Follow US:

Practice English Speaking&Listening with: Jupiter's Moons: Crash Course Astronomy #17

Difficulty: 0

This episode of Crash Course is brought to you by Squarespace.

As we saw in the last episode, Jupiter is by far the largest and most massive planet

in the solar system. That means it has a very strong gravitational field, which also means

it can hold on to a lot of moons. A lot. Right now, as we record this episode, there are 67 that have

been confirmed. And how many it really has depends on how small an object you're willing to call a "moon."

In 1610, Galileo pointed his telescope at Jupiter, and witnessed a revolution. Oh, hey, literally!

He saw three little stars lined up on either side of Jupiter, stars he could not see with

his naked eye. And they moved! A week later he saw a fourth one, and he knew he was seeing

objects revolving, orbiting around Jupiter. It was proof that not everything in the solar

system revolved around the Earth. That was a pretty big deal.

Those four moons are now called the Galilean moons in his honor. Not bad for a weeks work.

All four are really big, too. If Jupiter werent there, drowning them out with its glare, theyd

be visible to the naked eye. In that case we might even call them planets, too.

The biggest of Jupiters moons is Ganymede. At 5270 km across, its the biggest moon

of any planet. Its even bigger than the planet Mercuryin fact, in size its halfway

between Mercury and Mars!

Size isnt the only planet-like characteristic of Ganymede, either. Its mostly rock and

ice, but it probably has a liquid iron core. It even has a magnetic field, likely generated by that liquid core.

The surface is similar to our own Moon in that theres very old, cratered terrain

as well as smoother, younger areas. Ganymede is also criss-crossed with large grooves.

Its not clear what the origin of those grooves is, but it may be related to stress and

strain on the surface caused by the tides from the other large moons as they orbit Jupiter and pass each other.

Ganymede has a surprise well below its surface, too: Oceans of water! Measurements of Ganymedes

magnetic field, made during multiple passes by the Galileo spacecraft in the 1990s, combined with Hubble

observations of the moon, indicate Ganymede has quite a bit of salty liquid water, deep beneath

its surface! As well see in a sec, its not alone in that regard.

The next biggest moon is Callisto, at 4800 km in diameter. In many ways its similar

to its big brother Ganymede, mostly rock and ice. It probably has a rocky core, then a

layer of mixed rock and ice above that. The surface is mostly ice, but mixed with darker

material as well. It has a magnetic field, too, but it probably doesnt have a metallic core.

The surface is heavily cratered, and theres no indication of any volcanoes or tectonic

activity. That means the surface is very old, maybe as old as Callisto itself. It even has

an atmosphere, but its a tad thin: roughly one one-hundred-billionth the pressure of

Earths air at the surface!

Callisto orbits Jupiter farthest out of the four biggies, almost 2 million km away. Thats

too far to gravitationally interact with the other three; when I talk about the moons affecting

each other, its really the other three interacting.

Next up is Io. Its only a little bit bigger than our own Moon, and orbits Jupiter so tightly

it only takes about a day and a half to go around the planet.

When the Voyager 1 space probe passed Io in 1979 it revealed a surface that was really

weird. It was yellow and orange and red and black, and didnt seem to have any obvious

impact craters. An engineer, Linda Morabito, noticed that in one image there appeared to

be what looked like another moon behind Io, partially eclipsed by it. But that was no

moon: It was a volcano on Io erupting, its plume shooting up from the surface and opening

up into a wide arc.

Io is the most volcanic object in the entire solar system, with over 400 active volcanoes.

Quite a few of them are erupting at any given time, and images taken even a few months apart

show changes in the surface due to ejected material. A lot of the erupted material is

rich in sulfur, which is why the surface has all those odd colors on it.

The energy for all this activity comes from the other moons: As they pass Io in their

orbits they flex it via tides, heating its interior through friction.

A lot of that sulfur ends up as a very thin atmosphere around Io, and some of those sulfur

atoms are then picked up by Jupiters powerful magnetic field as it sweeps past Io and accelerates

them to very high speeds. This has created a tremendous donut-shaped radiation belt around

Jupiter, like Earths Van Allen belts, but far more powerful. The radiation there is

so intense it would kill an unprotected human in minutes. Of course, if youre floating

in space near Jupiter unprotected, you might have some more immediate concerns.

Oh, one more thing: Both Ganymede and Io are magnetically connected to Jupiter. Charged

particles flow from those moons along the lines of magnetism to Jupiter, which then

slams them down at Jupiters poles, just like the Earth does with the particles from

the solar wind. On Earth this creates the aurorae, the northern and southern lights,

and it does at Jupiter, too. You can even see the ultraviolet glow where each of the

moons connects to Jupiter; their magnetic footprints in the planets atmosphere!

And now we come to Europa, the smallest but perhaps most exciting of all the Galilean

moons. Slightly smaller than our moon, it was known for decades to be very reflective, meaning

its surface was probably loaded with water ice. But even so, the Voyager observations were shocking.

