Jupiter's Moons: Crash Course Astronomy #17

Summary

This Crash Course episode explores Jupiter's extensive moon system, highlighting the four largest: Ganymede, Callisto, Io, and Europa. Ganymede, the largest moon in the solar system, has a magnetic field and possibly an iron core. Callisto, the farthest out, has a thin atmosphere and a heavily cratered surface. Io, with its sulfur-rich surface, is the most volcanic object in the solar system, while Europa's subsurface ocean and potential for life intrigue scientists. The episode also touches on Jupiter's smaller moons, their diverse characteristics, and the potential for future exploration.

Takeaways

• 🌌 Jupiter is the largest and most massive planet in the solar system, with a strong gravitational field that can retain many moons, currently confirmed at 67.
• 🔭 Galileo's observation of four moons orbiting Jupiter in 1610 was a pivotal moment, demonstrating that not all celestial bodies revolved around Earth.
• 🌕 The Galilean moons are named in honor of Galileo, and are large enough to be visible to the naked eye if not for Jupiter's overwhelming brightness.
• 🏔️ Ganymede, Jupiter's largest moon, is bigger than the planet Mercury and has a liquid iron core that generates a magnetic field.
• 🌊 Ganymede is believed to have subsurface oceans of salty liquid water, inferred from its magnetic field measurements and Hubble observations.
• 🌍 Callisto, the next largest moon, has a heavily cratered surface and a thin atmosphere, and orbits farther from Jupiter than the other large moons.
• 🌋 Io is the most volcanically active object in the solar system, with its surface constantly reshaped by volcanic eruptions, primarily of sulfur.
• 🌀 Io's volcanic activity is fueled by tidal flexing from the gravitational interactions with the other Galilean moons, heating its interior.
• 🌌 Europa, slightly smaller than Earth's moon, is thought to have a subsurface ocean of water beneath its icy crust, indicated by its smooth and resurfaced appearance.
• 🧬 Europa's potential for life is of great interest to astrobiologists due to the presence of water, salt, and organic materials on its surface.
• 🌐 Jupiter's moons, including the smaller ones, are tidally locked to it, rotating once for every orbit around the planet, a result of Jupiter's strong tides.

Q & A

How many moons does Jupiter currently have confirmed?

As of the recording of this episode, Jupiter has 67 confirmed moons.

What significant discovery did Galileo make when he observed Jupiter with his telescope in 1610?

Galileo discovered four moons orbiting Jupiter, which came to be known as the Galilean moons, providing evidence that not everything in the solar system revolved around the Earth.

Which is the largest moon of Jupiter and what is its size comparison to other celestial bodies?

Ganymede is the largest moon of Jupiter, with a diameter of 5270 km, making it larger than the planet Mercury and about halfway in size between Mercury and Mars.

What unique characteristic does Ganymede possess that is similar to Earth's moon?

Ganymede has very old, cratered terrain on its surface, similar to that of Earth's moon, as well as smoother, younger areas and large grooves.

What evidence suggests that Ganymede has oceans of water beneath its surface?

Measurements of Ganymede's magnetic field during the Galileo spacecraft's passes in the 1990s, along with Hubble observations, indicate the presence of salty liquid water deep beneath its surface.

What is the farthest moon from Jupiter among the Galilean moons and what is its orbital distance?

Callisto is the farthest moon from Jupiter among the Galilean moons, orbiting at a distance of almost 2 million km away.

Why is Io considered the most volcanic object in the solar system?

Io has over 400 active volcanoes and is the most volcanically active object in the solar system due to tidal flexing from the gravitational interactions with the other Galilean moons.

What is unique about Europa's surface and what does it suggest about its interior?

Europa's surface is covered in long cracks, dark streaks, and complex ridges, suggesting that water from an underlying ocean is welling up and forming a new surface.

Why are scientists particularly interested in Europa's subsurface ocean?

Europa's subsurface ocean is of interest because it contains a large amount of water, possibly more than all of Earth's oceans combined, and may have the right conditions, including the presence of carbon-based compounds, to potentially support life.

What is the concept of the 'habitable zone' and how does Europa challenge this concept?

The 'habitable zone' is the range of distances from a star where a planet's surface temperature could support liquid water. Europa challenges this concept as it is located well outside the Sun's habitable zone yet has a subsurface ocean, indicating that life could potentially exist in unexpected places.

What are some of the characteristics of Jupiter's smaller moons, and how might they have formed?

