Title: Real Images of What Nasa Discovered On Callisto
“Through Voyager, Callisto, Calliope Betelgeuse, the neon moons
We're a slow-burning tune But we'll touch down soon”
This is part of the lyrics from Coloratura, a new song from Coldplay.
We all know music connects people, and Coldplay is very good at doing so.
Their subtle references to space exploration missions, celestial objects and stars are
so evocative and inspiring. It's like they are pushing us
towards the future when humanity will be hopefully part of something bigger.
Maybe a future in which interstellar travelling will be possible.
In the lyrics, there are references to Voyager, the fantastic NASA mission that
exited the Solar System (and was the first human-made mission to do so);
then there are references to Calliope and Betelgeuse.
Calliope is an asteroid from the asteroid belt in our Solar System,
discovered in 1852, and Betelgeuse is a star in the Orion constellation
that is going to be a supernova, although we don't know exactly when.
If you are interested, you can find out more about Voyager and Betelgeuse
at the links in the description.
Today we want to talk about Callisto.
This will conclude our grand tour of the Galilean satellites, the 4 moons of Jupiter.
Follow me to get to know more about the second largest Jupiter's moon.
(roll intro)
So far, in our previous videos, we had the chance to better understand the chemical,
physical, dynamical characteristics of Ganymede, Europa and Io, 3 of the 4 Galilean satellites.
Each one of them has its own special features, which make it a unique moon.
Europa has freckles on its surface, Io's is covered with volcanoes that produce plumes of
sulfur and sulfur dioxide, and Ganymede has an ocean of liquid water beneath its icy crust.
What about Callisto? First thing's first,
Where does the name Callisto come from?
Well, you should know that, according to Greek Mythology, Zeus had many lovers.
Callisto was one of them. She was a nymph,
a beautiful being associated with the goddess of the hunt, Artemis.
The name was once again suggested by astronomer Simon Marius,
but it was not in common use until the mid 20-the century. Before that,
they used to call him with its roan designation: Jupiter IV or the fourth satellite of Jupiter.
The origin of names is very important from a cultural point of view.
But when it comes to astronomy and astrophysics, we need something more.
We want to have the entire identity card of Callisto.
We wish to know, for example, its distance from Jupiter.
As of today, we know that Callisto is the outermost of the four Galilean moons of Jupiter.
It is very far from the gaseous planet, orbiting it at 1 880 000 km.
This is a huge distance, almost 26 times greater than the radius of Jupiter itself.
In the videos about Europa, Io and Ganymede we saw that all of them experience some sort of
resonance, meaning that their orbits are somehow connected in a precise way. These orbit resonances
are locking the satellites together, and the tidal locking phenomenon creates some internal structure
mechanisms which could contribute to increasing the heating of the inner part of the moons.
However, Callisto is so far from the other three moons that it does not participate
in the mean motion resonance.
Also, a high distance from the planet means a high orbital period, that lasts about 16.7
Earth days because Callisto has to travel a huge distance before ending an entire orbit.
During the years, we had some missions that encountered Callisto, taking amazing
pictures and collecting precious data for us. We will talk about it later in this video, and
I'll show you the amazing pictures, as promised. But first, it is worth stressing that all of these
missions helped us to understand better the physical and chemical properties of the moon.
For example, we know that the average density of Callisto is 1.83 g/cm^3.
If you think that the water density is 1 g/cm^3, this means that, on average,
Callisto is denser than water. This tells us that it is made
of rocky material and water ice, with some additional volatile ices, such as ammonia.
How do we know that Callisto is made of some water ice?
Well, it's all about spectroscopy. When astronomers looked at the moon
through near-infrared filters, they found the presence of ice absorption bands,
suggesting the presence of ice. But this is not the end of the story!
More accurate measurements are telling us that ice water is ubiquitous on its surface.
Talking about the surface, we know that Callisto is asymmetric: the leading hemisphere is darker
than the trailing one. This is peculiar since the reverse is true for Europa, Io and Ganymede.
But we know a lot more about Callisto. For example, we can infer some interior
characteristics from gravity field and magnetic field measurements.
We found that Callisto's radius is 2403 km, larger than our Moon's radius and that we
can think of its interior as a relatively uniform mixture of comparable amounts of ice and rocks.
The surface layer may differ from the underlying ice/rock layer in a variety of ways
including, for example, the percentage of rock it contains.
The lack of orbital resonances with other moons, and so the lack of tidal heating,
makes Callisto's geological history a really simple one if compared to the one of Ganymede,
although the two moons are very similar in bulk properties.
We think Callisto has never been heated enough to melt its ice component.
However, this doesn't mean we don't have an ocean of liquid water in its interior.
