So now I want to turn to the moons of our solar
system and particularly focus on the moons of the other planets, alright?
Here are the major moons of our solar system.
The Earth's moon.
Mars' two tinies, tiny moons, Phobos and Deimos.
Jupiter's four most significant moons. Saturn's largest moons.
And here are the moons of
Uranus, Neptune and of two of the Kuiper belt objects, Pluto and Eris.
And this is to scale so you can see how the moons compare in size to the Earth.
And you'll see that some of the Jovian and Saturnian moons are bigger than
the Earth's moon and, well, smaller than the Earth and quite significant in size.
Let's start
by talking about the Jovian moons.
The Jovian moons were discovered by Galileo when he pointed a telescope,
for the first time, out to look at what's in the heavens.
And Galileo discovered something truly remarkable.
He discovered that around Jupiter was
another, in a sense, miniature solar system.
So we think of the Sun with
the inner planets of Mercury, Venus, Earth, and Mars orbiting it.
The outer planets of Jupiter, Saturn, Uranus and Neptune orbiting it,
surrounded by things like the Kuiper belt and the Oort Cloud.
Jupiter is a system like that in miniature.
Orbiting around Jupiter are these four major moons
and then a collection of many smaller moons.
And then surrounding all that are distant moons on their regular
orbits, that form a sphere around Jupiter. But this is
what Galileo saw. And here is
Cosimo de' Medici. And Medici was Galileo's patron.
Galileo was Medici's tutor when Medici was a Cosimo was a young boy.
When Cosimo became ruler, he helped sponsor Galileo's research.
And these moons were called by Galileo, we call them the
Galilean moons, but Galileo called them the Medicean moons, after his sponsor.
And, one of the things I strongly recommend doing is taking your own
telescope if you have access to one, or even a pair of good binoculars, and
looking at Jupiter.
And if you look at Jupiter, what you'll see, just like Galileo, are these moons.
These moons move fairly quickly.
So if you observe Jupiter night to night,
you will see the position of these moons change.
And you can track the moons, and do what Galileo
did, and discover that they, too, orbit around a planet.
Before going on to talk about
the moons' properties, I just wanted to bring up this wonderful quote that I got
from the Medici Archives. To me, this represents one of the best
pieces of playing up to your sponsor that I've ever seen.
I just love this.
So I, so I'm going to read this to you.
Behold, therefore, four stars reserved for your illustrious name.
Remember, that Galileo were, named these after Medici.
And not of the common sort and multitude of the less notable fixed stars, but the
illustrious order of wandering stars. Wandering stars are, of course, planets.
And what Galileo realized is that these moons,
like planets, were nearby objects moving through the sky.
Which, indeed make their journeys
in orbits with the marvelous speed around the star of Jupiter.
As I noted, if you watch these planets night to
night, you will see how rapidly they move around Jupiter.
The most noble of them all, with mutually different motions,
like children of the same family while meanwhile, altogether, in
mutual harmony, compute, complete their great revolutions every 12 years
around the center of the world, that is, around the
Sun itself.
So this is Jupiter orbiting around the Sun on a rel, on a relatively long time scale.
Same time, the moons are orbiting around Jupiter.
Indeed, it appears that the Maker of
the Stars himself, by clear arguments, admonished me,
Galileo, to call these new planets by the
illustrious name of Your Highness before all others.
For as these stars, like the offspring worthy of Jupiter,
never depart from his side except for the smallest distance.
So who does not know the clemency, the gentleness of spirit,
the agreeableness of manners the splendor of the royal
blood, the majesty in action and the breath of authority
and rule over others, all of which qualities find
a domicile and an exaltation for themselves in Your Highness.
This is really sending a nice thank you note to your sponsor.
You know, NASA funds our research on the the planets.
Other for Americans, we have other funding
agencies for astronomers working in space from
other nations that are, that are exploring
space Funding, sometimes, comes from private agent, sources.
None of us, I think, have composed something as impressive
as this, as a way of thanking them for funding.
Alright now let's turn to these moons.
We talked a little bit about Io, the
innermost moon and it is a, an amazing place.
It has 400 active volcanoes going off on it.
As I mentioned, it is tidally heated by Jupiter.
If you look at the surface, one of the things you see are there are no craters.
Remember how many craters we saw on moon and on the Mercury, on
Mercury?
When you see lots of craters, that means it's an old system.
The fact that you've got no impact craters at all, means
that Io is being constantly resurfaced by material from the volcanoes.
Its composition is actually mostly silicates.
It's a, it's going to, it is a denser planet,
a denser moon than the other moons we'll look at.
Most of the outer moons
are mostly made of ice.
Their densities are relatively low, one or two grams per cubic centimeter.
The density of Io is over three grams per cubic
centimeter.
Io, through its volcanoes, actually feeds a
large plasma torus around Jupiter that some of
the material from the volcanoes are ejected up
into space, and go into orbit around Jupiter.
It's pretty big, right?
That's bigger than our moon.
