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Ganymede. The third of Jupiter’s Galilean moons. The largest moon that we know of, and
home to the largest water ocean in the solar system. But there’s something else that
sets Ganymede apart from anything else we’ve seen before, this Jovian giant contains an
intriguing mystery buried deep beneath its surface. I’m Alex McColgan and you’re
watching Astrum. Come with me on a journey as we investigate this strange phenomenon,
as well as learn what other incredible features and characteristics this truly special moon
possesses. I hope by the end of this video, I earn your like and subscription! There are
79 different moons of Jupiter, and Ganymede is the 3rd of a group known as the Galilean
moons, which are its 4 largest satellites. The other three Galilean moons are Io, Europa,
and Callisto. Ganymede is the largest of these four, with an impressive diameter of 5268
km. For point of comparison, this is 0.41 times the size of Earth’s diameter, and
1.02 times greater than the previously thought to be largest moon, Titan. It initially appeared
bigger because of its thick atmosphere which stretches hundreds of kilometres into space.
Ganymede’s volume is even 26% larger than Mercury’s, although it is does not contain
as much mass. Ganymede’s average density is 1.9 g/cm³ compared to Mercury’s 5.4
g/cm³. This is because of its composition. Like Europa, Ganymede’s surface is a thick
crust of water ice, extending 150km deep, underneath which is believed to lie a vast
ocean of liquid water. And when I say vast, it really is. Because while Mercury has very
little water, and is rich in dense metals, the abundance of water on Ganymede reduces
its average density. The ocean of Ganymede is so big that it contains more water than
all the oceans and seas on Earth, and it is estimated to be 100 km in depth on average
– 10 times deeper than the deepest point in our ocean. All of this water means that
Ganymede is only 50% rock, the rest being water and small amounts of metals and other
ices. This seems appropriate to me, as the name Ganymede comes from classical mythology,
where Zeus (or as the Romans called him, Jupiter) claimed a young boy called Ganymede and took
him to be a cupbearer for the gods. It seems fitting that the moon Ganymede would also
carry so much water around for Jupiter. Interestingly, Ganymede has an atmosphere that contains oxygen.
Now you might be wondering, with all this water – and an oxygen atmosphere – is
it possible that life exists on Ganymede? While it’s certainly possible, there are
some features of Ganymede that make this unlikely. To begin with, the oxygen atmosphere is very
thin. It is estimated to be somewhere between 0.2-1.2 micropascals, or about 100,000,000,000
to 500,000,000,000 times less than earth’s atmospheric pressure at sea-level. That would
be impossible to breathe! And while our own planet Earth has certainly demonstrated that
ecosystems can flourish in the depths of oceans without sunlight to sustain it, there could
be a big problem that prevents this from happening on Ganymede. This is because the ocean of
Ganymede is so deep that water down at its bottom would likely be compressed back into
ice through the sheer pressure. Life in the deepest parts of our ocean can survive thanks
to minerals being ejected from geothermal vents. With such a thick ice layer between
the core and the ocean, it is unlikely that this would occur on Ganymede. Europa, Ganymede’s
neighbouring moon, is considered a more likely candidate for life because of this. However,
if Ganymede’s ocean is salty, which there is an increasing amount of evidence for, it
could change the interior makeup of Ganymede drastically. Models suggest that with a salty
ocean, it could be that there are multiple layers, divided by sheets of ice. If this
is the case, the most internal layer could indeed be in contact with the rocky core,
increasing the chances of life existing there. But underneath Ganymede’s ice and water
exists something else truly surprising, something that scientists do not have an explanation
for. Somehow, Ganymede is producing a magnetic field. The magnetic field of Ganymede was
first discovered by scientists in 1996 when the Galileo spacecraft began a series of fly-bys
of the icy moon. The big indicator of a magnetic field is the presence of auroras, and incredibly,
auroras were detected in Ganymede’s tenuous atmosphere. Not only that, but as Hubble studied
Ganymede over an extended period, it became clear that these auroras didn’t wobble as
much as expected, likely a result of something known as “magnetic friction” in a salty
water ocean under the surface. A magnetic field is significant, as it’s the feature
on Earth that shelters the planet from solar radiation, enabling all life to flourish.