They showed a surface completely lacking in craters, meaning something had resurfaced

the moon like Io or Venus; but Europa has no volcanoes. Even more intriguing, the surface

was covered in long cracks, dark streaks all over the moon, as well as complex ridges.

These and other features appear to be due to material from the interior of Europa welling

up and forming the new surface, kind of like the way lava does on Earth.

But in this case, the material is water. Its now thought that Europa has an entire ocean

of water, sealed up under a solid crust of ice several kilometers thick. Water welling

up and moving under the crust causes it to shift, creating all the various surface features.

The amount of water that may be locked up on Europa is staggering; easily more than

all the water in all the oceans on Earth! Like Ganymede and Io, the interior of Europa

is kept warm by tidal flexing from the other moons, keeping the ice melted.

Now get this: A lot of Europas material is silicate rock, like on Earth and other

terrestrial planets, located in a layer under the ocean. If this interacts with the ocean

in the same way Earths oceans interact with the sea floor, this could make the subsurface

Europan water salty. In fact, those dark cracks on the surface have been found to be rich

in salt and organic materials - in other words, carbon-based compounds!

This is pretty exciting. We think Earths life originated in salty ocean water. If there

are carbon-based molecules actually in Europas water, its not too crazy to wonder if the

same spark that occurred here also happened there. We think Europa has everything it needs

to spawn life. We just dont have any direct evidence of it yet.

Some people have proposed sending a space probe to Europa specifically to look for life.

It would land near a crack in the ice, where the crust is thinner, and somehow penetrate it

(perhaps melting its way down). Chemical sampling could then look for signs of biological activity.

Thats amazing to me: The idea of life in Europa, even if its just microbial life,

is taken very seriously by astrobiologists, scientists who study the possibility of life

in space. It used to be science fiction. Now its a topic of scholarly research.

Astronomers have a concept called the habitable zone: The distance a planet can be from its

parent star where the temperature on the planets surface can support liquid water. Its a

fuzzy concept; Venus and Mars are both technically in the Suns habitable zone, but Venus is

too hot and Mars too cold for liquid water. Atmospheres make a big difference. But its

still a useful concept as rule of thumb for potential habitability.

But Europa changed that. Jupiter is way, way outside the Suns habitable zone, yet theres

Europa, all wet. Its a great example that we need to let our ideas breathe a bit sometimes,

let them relax and flow outside the boundaries we set for them. When we look for signs of

life on planets orbiting other stars, I bet well have to keep our minds open to types

of life weve never considered before.

Those are just the four big moons of Jupiter, each thousands of kilometers across. They

probably formed along with Jupiter, coalescing from the eddies and whorls around the protoJupiter

as it formed billions of years ago.

But the planet has dozens of other moons, too. About the only thing they all have in

common is that theyre tidally locked to Jupiter; they all rotate once for every time

they go around the planet. Jupiters tides are hundreds of times stronger than Earths,

so no surprise there.

The next biggest moon after The Big Four is way smaller; named Amalthea, its an irregular

lump about 250 km across its longest dimension. It was discovered in 1892, and its redprobably

polluted by sulfur from Io. It orbits just over 100,000 km from Jupiters cloudtops;

if you stood on Amaltheas surface, Jupiter would fill half the sky.

The moons get smaller and more irregularly shaped from there, with Himalia and Thebe

and Elara and Pasiphae, down to Hegemone, Kale, and Kallichore, which are no bigger than hills.

Many of the irregular, distant moons of Jupiter orbit the planet backwards relative to the

others, in what are called retrograde orbits. They may be captured asteroids from the nearby

asteroid belt. Many of the moons have orbital characteristics that are very similar, too,

which may indicate they were once a single object that broke up. Several such families

of moons orbit Jupiter.

The smallest moons weve seen are roughly a kilometer across. If they were sitting on

Earth they might be hard to pedal up on a bicycle, but orbiting Jupiter they hardly

rate as more than debris. There are probably thousands of moons the size of houses circling

the planet, and who knows, maybe millions the size of tennis balls.

Should we even call those moons? Maybe. But I dont really worry about that kind of

thing. The important thing to remember is that these are worlds, big and small, each

fascinating, rich, and diverse. And theres still a lot more left to explore about them.

Today you learned that Jupiter has lots of moons, and four big ones. Theyre mostly

rock and ice, though Ganymede, the biggest, may have an iron core. Io is riddled with

volcanoes, and Europa has an undersurface ocean that is the object of intense study

for scientists looking for life in space. Io, Europa, and Ganymede are close enough

to interact gravitationally, providing a source of heat for their interiors. There are lots

and lots of littler moons, but at the moment we really dont know much about them. Someday.

Crash Course Astronomy is produced in association with PBS Digital Studios, and you can head

over to their channel and find even more awesome videos. This episode was written by me, Phil Plait.

The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller. It was directed

by Nicholas Jenkins, edited by Nicole Sweeney, and the graphics team is Thought Café.

The Description of Jupiter's Moons: Crash Course Astronomy #17