Video summary
This lecture introduces the medium-sized moons of the solar system, distinguishing them from the large regular satellites and the numerous small captured objects. While there are approximately 200 known moons in total, only three reside in the inner solar system, leaving the majority in the outer regions around Jupiter, Saturn, Uranus, and Neptune. The focus here is on bodies that are too small to be compared directly with Earth's moon but too large to be considered mere debris; however, it is important to note that even among these medium-sized satellites, many have not been extensively studied by spacecraft. Consequently, our understanding of their origins and surface histories relies heavily on limited data, particularly for moons orbiting Uranus and Neptune, which were primarily observed during the Voyager 2 flyby in 1986.
Jupiter hosts two such medium moons, Amalthea and Himalia, both characterized by heavily cratered surfaces indicating great age. Amalthea, discovered visually in 1892, is a small, red body likely containing sulfur from its neighbor Io, while Himalia remains largely unstudied with no high-resolution images available. Saturn presents a more diverse group of medium moons, ranging from the heavily cratered Mimas, nicknamed the "Death Star Moon" due to a massive impact scar, to Enceladus, which displays a mix of old and young terrain. Enceladus is particularly significant because its active south pole erupts with water plumes, suggesting a subsurface ocean that has resurfaced parts of the moon by filling in craters with frozen water. Other Saturnian medium moons like Tethys, Dione, Rhea, and Iapetus further illustrate this variety, with some showing fault lines, others possessing two distinct hemispheres due to orbital debris accumulation, and Titan being noted separately as a large moon despite its classification context.
Moving inward to Uranus, the system features five medium moons that defy simple categorization based on size alone. Miranda stands out as the smallest yet most geologically active, displaying intense faulting and shifting material despite its diminutive scale. Larger companions like Ariel show evidence of icy volcanoes and tectonic cracks that have wiped out craters, while Umbriel presents a very dark surface with impact scars, and Titania and Oberon offer further examples of icy bodies with varying degrees of volcanic or impact-related activity. Neptune's two medium moons, Nereid and Proteus, are even less understood; Nereid has a highly eccentric orbit and remains poorly studied, while Proteus appears to be a heavily cratered captured object that likely did not form alongside the planet.
The origins of these medium moons remain an open question in planetary science, contrasting with the clearer formation theories for large moons and small captured objects. While large moons are thought to have formed alongside their planets and small moons were likely captured from asteroid or Kuiper belts, medium-sized moons may represent a hybrid population. Some could be captured bodies similar to the smaller satellites, while others might be natural satellites that accreted during the planet's formation. The lecture concludes by emphasizing the wide variety of surface features observed across these worlds, ranging from ancient, heavily cratered landscapes to geologically active regions with subsurface water and icy volcanism. This diversity suggests a complex history involving tidal forces, internal heating, and capture events, highlighting that even within a specific size category, the moons of the outer solar system tell vastly different stories about their evolution and origins.
Read the full video transcript
Greetings and welcome to the
introduction to astronomy. In this
lecture, we are going to talk about the
medium-sized moons in the solar system.
We've looked at the large moons. Now,
let's look at those that are not quite
as large. And again, we're not going to
really talk about the small moons.
There's a lot of them, but not many of
them well studied by spacecraft. And in
fact, we'll start to see that even with
the medium-sized moons that not all of
them have been well studied. Now, just
as a reminder, looking at moons in the
outer solar system, there are 200 moons
in the solar system. Only three of these
are in the inner solar system, our moon
and the two small ones of Mars.
There are six more large moons that are
comparable to Earth's moon in size.
There are the medium moons, which we
will look at this time. And there are
small moons, which are often captured
objects.
So, let's go ahead and start out and
we'll work our way out in the solar
system starting with Jupiter.
Jupiter has two medium-sized moons. As
you may recall, Jupiter had four very
large moons that we've talked about
previously.
This is the this these are the
medium-sized moons. So, these are things
that are in this case a few hundred
kilometers. Amalthea is 200 kilometers,
discovered in 1892 and was the last
satellite to be discovered visually.
That means by looking through a
telescope. So, not recording it on a
photographic plate or a CCD, not
recording an image of it and finding
that the moon was there, but to actually
see it. It was the last of the moons
that was discovered visually.
It has a red surface and is heavily
cratered. Likely, possibly part of this
is sulfur from Io. It's a very inner
moon, but a very old and active surface.
So, it's a very small area, not going to
be not going to tend to be very active.
Himalia, the second one, has not been
closely studied by spacecraft, so I
don't even have an image of it to share
with you.
Now, Saturn had only one large moon, but
has a lot of medium-sized moons. So,
we'll see several things in the few
hundred kilometer range that we look at.
First, we have Mimas, which is a heavily
cratered moon, and we see that here,
including a very large impact off to the
right-hand side, sometime giving it the
nickname of the Death Star Moon after
the Death Star from Star Wars. And it
somehow maybe resembles that, although
we didn't actually discover that moon
until after the movie original Star Wars
movie came out.
Um we So, that's a very heavily cratered
moon, very old surface.
Enceladus is a little bit larger, 500
km, has a very icy surface. So, we see
already very big differences between
these two.