This is due to the anomalous behaviour of ice melting temperature,
which decreases with pressure. This critical temperature is reached
in the layer between 100 and 200 km. Also, the presence of a small amount of
ammonia almost guarantees the liquid's existence because ammonia has the magic power to lower the
melting temperature even further. And the most interesting thing is that there could be life
in Callisto's subsurface ocean. Indeed, a possible subsurface ocean might be composed of saltwater.
It is possible that some organisms like halophiles could thrive in that ocean.
Those are extremophiles that thrive in high salt concentrations.
Perfect, I think I told you a lot of things about Callisto.
Now it's time to talk about exploration missions that gave us this giant knowledge and
see the real pictures NASA took of this astonishing moon.
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Are you ready? Let's go!
The history of Callisto's explorations starts with the Pioneer mission.
Pioneer 10 and 11 were two spacecraft that reached Jupiter in the early 70s
but contributed little new information about Callisto,
in comparison with what was already known from Earth-based observations.
It was only in 1979, with the advent of Voyager 1 and Voyager 2 flybys,
that the real breakthrough happened.
The two spacecraft, which aim was to exit the solar system,
passed close to the moon, and imaged more than half of the Calistoan surface,
with a resolution of 1-2 km. They also measured mass, temperature and shape.
This picture of Callisto was taken by Voyager 1 Tuesday, March 6, 1979,
during its approach to Jupiter's outer large satellite. The spacecraft was 350,000
kilometres from Callisto at the time and shows features about 7 kilometres across the surface.
The prominent bullseye type feature in this picture is believed to be a large impact basin,
similar to Mare Oriental on our Moon. The brighter circular spot is about
600 kilometres across, while the outer ring is about 2600 kilometres across. This is the first
recognized basin in the Jovian system and supports the concept of an old surface for Callisto.
The darker colour of Callisto (about half as reflective as Ganymede but still twice
as bright as our Moon) suggested that its upper surface was a "dirty ice" or
water-rich rock frozen at Callisto's cold surface,
therefore with a temperature of about 120 degrees K (or -243 degrees F)at the equator.
After Voyager encounters with Callisto, we had to wait until 1995
to have a second round of exploration. It was 1994 when Galileo Spacecraft flew by it 8 times,
the last having place in 2001, at a distance of 138 km to the surface.
Galileo orbiter gave us amazing pictures of Callisto. Voyager had a resolution of 1-2 km,
as we've already said. But Galileo delivered a number of pictures with a resolution as high as
15 meters of selected areas of the surface. The analysis of
high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft has revealed various
non-ice materials, such as magnesium silicates and possibly ammonia and organic compounds.
Here are of course some pictures taken from Galileo.
This one, for example, shows bright scars on a darker surface
that testify to a long history of impacts on Jupiter's moon.
As we can read on NASA's website,
“The picture, taken in May 2001, is the only complete global colour image
of Callisto obtained by Galileo, which has been orbiting Jupiter since December 1995.
Callisto's surface is uniformly cratered but is not uniform in colour or brightness. Scientists
believe the brighter areas are mainly ice and the darker areas are highly eroded ice-poor material.”
These four views of Jupiter's second-largest moon, Callisto,
highlight how increasing resolutions enable interpretation of the surface.
In the global view (top left) the surface is seen to have many small bright spots,
while the regional view (top right) reveals the spots to be the larger craters. The local
view (bottom right) not only brings out smaller craters and detailed structure of larger craters
but also shows a smooth dark layer of material that appears to cover much of the surface.
The close-up frame (bottom left) presents a surprising smoothness in this highest-resolution
view of Callisto's surface - 30 meters per picture element. Isn't it crazy to
think that we have pictures of other objects in the solar system, with such a high resolution?
Last but not least, in 2000, the Cassini spacecraft en route to Saturn
acquired high-quality infrared spectra of the Galileian satellites, including Callisto.
It acquired high-quality infrared spectra of the Galilean satellites.
Also new horizon probe on its way to pluto obtained new images and spectra of Callisto.
Before ending this video, we want to say that in 2003
NASA conducted a conceptual study called HOPE, which stands for Human Outer Planets Exploration.
This regarded the future of human exploration of the outer solar system,
and the target chosen to consider in detail was Callisto!
In December 2003, NASA reported that a manned mission to Callisto
might be possible in the 2040s.
Building a base on Callisto could facilitate remote exploration of Europa, or be an ideal
location for a Jovian system waystation servicing spacecraft heading farther into the outer Solar
System, using a gravity assist from a close flyby of Jupiter after departing its moon.
"This video ends here! Thanks for watching everyone! Are you guys in love with Europa?
Let us know in the comments below! See you soon on the channel!