Jupiter's smaller moons are irregularly shaped and many are tidally locked to Jupiter. Some may be captured asteroids or remnants of a single object that broke up, indicating a variety of formation processes.

Outlines

00:00

🌌 Jupiter's Moons and Their Unique Characteristics

The script introduces Jupiter as the solar system's largest and most massive planet with a strong gravitational field that retains 67 confirmed moons. It discusses the historical significance of Galileo's discovery of the Galilean moons and their individual features. Ganymede, the largest moon, is highlighted for its size, composition, magnetic field, and potential for subsurface oceans. Callisto is noted for its heavily cratered surface and thin atmosphere. Io is described as the most volcanic object in the solar system, with its activity powered by tidal flexing from other moons. Europa is presented as potentially the most exciting moon, with a subsurface ocean and the possibility of life due to its salty, organic-rich water.

05:03

🌿 The Search for Life on Europa and the Redefinition of Habitability

This paragraph delves into Europa's unique geology, marked by cracks and ridges indicative of an underlying ocean. The vast quantity of water on Europa, possibly exceeding Earth's total, is underscored. The potential for life on Europa is explored, considering the presence of salt and organic materials on its surface. The concept of habitability is challenged by Europa's existence outside the Sun's habitable zone, prompting a broader perspective on life's potential environments. The paragraph also touches on Jupiter's smaller moons, their diverse characteristics, and the possibility of further exploration and discovery.

10:06

🎬 Behind the Scenes of Crash Course Astronomy

The final paragraph provides credits for the production of the Crash Course Astronomy episode. It mentions the script editor, Blake de Pastino, and the consultant, Dr. Michelle Thaller. The direction by Nicholas Jenkins, editing by Nicole Sweeney, and the graphics by Thought Café are also acknowledged, showcasing the collaborative effort behind the educational series.

Keywords

💡Jupiter

Jupiter is the largest and most massive planet in our solar system, with a strong gravitational field that allows it to have numerous moons. In the video, Jupiter's size and gravitational influence are central to understanding its moons' characteristics and behaviors. The script mentions Jupiter's role in the discovery of the Galilean moons and its effect on their tides and magnetic interactions.

💡Galilean moons

The Galilean moons are the four largest moons of Jupiter, named after Galileo Galilei who discovered them. They are Ganymede, Callisto, Io, and Europa. The term is significant in the script as it represents a historical discovery and a key part of Jupiter's system. The video discusses each moon's unique features and their interactions with Jupiter.

💡Ganymede

Ganymede is the largest moon of Jupiter and the largest moon in the solar system. It is notable for being larger than the planet Mercury and having a possible liquid iron core and magnetic field. The script describes Ganymede's size, composition, and the presence of subsurface water oceans, making it a fascinating subject for study in the context of the video.

💡Callisto

Callisto is another of Jupiter's Galilean moons, known for its heavily cratered surface and old age. The script mentions Callisto's distance from Jupiter, which prevents gravitational interaction with the other large moons, and its thin atmosphere, providing insight into its geological and atmospheric characteristics.

💡Io

Io is a Galilean moon of Jupiter known for being the most volcanically active object in the solar system. The script describes Io's unique surface colors due to sulfur and its lack of impact craters, indicative of continuous volcanic activity. Io's role in the video is to illustrate the extreme geological activity possible on a moon.

💡Europa

Europa is a Galilean moon that has gained attention for its potential to harbor life due to the presence of a subsurface ocean. The script highlights Europa's smooth surface, lack of craters, and the intriguing possibility of an environment suitable for life, making it a central topic in the search for extraterrestrial life.

💡Volcanism

Volcanism refers to the geological phenomenon of magma reaching the surface, creating volcanic eruptions. In the script, Io's extreme volcanism is a key point, with over 400 active volcanoes contributing to its surface features and sulfur-rich atmosphere, illustrating the diverse geological activity in the solar system.

💡Magnetic field

A magnetic field is a region around a planet or moon where its magnetic force affects the movement of charged particles. The script discusses the magnetic fields of Ganymede and Io, which interact with Jupiter's magnetic field, leading to auroras and influencing the moons' geological features.

💡Tidal flexing

Tidal flexing is the deformation of a celestial body due to the gravitational pull of another body. In the script, the Galilean moons' proximity to each other and Jupiter causes tidal flexing, which is suggested as a source of heat for their interiors and a driving force behind their geological activity.

💡Habitable zone

The habitable zone, or 'Goldilocks zone,' is the region around a star where conditions might be right for liquid water to exist on a planet's surface. The script uses the concept to highlight that Europa, despite being outside Jupiter's orbit and the Sun's habitable zone, could still have liquid water, challenging traditional notions of habitability.