The field around Ganymede does not completely protect it from such radiation – being tucked
within Jupiter’s incredibly powerful magnetic field and radiation belt means it’s still
getting pelted with a load of radiation. Ganymede’s surface still has about 5-8 rem, enough to
make a human severely ill or dead in just two months – but it’s still better than
its closer orbiting neighbour moons. Scientists are not quite sure why this magnetic field
is here at all. Our planet’s core is hot and molten. Convection currents within it
move electrons, which in turn produces the magnetic fields that surround us here. However,
this shouldn’t be happening on Ganymede. Ganymede is smaller than Earth, and given
its size and composition, scientists believe that the core should have cooled down enough
by now that it should be a solid mass, not a liquid. This would prevent electrons moving
through convection and would prevent a magnetic field. And none of the other moons of Jupiter
have a magnetic field. In fact, Ganymede is the only moon in the entire solar system that
has one. So, what is going on? Scientists do not know for sure, but the answer might
lie in Ganymede’s incredible relationship with its planet and neighbouring moons, and
a process known as tidal heating. Ganymede orbits around Jupiter roughly once every 7
days in an eccentric orbit. This means that at some points of its orbit, it is closer
to Jupiter than at others. When an object comes under a strong gravitational force,
it will stretch in the direction of that force, as mass is pulled in the direction of gravity.
However, the further an object gets from the source of gravity, the more it will compress
back to its original shape. As Ganymede has an eccentric orbit around Jupiter, it is constantly
coming under more and then less gravity, and is constantly stretching and contracting.
Have you ever pulled and stretched blue-tac repeatedly in your hands, before pushing it
back to a ball shape? Then you might have noticed that after a while it gets surprisingly
warm. This is because stretching produces friction as the material rubs up against itself.
And on the scale of moons and planets, this friction adds up. This tidal force creating
heat through friction is known as tidal heating. On top of that, in a process I find fascinating,
Ganymede has formed an orbital resonance with two of its fellow Galilean moons Io and Europa,
in what is called a Laplace Resonance. For each time Ganymede orbits Jupiter, Europa
will orbit exactly twice, and Io exactly 4 times. This mathematically precise configuration
has not happened by coincidence, but is evidence of the moons’ gravities pulling on each
other, and the whole system attempting to conserve the resulting momentum. However,
it means that Ganymede is constantly being pulled by its neighbours’ gravity, and is
under a lot of gravitational stress. So perhaps tidal heating is warming up Ganymede enough
that its core remains a liquid after all, helping it continue to produce its magnetic
field? Scientists do not know for sure. But it would perhaps explain why the surface of
Ganymede is so interesting. You may have noticed that Ganymede’s surface is split into large
dark regions that cover about a third of its surface, and lighter regions that make up
the other two thirds? Through examining the number of impact craters on these two sections,
scientists can tell that the dark regions are actually older than the lighter ones,
as they contain more craters. The lighter regions might have fewer craters, but they
do contain long ridges and grooves up to 700 m high and thousands of km long. Truly an
impressive sight. But scientists are not yet sure how these ridges were formed. One explanation
is that tidal forces stretched out the surface of the moon in an unstable period of Ganymede’s
ancient history. Perhaps this same tidal force could have also warmed Ganymede’s core and
preserved its magnetic field? Whatever its cause, the magnetosphere of Ganymede has been
instrumental in helping scientists understand the composition of the moon. By measuring
the areas that auroras appeared in Ganymede’s atmosphere, scientists were able to confirm
the existence of Ganymede’s subsurface oceans, all without ever having set foot there. And
as for the rest of Ganymede’s mysteries, maybe JUICE (the Jupiter Icy Moon Explorer)
will find us the answers. The ESA spacecraft is set to launch in 2022, and though it won’t
reach an orbit around Ganymede for another 10 years after that, its mission is to investigate
the inner workings of a number of Jupiter’s moons, including Ganymede. Until then, we
may have to let Ganymede’s mysteries remain just that – a mystery. Looking at various
places in the solar system can give us a much better understanding of how we formed on Earth.
The Jovian system is full of clues, having four large moons, and having types in other
nearby solar systems that we can compare against. I found an interesting documentary on MagellanTV
called “The Jupiter Enigma” where it explores how Jupiter has influenced Earth ever since
the formation of the solar system. Sound like something you’d be interested in? Well,
you can try MagellanTV for one month for free with my link in the description so you can
give it a watch. MagellanTV is a subscription streaming service that has over 3000 science
documentaries to choose from, with more added weekly. They have a whole section dedicated
to space too, which a lot of fascinating insights into our universe. For more information, and
to claim your trial month, click the link in the description!
Thanks for watching. If you enjoyed this video, check out this playlist for some of the other
moons in our solar system. Also, a big thanks as always to my patrons and members for supporting
the channel. If you want to support too, check the links in the description. All the best,
and see you next time.