Saturn: Crash Course Astronomy #18

Summary

This episode of Crash Course explores Saturn, the solar system's second largest and least dense planet, known for its stunning rings and rapid rotation. Saturn's atmosphere, composed mainly of hydrogen and helium, hides a likely rocky core. The planet's oblate shape and storms are visible through telescopes. Its hexagonal vortex and massive rings, made of countless ice particles, are shaped by gravitational interactions with its moons. Saturn's moons, particularly Titan with its thick atmosphere and methane lakes, and Enceladus with its subsurface ocean and geysers, present intriguing possibilities for life beyond Earth.

Takeaways

• 🌀 Saturn is the second largest and least dense planet in the solar system, with a density lower than water.
• 🌌 It is a gas giant with a thick atmosphere composed mostly of hydrogen and helium, along with trace gases like methane and ammonia.
• 🌀 Saturn has a rapid rotation, completing a day in just over ten and a half hours, resulting in an oblate shape.
• 🌪 The planet features storms and a unique hexagonal vortex at its north pole, with winds reaching up to 500 kilometers per hour.
• 💍 Saturn's rings are expansive, spanning 250,000 kilometers across, and incredibly thin, averaging only about ten meters in thickness.
• 🚀 The rings are believed to be composed of countless small chunks of nearly pure water ice, each orbiting the planet independently.
• 🌑 Saturn has a complex system of rings with gaps and divisions, such as the Cassini Division, caused by the gravitational influence of its moons.
• 🌍 Titan, one of Saturn's moons, is larger than Mercury and has a thick atmosphere and surface lakes of liquid methane.
• 💧 Enceladus, another Saturnian moon, has geysers of water erupting from its south pole, indicating a subsurface ocean.
• 🌌 Saturn's system includes over 60 known moons, each with unique characteristics and potential for further scientific discovery.
• 🔭 Observing Saturn through a telescope has been a significant source of inspiration for many astronomers and space enthusiasts.

Q & A

What is the most striking feature of Saturn's atmosphere?

Saturn's atmosphere is most notably characterized by its thick atmosphere composed mostly of hydrogen and helium, with trace gases like methane and ammonia. It also has a banded pattern, similar to Jupiter's, but fainter due to its greater distance from the Sun and a deeper atmosphere.

Why is Saturn considered to be the least dense of the planets in the solar system?

Saturn is considered the least dense planet because its density is about 2/3rds that of water, which is less than the density of water on Earth.

How long does it take for Saturn to complete a day?

Saturn spins very quickly, completing a day in just 10.5 hours.

What is the nature of Saturn's storms?

Saturn experiences oval hurricanes that pop up occasionally. In 2010, a massive storm erupted in its northern hemisphere, which grew so large it wrapped around the planet, reaching over 300,000 kilometers in length.

What is the unusual feature at Saturn's north pole?

Saturn's north pole has a huge hexagonal vortex, a well-defined six-sided shape that is a natural outcome of a spinning fluid in the planet's atmosphere.

What are Saturn's rings made of and why are they not solid?

Saturn's rings are made of countless small chunks of nearly pure water ice. They are not solid because if they were, the inner edge would orbit the planet much faster than the outer edge, causing them to be torn apart.

How large are Saturn's rings in comparison to the distance between Earth and the Moon?

Saturn's rings are 250,000 kilometers across, which would cover ⅔ of the distance between Earth and the Moon if Saturn were placed between them.

Why are Saturn's rings so thin?

Saturn's rings are incredibly thin, averaging about ten meters in thickness, due to the gravitational effects of Saturn and the original formation process of the rings, which could have involved a disrupted icy moon or a differentiated moon that lost its icy layer.

What is the Cassini Division and how was it discovered?

The Cassini Division is a gap between Saturn's A and B rings, about 5,000 kilometers across. It was discovered by the Italian astronomer Giovanni Domenico Cassini and is maintained by the gravitational influence of the moon Mimas.