One was very heavily heavily cratered,
and this one has a mixture of some
cratered regions in the northern section
up here,
and some regions with very few craters
down towards the middle and lower lower
sections. So, it is a mixture of old
terrain and young terrain. And we
believe this is because it has
subsurface water. We've actually seen
water plumes erupting from the south
polar region. So, there is water down
below. Now, that would explain why we
see very few craters. Remember that
water like this would then fill in
craters and freeze, and that's what
we're seeing is the frozen water from
the interior that has covered some of
the lower areas of the moon. So, we know
that there is some subsurface water,
likely giving it
its unusual surface features.
Now, these aren't the only two moons.
There's actually several larger ones,
too. We have Tethys, over 1,000 km,
twice as big as the other two, which is
an icy and combination of icy cratered
with some faults crossing its surface.
We have Dione,
which has again I again icy with a
subsurface ocean down below.
We have Rhea, heavily cratered surface.
So, again, we see that they're all very
different and that they're a mixture of
cratered surfaces and some very younger
surfaces as well. Remember, even the
heavily cratered surfaces still are all
ice. So, we're still looking at ice
here.
Iapetus
is the two-faced moon. It has a light
half and a dark half. So, you can see
part of that here. You can see this
whole section is covered in kind of a
uh carbonaceous material and the
rest of it is very light and bright. And
it may be due to the fact that this is
the way it orbits through around the
Saturn. Remember, the moons are always
locked, so one side stays facing Saturn.
So, the darker a side could be the
leading edge traveling around in the
orbit and therefore picks up more
material debris from other from other
areas and maybe it's been able to pick
up some debris uh
and at over time and given it that
darker half of a surface.
So, Titan while sorry, while Saturn had
the one large moon Titan that we looked
at previously, we get to see six that
are classified in the middle-sized range
here.
Four in the 1,000 to 1,500 km range and
a couple in the few hundred kilometer
range.
Now, one planet we did not talk about
with large moons was Uranus. Uranus does
not have any large moons, but does have
several small uh medium moons and some
small moons.
So, we have Miranda here. Miranda is a
very unusual moon in that it is the
smallest of the five moons of Uranus.
It is mostly water ice, but it has signs
of intense geological activity. Now,
it's only a few hundred kilometers, less
than 500 kilometers across, but there
has definitely been some faulting and
shifting of material, and perhaps
material coming up from inside. So, it's
a very unusual. Most of the small
smaller size of the medium moons we'd
expect to be heavily cratered. This one
is not.
Now, we also have Ariel, about twice
more than twice as big, and again, now
this might expect a little more of the
icy geology that we see. We get some of
those tectonic cracks that you can see
stretching across the moon, and icy
flows from the interior that have filled
in and wiped out craters.
Now, there's a couple more. I said there
are five of these moons to look at, and
let's take a look at the others.
Here we have Umbriel, a very dark
surface, the darkest surface among these
moons. It is icy, and we can see some of
the impacts there.
And we have Titania.
Again, the largest of the the largest of
these moons at about 1,500 kilometers,
and signs of impacts, and again, more
icy volcanoes. We can see those scarring
impacts in the bright colors there, but
also some signs of other activity. So,
even though these are not the largest
moons, we still see some signs of
activity within them.
And finally, we have Oberon.
Oberon is a has a dark reddish surface,
and is primarily impact, so less
volcanic activity. So, the question
could be why are some of these
volcanically active and and others not?
It may have to do with the tidal forces
or other things that are going on with
these moons that churn them up inside.
Because some of them are very active,
whereas others are very inactive and
heavily cratered. Now, you do have to
recall that for Uranus and for Neptune
coming up, we have only looked at these
with one spacecraft. And you'll note
that not all of them are really high
detailed, high resolution images.
And that's because they're images of the
configuration when Voyager 2 happened to
pass through in 1986.
So, those are the only images
we have to study these moons.
Not that the moons have changed
significantly since then,
but we don't have detailed high
resolution images as we might have had
with with other the moons closer in.
So, finally, let's look at Neptune here.
Neptune has two moons, Nereid, which has
not been well looked at, and you can see
that here again, but it has a highly
eccentric orbit,
but not very, very well studied, only
about 170 km across.
And then we have Proteus.
Again, a heavily cratered, likely a
captured object, probably did not form
with Neptune.
So, not as well here. We did look at the
one large moon of Neptune, but there are
several medium-size ones as well. And
again, with all of these, there are
small moons, too, that we're really not
going to get into in these lectures.
So, where did the medium moons come
from? Well, we think perhaps the large
moons formed with the planet, and the
small moons were captured. So, small
moons were captured objects from the
asteroid belt or the Kuiper belt,
depending on the planet.
Medium moons, maybe it's some kind of
combination. Maybe some of these are
captured, maybe some of them formed with
the planets themselves. But it's still a
good question as we look at these. We
have a pretty good idea for the large
and small moons, but less of an idea of
how the medium moons may have formed.
So, let's go ahead and finish up with
our summary. And what we've looked at
this time is the medium moons of the
solar system
show a really variety variety of surface
features. Wide variety. Some are heavily
cratered. Well, some show signs of
volcanic type activity with ice.
Many of these could be captured like
some of these may have been captured
like many of the smaller moons. Some of
them may be natural satellites that
formed with the planets.
So,
that concludes this lecture on the
medium moons of the solar system. We'll
be back again next time for another
topic in astronomy.
So, until then, have a great day,
everyone, and I will see you in class.