Lecture 14 -- Minions of the Giant: The Galilean Satellites
Monday, June 24
When you look at Jupiter with a telescope, you see 4 moons.
Sometimes one or more of them are behind the planet
Sometimes they are in front of the planet, but you may be able to see their shadow on the cloud tops of Jupiter
The 4 moons are in a straight line, aligned with the bands of Jupiter (it's rotation)
The inner ones move faster, like the planets around the sun.
They all move in the same plane as the rotation of Jupiter, like the planets around the sun.
The 4 moons are called the Galilean satellites, after Galileo who first saw them in his telescope
He recognized it as something like a solar system
From his view that the Earth's moon was a distinct geological world, it was possible that there were many other worlds
From Jupiter out, they are Io, period 1.75 days, Europa, period 3.5 days, Ganymede, period 1 week, Callisto 16.5 days.
Note the doubling of the periods as we go out from Jupiter
Voyager 1 and Voyager 2 from 1979 - 1989 took close views of all the Galilean satellites.
Mapped parts of them in detail, sensed the huge magnetic fields and intense particles in the spaces between the moons
Along with all the other myriad of small moons in the outer solar system, what we saw were among the most stunning and extraordinary images that humankind has ever rendered.
We saw 4 completely different worlds with highly varied and complex landscapes
3 of them larger than our moon, Ganymede, the largest, is bigger than Mercury, and twice as big as Pluto! Callisto is the same size as Mercury!
So here we have two moons of Jupiter larger than two of our planets
They were truly fantasy ice worlds -- cold, semi-lit places with no atmospheres and stark starry skies. Jupiter dominates the sun, rising huge and majestic into the Galilean skies from E to W.
The analogy inspired by the appearance of moon systems around the outer planets as solar systems goes further
In origin, they are in effect like miniature solar systems
Like little eddies embedded in the large gaseous eddy (the solar nebula), that formed out solar system
Because the outer planets generated a lot of heat when they formed, there was a pattern of a hot center with cooler edges, just like in the solar nebula
Heavier substances like rocks condensed out at the higher temperatures inside the Jovian nebulas, carbonaceous material, and lighter ice froze further out.
All the moons are mixtures of ice and rock, (except Io), with more fraction of ice as you go out away from Jupiter
The result is Io is denser than Europa, which is denser than Ganymede, with is denser than Callisto.
As we look at the whole system of stuff orbiting Jupiter, we find that we can divide it into 4 kinds of objects:
The dust and tiny moonlets that make up the ring system closest in.
This is inside the Roche limit, where moons cannot form by accretion
The small moons just on the outer edge of the ring system. These are the things that are crashing into each other, making the ring. Like the terrestrial planets (4 of these)
Large moons at intermediate distance. These are the largest ones -- the Galilean satellites. Like the giant outer planets of the solar system
Small moons at the edge of Jupiter's gravitational sphere of influence. These may just be passing asteroids that get snared. Like the Oort cloud in the solar system (about 8 of these)
The moons are made of varying proportions of rock, carbonaceous material, ice and frozen gases
Recall that when a forming body gets big enough, it begins to heat up due to the compression caused by gravity
We know from the planets that this caused differentiation.
The heavy stuff sinks to the bottom, and the light stuff floats to the top
In the case of the Galilean moons, the cores are rock, the mantle is a mixture of ice and carbonaceous stuff, and the crust is ice.
Ice at these temperatures (-250 F) is just like rock
In fact, it is the 'magma' on these worlds. Icy crust and mantle, perhaps icy volcanoes on the alien landscapes
When Voyager first imaged Io, people noticed odd, changing shapes on the limb of the world. Soon figured out that these were volcanoes
Probably the most bizarre and alien world yet found
Covered in various shades of white-yellow-orange-red, due to sulfur
Because of the intense gravity of Jupiter, and because it is close in, it gets squeezed this way and that, heating the interior
As a result, it is the most volcanically active body in the solar system
It has enormous sulfur volcanoes going off on the surface all the time
All the dark spots are probably volcanic vents. The dark red areas are the hottest lava, with yellow the coolest.
The white stuff is frozen sulfur dioxide gas that condenses during the night, if volcanoes aren't in the immediate vicinity
It has no impact craters at all it is the youngest planetary surface in the solar system
It has turned itself inside out several times in the history of the solar system
The reason it doesn't have any ice or carbonaceous material is that it has all evaporated and been driven off the planet. The sulfur and other materials in the surface are still being driven off. They get trapped in Jupiter's enormous magnetic field, forming a donut around Jupiter called the 'Io Torus'
Galileo went to Jupiter, and one of its big goals was to get some really close up pictures of Io. But because of a combination of problems, no images were taken when Galileo passed by.
There may be an opportunity in a few years, if funding isn't withdrawn from the mission
Europa is a little smaller than Io, and not subjected to the strong tidal stresses that Io is
Europa is smooth and white, with no dark markings, few craters, no mountain ranges
But it is covered with a complex system of cracks -- ice-tectonics
If you count the craters, you see a very young surface, about the same age as young Earth crust. So it seems to form crust about as fast as Earth.
What drives it is the same thing that heats up Io, on a much smaller scale.
The water ice still exists on Europa, in fact it seems to continually erupt out onto the surface via ice volcanism.
Beneath a global layer of ice 100 km thick, there is probably a deep water ocean, or mantle of water.
Enormous world, largest moon in the solar system
It has a geologically varied surface, with plains of ice erupted from ice volcanoes, but older, darker, dusty regions also
The older dark regions have lots of craters, almost as old as lunar surface
The grooved, lighter, younger terrain are probably water rift zones, where the ice melted and came up from the interior
You can see places on the surface where the ice erupted through giant cracks, and then covered the old dark areas with a thin veneer of ice.
You can see the ghost of old craters beneath the ice, telling us the story of what happened on Ganymede
We also see evidence some ice plate tectonics. Huge plates of ice that floated on top of a slush mantle.
But you can see where the plates moved against each other in the Voyager images. Trouble is, on Earth, the cooler a plate gets, the denser it gets, so it tends to subduct. For water ice, the cooled magma would turn to ice, which is less dense than water. It seems that subduction may not happen
Same size as Mercury
It is the lightest of the Galilean satellites, being the furthest out and therefore having more ice to rock than the closer in moons
It has about half ice and half carbonaceous material
It is heavily covered in impact craters, showing a surface about like the moon
No interior melting or signs of volcanism on its surface
There's a dark carbonaceous layer all over the planet, except where there are large impacts -- brighter, cleaner water ice shows through
Least altered of all the Galilean satellites, with oldest surface
VIDEO: TO BOLDLY GO, NOVA (Second half)