Thanks to World of Tanks for sponsoring today’s video. The largest Jovian moons, known as
the Galilean moons, are perhaps some of the most well known in the entire solar system.
However, of the four, there is one that hardly gets mentioned at all, Callisto. Despite the
fact it is the third largest moon that we know of, I think the majority of people would
be hard pressed to say any facts about it at all. Is it just a boring world, or do we
simply not know much about it? Actually, we know more than you may first assume, and it
is far from boring. I’m Alex McColgan, and you’re watching Astrum, and in this video
we will take a deep dive into this mysterious moon, explaining what it’s like there, what
makes it unique, and why it has been one of the most requested topics on this channel.
Let’s start with where it fits into the Jovian system. Callisto is the third largest
moon in the entire solar system, and just smaller than Mercury, at 4820km across. However,
while its diameter is only 58.4 km (36.2 mi.) less than Mercury’s, it is only ⅓ the
mass of the planet, meaning it’s less dense, and its gravity is a lot weaker. It is the
second largest of the Galilean moons and orbits Jupiter much further out than the other three,
taking 17 Earth days to do so at an average distance of 1.9 million km (1,170,000 mi).
Even at this distance, it is still tidally locked with Jupiter, meaning the same side
of Callisto always faces its parent planet. However, it does mean that it is not locked
in an orbital resonance with the other three moons, nor do we believe that it ever was.
Callisto has a very tenuous atmosphere composed of carbon dioxide and possibly oxygen too,
although oxygen has never actually been detected yet. We know, however, that the atmosphere
is so thin that the molecules contained within it do not collide, and theoretically the atmosphere
should be stripped away by atmospheric loss processes in just 4 years. Scientists believe
that Callisto's crust is replenishing the lost atmospheric particles though sublimation
of carbon dioxide surface ices, evidenced by some interesting surface features. Now,
the surface of Callisto is one of the most ancient in the Solar System, with evidence
placing it at over 4 billion years old. You’ll immediately notice the speckled nature of
Callisto. Its surface is completely covered with various sized impact craters, more so
than any other object we’ve observed. In fact, it’s close to saturation, any new
crater will probably just overlap another one at this point. Without any geological
activity, cratering is perhaps the only process that has vastly been impacting Callisto’s
appearance over its lifetime. Why is Callisto’s surface so old compared to a lot of other
bodies in the solar system? Well, on geologically active planets like Earth, there are processes
that can erase almost all evidence of past impacts. Earth has one of the least cratered
surfaces in the solar system because so much happens on its surface. It has weather, water,
plants, volcanism, tectonic plates, and human activity. These act together to break apart,
wear down and lift up the ground. Even other Jovian moons like Europa or Io have comparatively
fewer craters thanks to tidal forces which cause geological activity on their surfaces.
You see, the first 3 Galilean moons Io, Europa and Ganymede, are in locked what’s called
a 4:2:1 orbital resonance. These moons exert a gravitational influence on each other. This
means that every time Ganymede orbits Jupiter once, Europa orbits twice, and Io orbits four
times. The orbital resonance exerts tidal forces on these moons, their constant pulling
never allowing any of them to get a perfectly circular orbit. This means the magnitude of
the gravitational force acting on the moons increases and decreases as they orbit around
Jupiter. This creates tides that stretch the moons’ surfaces and also heat up the moons’
cores. This doesn’t happen with Callisto. Callisto does not show any signs of geological
processes such as volcanism or plate tectonics and so its surface has remained intact after
all these years. Limited tidal forces and thus no geological activity impact Callisto
in another very unique way: there are no mountains to speak of on its entire surface. When Callisto
initially formed at the very beginning of the solar system, it was likely an ocean world
that has since frozen over, and apart from the bombardment of meteors, it has stayed
exactly the same ever since. So, let’s have a look at some of these craters, because some
of the biggest are truly impressive structures. This is Asgard, the second-largest impact
crater on Callisto, measuring 1,600 kilometres (990 mi.) in diameter. And this is Valhalla,
the largest multi-ring impact crater in the entire Solar System with a diameter of 3,800
km. Its these craters that add to the unique appearance of Callisto, as they contain more
rings than craters anywhere else. With these craters, the impactor was large enough that
it may have completely punctured the thin crust, with it eventually refreezing over
in the light patches you can see in the middle. Zooming in on these middle regions, you’ll
notice they have a mottled appearance, there seems to be a big difference in contrast between
the bright knobs and the darker plain. These regions are a lot less cratered than the rest
of Callisto’s surface, which would make sense if the plain truly is a refrozen surface,
likely making it two billion years younger than the rest of Callisto. The rings are likely
fractures in the crust, a concentric failure in the brittle shell of the moon. Interestingly
though, within the rings, these bright knobs are still visible. What are the knobs? Well,
they are believed to be the degraded remains of the millions of crater rims from Callisto’s
past. We don’t know exactly why they have degraded so much over time, but perhaps it’s
due to micrometeor impacts, or simply the ices slowly sublimating over time. They are
brighter than the lower plains because the rocky debris from the meteors and micrometeors
will have fallen down the knobs over time, leaving the pure ice exposed at the top. Smaller
impact craters are also fascinating due to their uniqueness. Most impact craters are
shallower on Callisto compared to our Moon, for instance the Lofn impact structure. It’s
well over 100km wide, however it’s only 600m deep. This could be because the impactor
was breaking apart before it impacted, causing more of a spread-out shotgun effect on the
surface, or it could be that the surface has since levelled off from other, larger nearby
impacts that occurred later on. This crater is believed to be over a billion years old
at least, yet you can still clearly see the ejecta that has streamed away from the impact,
clear evidence of how unchanging this remarkable moon is. With some craters, like Hár crater,
there is even a large dome found in the middle. With typical large impacts on other worlds,
you will see a few concentric rings and a raised peak in the centre. This is very much
the case on our Moon. However, Callisto has some remarkable examples of large craters
where just the opposite happens. Look at Tindr here, instead of a peak there is actually
a pit in the centre. Why does Callisto have such unique craters? It could be due to the
fact that Callisto’s crust is not just brittle, but pretty thin too, with either soft ice
or a salty ocean underneath. Estimates put the crust at 80-150 (40 - 90 mi.) km thick.