💡Astrobiologists

Astrobiologists are scientists who study the possibility of life in the universe, including the origins, evolution, and distribution of life. The script mentions astrobiologists' interest in Europa's subsurface ocean, emphasizing the scientific community's pursuit of understanding life beyond Earth.

Highlights

Jupiter has the most moons in the solar system with 67 confirmed moons, and potentially more depending on the definition of a 'moon'.

Galileo's observation of four moons orbiting Jupiter in 1610 provided evidence against the geocentric model of the universe.

The four largest moons of Jupiter, known as the Galilean moons, are visible to the naked eye if not for Jupiter's glare.

Ganymede, Jupiter's largest moon, is larger than the planet Mercury and has a liquid iron core and a magnetic field.

Ganymede's surface features include old cratered terrain, younger smooth areas, and large grooves possibly caused by tidal stress.

Ganymede harbors subsurface oceans of salty liquid water, indicated by magnetic field measurements and Hubble observations.

Callisto, the second-largest moon, has a heavily cratered surface and the thinnest known atmosphere, with no volcanic or tectonic activity.

Io, the most volcanic object in the solar system, has over 400 active volcanoes and a surface colored by sulfur.

Europa, slightly smaller than Earth's moon, is thought to have a subsurface ocean of water beneath its icy crust.

Europa's surface features, such as cracks and ridges, are due to water welling up from its interior ocean.

Europa's potential for life is of great interest to astrobiologists due to the presence of water, salt, and organic materials.

Jupiter's habitable zone concept is challenged by the presence of Europa's subsurface ocean, which is outside the Sun's habitable zone.

Jupiter has dozens of smaller moons, many of which are tidally locked and may have been formed from the proto-Jupiter's eddies.

Amalthea, the next largest moon after the Galilean moons, is irregularly shaped and orbits close to Jupiter, appearing half the sky from its surface.

Many of Jupiter's smaller moons are in retrograde orbits, suggesting they may be captured asteroids from the asteroid belt.

The smallest known moons of Jupiter are about a kilometer across, with potentially thousands or millions of even smaller moonlets.

The moons of Jupiter, from the largest to the smallest, offer a diverse range of scientific opportunities for exploration and study.

Transcripts

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 week’s work.

All four are really big, too. If Jupiter weren’t there, drowning them out with its glare, they’d

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

The biggest of Jupiter’s moons is Ganymede. At 5270 km across, it’s the biggest moon

of any planet. It’s even bigger than the planet Mercury—in fact, in size it’s halfway

between Mercury and Mars!

Size isn’t the only planet-like characteristic of Ganymede, either. It’s 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 there’s very old, cratered terrain

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

It’s 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 Ganymede’s

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 we’ll see in a sec, it’s not alone in that regard.

The next biggest moon is Callisto, at 4800 km in diameter. In many ways it’s 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 doesn’t have a metallic core.

The surface is heavily cratered, and there’s 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 it’s a tad thin: roughly one one-hundred-billionth the pressure of

Earth’s air at the surface!

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

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

each other, it’s really the other three interacting.

Next up is Io. It’s 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 didn’t 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 Jupiter’s 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 Earth’s Van Allen belts, but far more powerful. The radiation there is

so intense it would kill an unprotected human in minutes. Of course, if you’re 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 Jupiter’s 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 planet’s 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. It’s 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 Europa’s 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 Earth’s 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 Earth’s life originated in salty ocean water. If there

are carbon-based molecules actually in Europa’s water, it’s 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 don’t 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.

That’s amazing to me: The idea of life in Europa, even if it’s 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 it’s 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 planet’s surface can support liquid water. It’s a

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

too hot and Mars too cold for liquid water. Atmospheres make a big difference. But it’s

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

But Europa changed that. Jupiter is way, way outside the Sun’s habitable zone, yet there’s

Europa, all wet. It’s 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 we’ll have to keep our minds open to types

of life we’ve 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 they’re tidally locked to Jupiter; they all rotate once for every time

they go around the planet. Jupiter’s tides are hundreds of times stronger than Earth’s,

so no surprise there.

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

lump about 250 km across its longest dimension. It was discovered in 1892, and it’s red—probably

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

if you stood on Amalthea’s 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 we’ve 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 don’t 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 there’s still a lot more left to explore about them.

Today you learned that Jupiter has lots of moons, and four big ones. They’re 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 don’t 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.