What are some unique features of Saturn's moons Titan and Enceladus?

Titan is Saturn's largest moon, bigger than Mercury, with a thick atmosphere and lakes of methane. Enceladus is smaller but has an undersurface ocean and eruptions of water geysers, making both moons interesting candidates for potential life.

What is the significance of the statement 'Saturn is the crown jewel of the solar system'?

The statement highlights the visual beauty and scientific intrigue of Saturn. Its stunning rings, diverse moons, and atmospheric phenomena make it a highlight of astronomical study and observation.

Outlines

00:00

🪐 Saturn: The Gas Giant with a Unique Hexagon

This paragraph introduces Saturn as the last of the naked-eye planets, highlighting its status as the second largest but least dense planet in the solar system. It delves into Saturn's composition, including its possible rocky core, layers of ice and metallic hydrogen, and a thick atmosphere primarily of hydrogen and helium. The summary mentions Saturn's rapid rotation, resulting in an oblate shape, and its visible cloud tops made of ammonia ices. The paragraph also covers Saturn's atmospheric banding, storms, and the distinctive hexagonal vortex at the north pole. The iconic rings of Saturn are discussed, explaining their composition of water ice chunks and their surprisingly thin profile, as well as theories about their formation.

05:01

🌀 Saturn's Rings and Moons: A Cosmic Dance

This section explores the intricacies of Saturn's ring system, including the discovery of the Cassini Division and the role of Saturn's moons in creating gaps and ringlets within the rings. It describes the dynamics of the F ring, maintained by the gravitational influence of moons Prometheus and Pandora. The paragraph also discusses the phenomena of wave formations in the rings caused by the interaction of small moons with the ring particles. Two of Saturn's moons, Mimas and Titan, are highlighted: Mimas for its massive crater and role in the Cassini Division, and Titan for its substantial atmosphere, hydrocarbon dunes, and potential for subsurface liquid water. The possibility of life on Titan, given its weather, atmosphere, and organic molecules, is also considered.

10:04

🛸 Saturn's Moons: A Menagerie of Celestial Oddities

The final paragraph delves into Saturn's diverse moons, focusing on Enceladus with its reflective surface, potential for cryovolcanism, and discovery of geysers, suggesting an undersurface ocean. It touches on the possibility of life on Enceladus, given the presence of organic molecules in its geysers. The paragraph also mentions other peculiar moons of Saturn, such as Iapetus with its walnut-like shape and Hyperion with its sponge-like, porous nature. It concludes with a personal note from the author about the inspirational impact of observing Saturn through a telescope and the scientific and aesthetic wonder it represents, ending with credits for the Crash Course Astronomy production team.

Keywords

💡Saturn

Saturn is the sixth planet from the Sun and the second largest in our solar system. It is a gas giant with a thick atmosphere, primarily composed of hydrogen and helium, and is known for its extensive ring system. In the video, Saturn's unique features, such as its low density and rapid rotation, are highlighted, making it an oblate spheroid shape. The planet's storms, hexagonal vortex, and rings are central to the video's exploration of its complex and fascinating environment.

💡Gas Giant

A gas giant is a large planet that is not primarily composed of solid materials but rather consists mostly of gases, such as hydrogen and helium. In the context of the video, Saturn is described as a gas giant, which means it lacks a solid surface and has a deep atmosphere with layers of ice and metallic hydrogen surrounding a possible rocky core.

💡Oblate

Oblate refers to a shape that is wider through the equator than through the poles, typically due to rapid rotation. The video explains that Saturn is oblate because it spins quickly, completing a day in just over ten hours, which causes it to be squashed at the poles and bulging at the equator.

💡Atmosphere

The atmosphere of a planet is the layer of gases surrounding it. Saturn's atmosphere, as mentioned in the video, is hundreds of kilometers deep and is composed mainly of hydrogen with trace amounts of other gases like methane and ammonia. The atmosphere plays a crucial role in shaping the planet's weather patterns and appearance.