The Galileo space probe, which spent several years around Jupiter, spent a significant
chunk of its time aptly studying the Galilean moons. Galileo found that Jupiter’s magnetic
field could not penetrate through Callisto, implying there is a highly conductive layer
under the surface at least 10km thick. This couldn’t have happened in ice or silicates,
unless the ice layer is at least partially molten or very large temperature gradients
can be maintained below the ice to create a conductive effect. Data also suggest that
Callisto has a small silicate core at its centre, with a radius of about 600km. Which
begs the question, can Callisto be conducive to life like other icy worlds with water mantles
under their surfaces? Well, life as we know it requires liquid water and energy to exist.
Callisto might have an ocean of liquid water under its surface but being about 800 million
kilometres (500 million mi.) away from the sun, it barely receives any heat from our
star. And the absence of tidal forces in doesn’t help either, leaving radioactive elements
the only source of heat, to warm the sub-surface ocean, if it exists. And unfortunately, even
visiting Callisto wouldn’t prove anything. Moons like Europa and Enceladus have vents
ejecting water directly from their subsurface oceans, meaning we can investigate habitability
prospects without ever going into the oceans themselves. No such vents exist on Callisto,
so scientists would have to penetrate the crust in order to find out. All these things
combined mean that, unfortunately, scientists think the environmental conditions necessary
for life appear to be less favourable on Callisto than on other icy worlds. Nevertheless, Callisto
shouldn’t be ignored. If we ever go to explore the outer solar system, Callisto would make
a perfect base. You see, orbiting at a distance of 1.9 million kilometres from Jupiter means
that Callisto is located beyond Jupiter’s main radiation belts, making its environment
thousands of times more conducive to human exploration than its inner moons. Callisto
also has a lot of water-ice, that can be used for propellant production, and of course is
the staple to keep humans alive. Separating the hydrogen from oxygen also leaves oxygen
for breathing. Callisto’s geological stability would make for building structures on the
surface relatively worry free, and the lack of huge mountains or deep trenches means it’s
faster, easier and more efficient to travel over. From Callisto, we could better explore
the inner Jovian system from a safe distance, or use it as a waystation for heading farther
into the outer Solar System. Lifting off from Callisto would be ideal thanks to its low
gravity, and you could also get a gravity assist from a close flyby of Jupiter. It’s
true that this is a futuristic prospect, although if we ever do become a space faring species,
this is a distinct possibility. Now, while Galileo did an admirable job studying Callisto
during its few flybys, there is still a lot of information gaps that need to be filled.
With any luck, we’ll get that from the European Space Agency's Jupiter Icy Moon Explorer (JUICE),
due to launch in 2022, which will reach the Jovian system by 2030 and will explore Jupiter
and three of its largest moons, Ganymede, Callisto and Europa. ESA has planned several
close flybys of Callisto during this mission. This mission might provide more insight into
questions like: does it have a sub-surface ocean? And if so, does it or can it sustain
life? Beyond that, there’s also the questions we haven’t thought yet to ask. Who knows
what more we will still discover about this incredible moon. So, there we have it, almost
everything you could want to know about the fascinating, yet underrated world of Callisto.
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If you enjoyed this video about Callisto, you’ll probably enjoy learning about other
moons in our solar system too, so check out this playlist here. A big thank you as well
to my patrons and members who support the channel. If you would like to support too,
check out the links in the description below. All the best and see you next time!