Real images of what NASA discovered on Jupiter's oceanic moon Ganymede
When we talk about space, we often think about giant planets, stars, black holes and the
curvature of space, what lies beyond what we can see, where the Big Bang happened, and
why we are here.
It's like our mind likes to “think big”.
But I can assure you that space is much more variegated and better than this.
Space is not only about big stars and black holes.
It is not only about merging neutron stars and supernovae explosions.
Indeed, we know very well that smaller objects exist and play a big role in our understanding
of the universe.
Some of them are, for example, comets and moons.
Each of them is a world apart, with its own secrets to be revealed.
Just to say, did you know that comet impacts on a planet surface could enrich the planet
with water?
Don't you think this is amazing?
And what about moons?
We don't have to go far to find a fantastic moon with interesting properties: we just
have to point our telescopes towards Jupiter.
There's absolutely no need to look for exotic exomoons in order to get caught in the sheer
beauty of these smaller celestial objects.
Today we are going to explore one of them.
Its name is Ganymede.
It is one of Jupiter's moons.
In this video, you will find real pictures of it, and you'll learn a lot about its physical
and chemical structure.
I'm sure it will blow your mind.
The moon Ganymede, as well as other famous celestial objects, takes its own name from
Greek mythology.
This often happens in astronomy, and it shows how astronomy is not just a physics and mathematical
thing, but it involves also some cultural factors, and we think this is wow.
Indeed, Ganymede took its name from a Greek mythological character who, according to Greek
mythology, was an exceedingly handsome aristocratic young Trojan, abducted by Zeus to become his
cupbearer.
Transforming himself into an eagle, the Mighty Thunderer swooped to earth and carried off
the boy to Mount Olympus, where his services were rewarded by immortality.
The famous “Rape of Ganymede” by Michelangelo's drawing captures the moment when Ganymede
has just been seized and is being borne aloft.
Far from showing fear, his body is yieldingly relaxed and his facial expression one of dreamy
submission.
However, the name Ganymede was not in common use until the mid 20-the century.
Okay, now that we are done with the presentations, let's see what we know about this famous Jupiter's
moon.
In order to describe celestial objects, astronomers give some crucial parameters which provide
us with important information.
Since it is a Jupiter moon, we want to know how far from the giant gaseous planet it is
from the centre.
Accurate measurements show that it orbits around Jupiter at a distance of about 1 million
kilometres, and it completes a revolution every seven days or so.
If Jupiter was the Sun and Ganymede was the Earth, you would have to celebrate your birthday
every seven days.
Doing a simple calculation, a 1 Earth-year baby would be approximately 52 Ganymede-Years
old.
Another important thing to say about Ganymede (and this probably applies to most known moons)
Ganymede is locked.
Well, tidally locked.
What does it mean?
It simply means that it has one side always facing toward the planet, just as the Moon
does with Earth.
This tidal locking behaviour is well described by celestial body mechanics, and it says that,
under certain initial conditions, the moon will “be locked” in a position so that
he will always show the same side to the planet, and this is due to the gravitational potential
of the planet acting on its own moon, and to the fact that the moon is not a perfectly
spherical object.
Despite being a small object, Ganymede is the largest and most massive moon in the Solar
System.
It has a huge diameter of 5268 km.
Just for comparison, the Earth has a diameter of about 12.742 km.
Accurate density measurements show that Ganymede average density is about 1.936 g/cm^3.
Now, I don't know if you are used to this kind of number, but the density of the water
is, for example, 1 g/cm^3.
This suggests a composition for Ganymede of about equal parts rocky material and mostly
water ices.
You could of course walk on Ganymede's surface.
Over the years, spending a lot of time unveiling the secrets of Ganymede, we learned that some
of the water composing this moon is in liquid form.
That's why we think that Ganymede owns an underground ocean.
The exact composition of Ganymede's rock is not known, but it is probably made out of
chondrites.
Chondrites are characterized mostly by Iron and Iron oxides.
We know chondrites very well since they significantly contribute to the total meteorite population
here on Earth (from 85.7 to 86.2 per cent!).
Here's a Chondrite picture:
Thank you, Insane Curiosity, for all of this information!
But what can we say in terms of observations?
What do we expect to see if we point our telescope towards Ganymede?
Is it bright or not?
Ganymede has an albedo of about 43%, meaning that it can reflect back to us the 43% of
the total solar radiation hitting on its surface.
Also, as they were suspecting from density measurements when astronomers studied Ganymede
by means of the Near-Infrared spectroscopy, they found strong water ice absorption bands,
thus confirming the presence of water ice on Ganymede.
However, the leading Hemisphere is brighter than the trailing one, and this is probably
due to the presence of extended ice caps in the leading hemisphere, while the trailing
one seems to be enriched in sulfur dioxide.