💡Rings

Saturn's rings are one of its most distinctive features and are a major focus of the video. These rings are made up of countless small chunks of nearly pure water ice, which orbit the planet independently. The rings are vast, spanning 250,000 kilometers across, yet incredibly thin, averaging only about ten meters in thickness.

💡Hexagonal Vortex

A hexagonal vortex is a weather phenomenon characterized by a six-sided shape, as seen at Saturn's north pole. The video describes this vortex as a natural outcome of the spinning fluid in Saturn's atmosphere, similar to Earth's jet stream but much larger, with wind speeds reaching up to 500 kilometers per hour.

💡Moons

Moons are natural satellites that orbit a planet. Saturn has a multitude of moons, some of which, like Titan and Enceladus, are highlighted in the video for their unique characteristics and potential for life. Moons can also influence the structure of Saturn's rings, creating gaps and resonances that affect the distribution of ring particles.

💡Titan

Titan is Saturn's largest moon and is larger than the planet Mercury. The video describes Titan as having a thick atmosphere composed mainly of nitrogen, with methane and hydrogen present. It also has a surface featuring lakes of liquid methane and potential cryovolcanism, making it a subject of interest for the search for extraterrestrial life.

💡Enceladus

Enceladus is another of Saturn's moons, noted in the video for its potential to harbor life. It is covered in water ice and has geysers of water erupting from its south pole, indicating an undersurface ocean. The detection of organic molecules in the plumes of water from Enceladus adds to the intrigue of its potential for life.

💡Cryovolcanism

Cryovolcanism refers to the phenomenon where liquid water, instead of lava, is erupted from a volcano. The video mentions cryovolcanoes on Titan and the possibility of such activity as a reason for the moon's smooth surface areas, suggesting an exotic form of geological activity.

💡Resonance

In the context of celestial mechanics, resonance refers to a specific orbital relationship between two bodies where the orbital periods have a simple integer ratio. The video explains that resonance is responsible for the gaps in Saturn's rings, such as the Cassini Division, where the gravitational pull of a moon repeatedly affects the ring particles.

Highlights

Saturn is the second largest planet in the solar system but has the least density, and would theoretically float in a large enough bathtub.

Saturn is a gas giant with a thick atmosphere, likely containing a rocky core surrounded by layers of ice and metallic hydrogen.

The atmosphere of Saturn is mostly hydrogen with trace gases like methane and ammonia, giving it a density about 2/3 that of water.

Saturn completes a rotation on its axis in just 10.5 hours, making it significantly oblate.

The cloud tops of Saturn are visible from Earth and are composed mostly of ammonia ices.

Saturn's atmosphere has faint banded patterns, likely due to its distance from the Sun and deeper atmosphere compared to Jupiter.

Saturn experiences oval hurricanes and a massive storm in 2010 that wrapped around the planet and persisted for years.

A unique hexagonal vortex is present at Saturn's north pole, a natural outcome of the planet's spinning fluid dynamics.

Saturn's rings, first seen by Galileo, were later identified by Christiaan Huygens as a ring system surrounding the planet.

The rings of Saturn are made of countless small chunks of nearly pure water ice, each orbiting the planet independently.

Saturn's rings are incredibly thin, averaging about ten meters in thickness, making them thinner than a sheet of paper when scaled.

Saturn's main rings are named A, B, and C, with the B ring being the broadest and the Cassini Division separating the A and B rings.

The gravity of Saturn's moons creates gaps and resonances in the rings, influencing their structure and stability.

Saturn's moon Titan is larger than Mercury and has a thick atmosphere composed mostly of nitrogen with traces of methane and hydrogen.

Titan has lakes of liquid methane on its surface, indicating weather patterns driven by methane rather than water.

Saturn's moon Enceladus has geysers of water erupting from its south pole, indicating a subsurface ocean.

The possibility of life on Saturn's moons Titan and Enceladus is considered due to their unique atmospheric and subsurface conditions.

