Jupiter's Moons: Crash Course Astronomy #17
Summary
TLDRThis 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
- đ 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.
- đ Galileo's observation of four moons orbiting Jupiter in 1610 was a pivotal moment, demonstrating that not all celestial bodies revolved around Earth.
- đ 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.
- đïž Ganymede, Jupiter's largest moon, is bigger than the planet Mercury and has a liquid iron core that generates a magnetic field.
- đ Ganymede is believed to have subsurface oceans of salty liquid water, inferred from its magnetic field measurements and Hubble observations.
- đ Callisto, the next largest moon, has a heavily cratered surface and a thin atmosphere, and orbits farther from Jupiter than the other large moons.
- đ Io is the most volcanically active object in the solar system, with its surface constantly reshaped by volcanic eruptions, primarily of sulfur.
- đ Io's volcanic activity is fueled by tidal flexing from the gravitational interactions with the other Galilean moons, heating its interior.
- đ 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.
- 𧏠Europa's potential for life is of great interest to astrobiologists due to the presence of water, salt, and organic materials on its surface.
- đ 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
đ 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.
đż 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.
đŹ 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.
Mindmap
Keywords
đĄJupiter
đĄGalilean moons
đĄGanymede
đĄCallisto
đĄIo
đĄEuropa
đĄVolcanism
đĄMagnetic field
đĄTidal flexing
đĄHabitable zone
đĄAstrobiologists
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 weekâs work.
All four are really big, too. If Jupiter werenât there, drowning them out with its glare, theyâd
be visible to the naked eye. In that case we might even call them planets, too.
The biggest of Jupiterâs moons is Ganymede. At 5270 km across, itâs the biggest moon
of any planet. Itâs even bigger than the planet Mercuryâin fact, in size itâs halfway
between Mercury and Mars!
Size isnât the only planet-like characteristic of Ganymede, either. Itâs 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 thereâs very old, cratered terrain
as well as smoother, younger areas. Ganymede is also criss-crossed with large grooves.
Itâs 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 Ganymedeâs
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 weâll see in a sec, itâs not alone in that regard.
The next biggest moon is Callisto, at 4800 km in diameter. In many ways itâs 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 doesnât have a metallic core.
The surface is heavily cratered, and thereâs 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 itâs a tad thin: roughly one one-hundred-billionth the pressure of
Earthâs air at the surface!
Callisto orbits Jupiter farthest out of the four biggies, almost 2 million km away. Thatâs
too far to gravitationally interact with the other three; when I talk about the moons affecting
each other, itâs really the other three interacting.
Next up is Io. Itâs 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 didnât 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 Jupiterâs 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 Earthâs Van Allen belts, but far more powerful. The radiation there is
so intense it would kill an unprotected human in minutes. Of course, if youâre 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 Jupiterâs 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 planetâs 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. Itâs 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 Europaâs 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 Earthâs 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 Earthâs life originated in salty ocean water. If there
are carbon-based molecules actually in Europaâs water, itâs 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 donât 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.
Thatâs amazing to me: The idea of life in Europa, even if itâs 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 itâs 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 planetâs surface can support liquid water. Itâs a
fuzzy concept; Venus and Mars are both technically in the Sunâs habitable zone, but Venus is
too hot and Mars too cold for liquid water. Atmospheres make a big difference. But itâs
still a useful concept as rule of thumb for potential habitability.
But Europa changed that. Jupiter is way, way outside the Sunâs habitable zone, yet thereâs
Europa, all wet. Itâs 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 weâll have to keep our minds open to types
of life weâve 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 theyâre tidally locked to Jupiter; they all rotate once for every time
they go around the planet. Jupiterâs tides are hundreds of times stronger than Earthâs,
so no surprise there.
The next biggest moon after The Big Four is way smaller; named Amalthea, itâs an irregular
lump about 250 km across its longest dimension. It was discovered in 1892, and itâs redâprobably
polluted by sulfur from Io. It orbits just over 100,000 km from Jupiterâs cloudtops;
if you stood on Amaltheaâs 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 weâve 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 donât 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 thereâs still a lot more left to explore about them.
Today you learned that Jupiter has lots of moons, and four big ones. Theyâre 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 donât 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.
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