And recall that the Earth's size is a bit bigger than that 6,000 kilometers.
So we're looking
at, with Io, a fairly large moon orbiting Jupiter.
Moving at, outward to look at the other planets in the the major Jovian moons, we
find that the three innermost planets are all
locked into what we call an orbital resonance.
What that means is there's a rational, ratio
between the orbital periods of the different moons.
So let's see how that works. So here's Io, Europa and
Ganymede. Every time Io goes around once,
Ganymede goes around a quarter, Europa goes around
half. So that when Io completes four orbits,
Ganymede has gone around one, Europa's gone around twice,
and the things come back to where they start.
So let's follow that picture.
Here's at the beginning.
Here's one Io orbit later. So Io has gone around, once.
Europa has gone around halfway. Ganymede here.
Here's two orbits. These two
are back here. Here's four orbits.
This is what we'd call a one to two to four resonance.
These resonances have significant effects on the
planet, as you could see in the moon.
As you can see, when you're in a
resonance like this, both of these moons are pulling,
on Io, and they keep pulling every single time.
Think of this as like a child on a swing and
every time you line up this way, you get a pull.
What that does is that takes Io's orbit and makes it somewhat non-circular.
Same time, Jupiter is trying to circularize and tidally lock Io.
The friction between these two competing effects is
the source of heat for Io, that tidal friction.
One of the things that we've discovered about many
of the extrasolar planets that we're going to be talking
about later in the course is that a number of
the ones we've found are locked into various resonant combinations.
So there, we've learned that planet formation and evolution often
drives systems into resonance.
And this has been an area of great fascination for
astronomers because these resonances have
interesting effects on the planetary system.
Let's now turn to the second moon, Europa.
Europa's slightly smaller than our moon. Its composition is very interesting.
It's got a silicate core, and then it's surrounded by a lot of water.
That water is mostly at the, on the surface in the form of ice.
But what's intriguing is, we think that underneath this ice may be liquid water.
When we look at the surface, what you see
in this picture of Europa, is a very smooth surface.
By now, I hope, when you see a smooth surface you think, young surface.
It has not been disturbed by
lots of meteors.
If it was an older surface, it would be pockmarked with meteors.
So, it must resurface often.
We see these cracks and features.
They look like the kind of cracking that we, you know, that
represents perhaps, almost plate tectonics going on with the ice on the surface.
When we zoom in more closely to look at the surface,
we see very complicated structure that looks a lot like ice flow that we see
in a place like the Arctic, where we have ice sitting on top of water.
And we see these complex structures seen on the surface.
Many people think that what's going on in Europa is we have a layer of ice, moving,
much like the continents move on the Earth's magma.
And underneath is a liquid ocean.
Now, there's some dispute on whether there's a, the wa, the ocean is
liquid, are kind of warm or lukewarm ice, kind of like a slushie.
So we don't know whether it's a slushie Europa or a liquid Europa but
the fact that we've got lots of liquid water
is one of the very intriguing things about this planet.
And one of the things that suggests that this might
be a place that might be capable of hosting life.
We find life in a remarkable range of environments.
When we start talking about extrema files toward the end of this
course, we'll, we'll talk about life in deep underground vents, life in places
like Lake Vostok buried deep underneath the
Antarctic ice, and we'll see that there are
environments that aren't so different from Europa in
some ways on the Earth where life thrives.
And that suggests that, perhaps, this is a planet that
may host life or a moon that may host life.
Next out is Ganymede.
Ganymede orbits Jupiter about seven days.
It's actually the largest moon in the solar system,
much bigger, definitely bigger than our moon.
Big enough, it's got a fully differentiated core and iron
in the center surrounded by silicates, then surrounded by water.
When we look at its surface, it's fairly old surface, it's pretty complex.
We see lots of structure from, meteor hits.
So it doesn't look like it's a place that's
resurfacing often the way that Europa and Io does.
And, finally, here's Callisto. Callisto also may have subsurface water.
It's the third
largest moon in the system but again, we see, it's a heavily crated surface,
suggesting that it doesn't have as active geology
as the inner moons of Io and Europa.
So, now let's go further out in the system.
And you can see that the system really looks like,
in many ways, a scaled down version of our own
solar system. In the center, we have Jupiter.
And then we have a host of these tiny
moons, with all kinds of intriguing names, around Jupiter.
Same thing's true of Saturn.
These tiny little moons don't lie in the
orbital plane formed by the Jupiter and the
Sun close to, nor the orbital plane formed by Jupiter and its major moons, those four
big Galilean moons, but orbit around in, you know, kind of random direction.
We think these represent asteroids that were captured by Jupiter.
And we continue to discover more and
more of these tiny moons orbiting around Jupiter.
Phobos and Deimos, in many ways, may be
more like this, these tiny captured moons, than some,
than a moon like our own moon, or
the moons that orbit around Jupiter and Saturn.
So we're going to ask look at some
questions about the property of these moons,
and then we're going to turn and talk about Saturn and the other solar system.
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