Saturn has a diverse array of over 60 moons, each with unique characteristics and potential for scientific discovery.

Personal anecdotes from astronomers often credit their first telescope view of Saturn as the inspiration for their career choice.

Transcripts

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

So now we’ve come to the last of the naked eye planets in the solar system: Saturn. It’s

the second largest planet, but ironically the least dense. In fact, if you could find

a bathtub big enough, Saturn would float in it!

But it would leave a ring.

OK, that’s the oldest joke in astronomy, but it’s true. Saturn is a gas giant, which

means it has a thick atmosphere. Like Jupiter, it probably has a rocky core several times

the mass of Earth, surrounded by layers of ice and weird, metallic hydrogen. On top of

all that is its atmosphere, hundreds of kilometers deep, composed mostly of hydrogen, with a

smaller amount of helium and a whole bunch of trace gases like methane and ammonia.

Overall, its density is about 2/3rds that of water. It wouldn’t really float, though;

its gravity would draw in all the water from the giant bathtub… although, actually, come

to think of it, a bathtub filled with water big enough to put Saturn in would have more

mass than the planet, so the water would pull Saturn in.

Astronomy is fun!

Saturn spins really quickly, completing a day in just ten and a half hours. Its rapid

rotation and low density means it’s really oblate, or squished; it’s 10% wider through

the Equator than through the poles! That’s really noticeable through a telescope.

When we look at Saturn, we only see its cloud tops. Those clouds are mostly ammonia ices,

with water clouds lower down. The atmosphere is banded, like Jupiter’s, but the pattern

is much fainter, probably due to Saturn being farther from the Sun and therefore colder,

as well as the atmosphere being deeper than Jupiter’s too.

That’s not to say Saturn doesn’t have storms. Oval hurricanes pop up every now and

again, and in 2010 a huge storm erupted in Saturn’s northern hemisphere. It was discovered

by an amateur astronomer, and Cassini spacecraft pictures showed it getting so big it literally

wrapped around the planet, reaching a length of over 300,000 kilometers! Traces of it persisted for years.

One of the most striking features on Saturn is a huge, hexagonal vortex at its north pole.

Vortices like this are common on planets; Earth and Venus have them too. That well-defined

six-sided shape is pretty freaky, but it’s actually a natural outcome of a spinning fluid—the

air circulating Saturn’s atmosphere—which itself is in a spinning system—in this case,

the planet itself. It’s not that different than Earth’s jet stream, but it’s a lot

bigger, about 20,000 kilometers across… wider than Earth. And right at its center,

over Saturn’s north pole, is a ridiculously beautiful storm system about 2000 kilometers

across. Wind speeds there have been clocked at 500 kilometers per hour.

Saturn as a planet is of course an amazing place, but c’mon. When you think Saturn,

you think rings. As well you should.

Saturn’s rings were seen by Galileo, but his telescope lacked the resolution to really

figure out what they were—he thought Saturn might be three objects in close proximity.

Some decades later the Dutch scientist Christiaan Huygens correctly interpreted their appearance

as a ring surrounding the planet.

They’re not solid. If they were they’d get torn apart, since the inner edge would

orbit the planet way faster than the outer edge. They’re actually made of countless

small chunks of very nearly pure water ice. Each particle orbits the planet independently…

so I guess Saturn wins in the “which planet has the most moons” category.

What really gets me about Saturn’s rings is their scale. They’re 250,000 kilometers

across. If you put Saturn between the Earth and Moon, the rings would cover ⅔ that distance.

And yet, despite their size, they’re incredibly thin: They average about ten meters thick.

And no, I did not misspeak, I meant ten meters. Ten meters.

That’s amazing. That’s thinner, to scale, than a sheet of paper. Way thinner: If you

shrank Saturn down to the size of a piece of paper, that paper would be ten thousand

times thicker than the rings.

So yeah, Saturn’s rings are flat. But why?