Talking about its internal structure, here's how Ganymede appears to be:
Differentiated layers appear to contribute to its composition, in particular, we have
an internal structure consisting of iron sulfide and iron core, a silicate mantle and outer
layers probably made of water ice and liquid water.
In the 70s, NASA scientists began to suspect that Ganymede had a thick ocean between two
layers of ice.
After 20 years, they sent a mission to confirm or reject this hypothesis.
The mission's name was Galileo.
Later in this video, we will see what Galileo found.
One thing left to discuss is whether Ganymede has an atmosphere or not.
When we launched HST, we found that Ganymede does have one!
It is actually a tenuous atmosphere, made of oxygen.
We know this because HST observed airglow of atomic oxygen in the Ultraviolet wavelengths.
Such airglows are excited when molecular oxygen I dissociated by electron impacts.
Spectroscopy is a really powerful instrument we have to investigate other worlds.
And it is based on the light emission or absorption of electronic transitions of elements.
So if spectroscopic studies suggest the presence of oxygen lines, then there is no doubt: Oxygen
is present on Ganymede!
Also, the existence of such an oxygen atmosphere implies that an ionosphere should exist.
We expect that high energy electrons coming from both solar radiation and magnetosphere
of the moon ionise oxygen, contributing to building an ionosphere.
Okay ladies and gentlemen, now we know a lot about Ganymede.
We introduced it to you, but until now you've barely seen it.
That's why we are going to show you the most beautiful pictures of Ganymede we have taken
so far.
All of the pictures we are about to discuss were taken from probes flybys all over the
years.
We never had a dedicated mission to Ganymede, meaning that no spacecraft has yet orbited
Ganymede, but there have been multiple proposals to do so, including the JUICE mission, which
is currently under construction.
This is one of the pictures taken from the Pioneer 10 mission, which performed a flyby
in 1973 as it passed through the Jupiter system at high speed.
Pioneer was the first spacecraft to obtain close-up images of Jupiter, Callisto, Europa
and Ganymede.
This mission was useful to determine the moon's physical characteristics we discussed so far!
Thank you, Pioneer!
Six years later, the famous Voyager 1 and 2 probes passed through the Jupiter system,
as they were designed to study the outer Solar System and interstellar space beyond the Sun's
heliosphere.
Data from those flybys were used to redefine the size of the moon.
This Voyager 2 colour photo of Ganymede, the largest Galilean satellite, was taken on July
7, 1979, from a range of 1.2 million kilometres.
Most of this portion of Ganymede will be imaged at high resolution during the closest approach
with the satellite on the evening of July 8, 1979.
The photo shows a large dark circular feature about 3200 kilometres in diameter with narrow
closely-spaced light bands traversing its surface.
The bright spots dotting the surface are relatively recent impact craters, while lighter circular
areas may be older impact areas.
The light branching bands are ridged and grooved terrain first seen on Voyager 1 and are younger
than the more heavily cratered dark regions.
The nature of the brightish region covering the northern part of the dark circular feature
is uncertain, but it may be some type of condensate.
Most of the features seen on the surface of Ganymede are probably both internal and external
responses of the very thick icy layer which comprises the crust of this satellite.
Here instead you can see a coo picture as acquired by Voyager 1 during its approach
to Ganymede.
The image shows detail on the surface in a northern hemisphere region.
It shows a variety of impact structures.
The most striking features are the bright ray craters, which have a bluer colour (appearing
white against the redder background).
Ganymede's surface is known to contain large amounts of surface ice and it appears that
these relatively young craters have spread bright fresh ice materials over the surface.
https://photojournal.jpl.nasa.gov/browse/PIA01515.jpg
However, the record for the closest flyby is owned by Galileo Spacecraft.
It passed just 264 km from the surface of Ganymede.
It actually did 6 close flybys of Ganymede.
Here's a simulated flyover of an area on Ganymede, based on images taken by NASA's Galileo spacecraft.
It takes the viewer over rugged, ancient terrain that is interrupted by a bright icy swath
of more recent surfacing.
Parts of the dark terrain are heavily cratered, testifying to their ancient age, but other
parts have been deformed into deep grooves by stretching and faulting.
Other missions that visited Ganmymede were New Horizons and Juno.
Both of them performed distant flybys, at a range of 100.000 km.
However, in June 2021 Juno performed a closer flyby (of about 1000 km).
On that occasion, additional images of the surface were collected.
Here are some:
Take a moment just to look at them and appreciate the beauty of our Universe!
Finally, I want to show you an image that is worth a thousand words: (
On the left side, you have a picture of Ganymede taken by HST in 1996, and on the right side,
you can see the giant steps technology made between 1996 and 2021.
In fact, the right panel picture was taken by Juno in 2021, and show features of Ganymede
as seen in the infrared domain.
What a time to be alive!
"This video ends here!
Thanks for watching everyone!
What are your thoughts about such amazing pictures?
Let us know in the comments below!
And don't forget to share!
See you next time for another amazing video.”