycle here on Earth
where you can have water that will rain
and then run to flow to rivers and to
lakes and then will be evaporated and
the process continues. Titan has the
exact same type of thing with methane.
And that gives us ideas that maybe Titan
could have some form of life. Maybe
there could be some life based with
methane as the liquid instead of water.
And we'll look at that again later in
the course. I will talk about that in a
future chapter.
So, let's move on to our last moon to
look at here and that is the large moon
of Neptune, Triton.
Uh Triton is the largest moon of
Neptune. It has ice volcanoes. Now, we
talk about this, the lava that we see is
a water or a water ammonia mixture. So,
that behaves much like the molten rock
we have on Earth. You have this slushy
kind of mixture that erupts up through
the volcanoes.
When we look at the icy surface, you can
see that it has all sorts of structures
to it. So, a very unusual area. So, a
lot of this has probably had material
flow on it and wipe out craters. We know
that it is not an old surface because it
is not covered in craters.
The icy surface itself has a mixture of
water, nitrogen, methane, and carbon
monoxide. We're getting out to the very
depths of the solar system, and
temperatures are so low that almost
everything is frozen.
We also note that it has some unusual
surface features, and that means that
it's active because we see the like of
like lack of craters.
Why it does it have this activity? Well,
some guesses, tidal heating from
Neptune, radioactive decay from
materials from its formation, some kind
of icy greenhouse effect where the icy
materials help absorb that material.
These are all good questions, but you
have to remember the only visits we've
had to visit we had to Triton was in
1989 when the Voyager craft flew by
Neptune. So, we can't really study it in
detail from this far, so there's still a
lot of questions about this large moon.
So, let's go ahead and finish up with
our summary, and we talked about the six
large moons in the outer solar system.
Most of them are icy, and most of them
show some kind of geological activity
based on ice, just as we have with rock
in the inner solar system.
The tidal interactions between moon and
planet can really play a large role in
developing these surface features. Those
tidal interactions can keep them the
planet active, the moon active, even
though it would normally be very cool
and dead otherwise.
So,
that concludes this lecture on the large
moons of the solar system.
We'll be back again next time for
another topic in astronomy.
So, until then, everyone, have a great
day, and I will see you in class.