Well that’s tied to their origin, though the specifics of how they came to be aren’t

exactly known. One idea is that they were originally an icy moon that got disrupted

by a huge collision. The shattered moon particles would have spread around Saturn, and any of

them that were on a tilted orbit would quickly collide with others, and in the end that would

average out all their orbits to form a flattened ring.

Or maybe there was once a large moon near Saturn, that was big enough to be differentiated;

that means the rocky bits fell to the moon’s core, while the ice stayed near the surface.

As it plowed through the material around the protoSaturn, the moon’s icy layer got stripped

off, forming the rings. And again, that would form a very flat disk.

However they formed, collisions would quickly flatten the ring out. And remember, Saturn

is oblate, so as the rings formed they would’ve felt a gravitational tug from Saturn’s equatorial

bulge, centering them right over the planet’s midline.

Saturn has three main rings, give or take. In classic imaginative astronomer fashion,

they’re given the names A, B, and C—in order of their discovery. B is the broadest,

roughly 26,000 kilometers wide. A is outside B, while the C ring is fainter and inside the B ring.

When the rings were first observed, the A and B rings were thought to be one ring. But

then the Italian astronomer Giovanni Domenico Cassini discovered a gap between them, and

we now call that the Cassini Division. It’s about 5000 kilometers across, but it’s not

empty! There are quite a few very narrow rings inside of it.

The gap is due to one of Saturn’s moons, Mimas. If you’re an ice particle in the

Cassini division, you orbit Saturn twice for every one time Mimas goes around. This is

called a resonance, and it means that the particles at that location get tugged repeatedly

by the gravity of Mimas, which in essence yanks them out of their orbit.

There are quite a few gaps in the rings caused by moons; in fact, there are hundreds of “ringlets”

in the main rings. Even weirder, the F ring is outside the A ring, and is very narrow,

just a few hundred kilometers across. It’s kept that way by two moons, Prometheus and

Pandora, orbiting just inside and outside the ring. They tug on the ring particles in

such a way that keeps them confined to very strict orbits; if a particle wanders out,

the moons’ gravity drags it back in.

As the moons and ring particles interact, they can also create weird rippling waves

in the rings. And this is really cool: when small moons orbit Saturn on slightly inclined

orbits; it can create waves in the rings, tremendous vertical excursions that can reach

several kilometers high.

Saturn is weird.

And so are its moons. I already mentioned Mimas, an icy moon that carves out the Cassini

Division with its gravity. It also suffered a massive impact long ago that created a single

huge crater, making it look somehow familiar.

But Saturn has two moons specifically you should know about. The first is Titan, a monster

satellite that’s actually bigger than Mercury, not that much smaller than Mars, and second

in moon size ranking only to Jupiter’s moon Ganymede. Titan not only has a substantial

atmosphere—the only moon to have one—but it’s thicker than our air on Earth! I wouldn’t

suggest breathing it, though: It’s mostly nitrogen, with a smattering of methane and

hydrogen. It’s also a hundred and eighty below zero Celsius there. Definitely don’t

stick your tongue on a flagpole on Titan.

Titan’s atmosphere has a thick layer of haze that prevents us from seeing the surface

using visible light. But using infrared and radar, though, the Cassini spacecraft, which

has been orbiting Saturn since 2004, is able to map the surface. It also deployed a probe

named Huygens to land on Titan’s surface, the first time humanity had ever landed on

an outer solar system moon.

What they saw was an odd world, Earth-like in some ways, and very alien in many others.

Titan has wind-sculpted dunes, but they’re made of hydrocarbon grains, not sand. There

are hints of volcanoes that spew liquid water instead of lava. These are called cryovolcanoes,

cold volcanoes, a term I love. This, together with measurements of Titan’s gravitational

field, have scientists thinking that Titan may have an underground ocean of liquid water.

Intriguingly, Titan also has long, winding channels indicating flows of some kind of

liquid on its surface. But it can’t be water; it’s far too cold on the surface for that.

So what kind of liquid could it be? Well, Cassini also spotted quite a few flat regions

near Titan’s north and south poles. These have since been confirmed to be lakes of liquid

methane—the first time any liquid has been seen on the surface of another world. The

shorelines of the lakes appear to change over time, too, hinting very strongly that Titan

has weather, driven by methane as opposed to water here on Earth.

Weather, atmosphere, surface liquids, and an abundance of organic, carbon-based molecules…

could there be life on Titan? The conditions there are…interesting. We obviously don’t

know yet, but it’s an exciting possibility.

And it’s not the only Saturnian moon scientists are looking at for signs of life. Enceladus

is a smaller, icy moon about 500 km across: About the size of my home state of Colorado

or the southern part of England. Enceladus is incredibly shiny, reflecting nearly all

the light that hits it: a strong indicator it’s covered in water ice.

Parts of it are cratered, but other parts are very smooth, indicating some event resurfaced

the moon. The surface also has an extensive series of cracks in it, much like Jupiter’s

moon Europa. Scientists suspected cryovolcanism, and that was confirmed quite dramatically

in 2005 when Cassini spotted active geysers of water spewing from the south pole of Enceladus!

The water is erupting from a series of cracks nicknamed “tiger stripes”, and is coming

from deep under the surface. Like Europa, Enceladus has an undersurface ocean, kept

liquid by Saturn’s ferocious tides, which squeeze the moon as it orbits the planet.

The plumes reach several hundred kilometers from the moon’s surface, and Cassini even

flew directly through the plumes, and detected organic molecules.

Could Enceladus be yet another harbor for life in the solar system? Again, no one knows,

but I’d put it in the top three places to look, including Europa and Titan. It’s weird,

isn’t it? We’ve always looked for planets where conditions were like here on Earth,

but it turns out extremely cold moons may be the best places to seek out new life.

Saturn has a huge retinue of other moons, too, and they’re all pretty weird. Iapetus

is shaped like a walnut, with a tall ridge of mountains encircling its equator; no one’s

quite sure how that formed. Hyperion looks like a huge piece of Styrofoam that’s been

hit repeatedly with a BB gun—and that’s not a bad analogy, since it has such a low

density it’s likely to be extremely porous, like a sponge. A lot of Saturn’s moons orbit

the planet backwards, retrograde; similar to several moons of Jupiter. These may be

captured asteroids, or even Kuiper Belt Objects, giant chunks of ice that we’ll learn about

very soon in a future episode. All in all, Saturn has over 60 moons we’ve discovered so far.

I want to add a personal note. When you ask astronomers or astronauts what got them started,

what was the very first thing that happened to them to inspire them to do what they do,

overwhelmingly they will tell you: “I saw Saturn through a telescope.”

It’s true. It’s true for me; I remember my parents buying a cheap department store

telescope when I was five or six, setting it up in our driveway, and aiming it at Saturn.

I still remember seeing Saturn in the eyepiece, a perfect jewel, the rings small but vivid

and sharp. It’s hard to believe it’s real when you see it, and even after all these

years, it’s my favorite target when I’m stargazing. I’ll never get tired of watching

peoples’ faces when they see it for the first time. They gasp, they their eyes get

wide, their mouths drops open—it’s honestly one of the greatest joys of life.

Saturn is the crown jewel of the solar system. It’s the best kind of astronomical object:

Visually stunning, artistically gorgeous, and filled to overflowing with incredible science.

Today you learned that Saturn is a gas giant, and has a broad set of rings made of ice particles.

Moons create gaps in the rings via their gravity. Saturn has dozens of moons, including Titan,

which is as big as Mercury and has a thick atmosphere and lakes of methane; and Enceladus

which has an undersurface ocean and eruptions of water geysers. And, though we don’t know

for sure, it’s possible either or both moons may support life.

Crash Course Astronomy is produced in association with PBS Digital Studios. Head on over to

their channel and discover 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.