From its discovery around Jupiter, to its
composition, and what mysteries it may hold,
and more!
Join us as we explore Ganymede: Jupiter's
Oceanic Moon!
8.
The Finding And Naming Of Ganymede
At present, Jupiter has 79 moons, some of
which have only been discovered recently.
But in regards to Ganymede, it may have an
origin that is far beyond the histories of
the other moons.
Chinese astronomical records report that in
365 BC, Gan De detected what might have been
a moon of Jupiter, probably Ganymede, with
the naked eye.
However, Gan De reported the color of the
companion as reddish, which is puzzling since
the moons are too faint for their color to
be perceived with the naked eye.
Shi Shen and Gan De together made fairly accurate
observations of the five major planets.
On January 7th, 1610, Galileo Galilei observed
what he thought were three stars near Jupiter,
including what turned out to be Ganymede,
Callisto, and one body that turned out to
be the combined light from Io and Europa;
the next night he noticed that they had moved.
On January 13th, he saw all four at once for
the first time, but had seen each of the moons
before this date at least once.
By January 15th, Galileo came to the conclusion
that the stars were actually bodies orbiting
Jupiter.
Thus, the discovery of the moons.
Galileo originally called Jupiter's moons
the Medicean planets, after the Medici family
and referred to the individual moons numerically
as I, II, III, and IV.
Galileo's naming system would be used for
a couple of centuries.
It wouldn't be until the mid-1800's that the
names of the Galilean moons, Io, Europa, Ganymede,
and Callisto, would be officially adopted,
and only after it became apparent that naming
moons by number would be very confusing as
new additional moons were being discovered.
In mythology, Ganymede was a beautiful young
boy who was carried to Olympus by Zeus (the
Greek equivalent of the Roman god Jupiter)
disguised as an eagle.
Ganymede became the cupbearer of the Olympian
gods.
The Greek/Roman pantheons are the epicenter
of many names of both planets and moons in
our solar system.
7.
Orbits and Rotations
Ganymede orbits Jupiter at a distance of 1,070,400
km, third among the Galilean satellites, and
completes a revolution every seven days and
three hours.
Which indeed means that a "Day" on Ganymede
is a week here on Earth, that would be something
that would take some getting used to no doubt.
Like most known moons, Ganymede is tidally
locked, with one side always facing toward
the planet, hence its day is seven days and
three hours.
Its orbit is very slightly eccentric and inclined
to the Jovian equator, with the eccentricity
and inclination changing quasi-periodically
due to solar and planetary gravitational perturbations
on a timescale of centuries.
Ganymede participates in orbital resonances
with Europa and Io: for every orbit of Ganymede,
Europa orbits twice and Io orbits four times.
Conjunctions (alignment on the same side of
Jupiter) between Io and Europa occur when
Io is at periapsis and Europa at apoapsis.
Conjunctions between Europa and Ganymede occur
when Europa is at periapsis.
The longitudes of the Io–Europa and Europa–Ganymede
conjunctions change with the same rate, making
triple conjunctions impossible.
Such a complicated resonance is called the
Laplace resonance.
There are two hypotheses for the origin of
the Laplace resonance among Io, Europa, and
Ganymede: that it is primordial and has existed
from the beginning of the Solar System; or
that it developed after the formation of the
Solar System.
A possible sequence of events for the latter
scenario is as follows: Io raised tides on
Jupiter, causing Io's orbit to expand (due
to conservation of momentum) until it encountered
the 2:1 resonance with Europa; after that
the expansion continued, but some of the angular
moment was transferred to Europa as the resonance
caused its orbit to expand as well; the process
continued until Europa encountered the 2:1
resonance with Ganymede.
Eventually the drift rates of conjunctions
between all three moons were synchronized
and locked in the Laplace resonance.
6.
Composition
Ganymede has three main layers.
A sphere of metallic iron at the center (the
core, which generates a magnetic field), a
spherical shell of rock (mantle) surrounding
the core, and a spherical shell of mostly
ice surrounding the rock shell and the core.
The ice shell on the outside is very thick,
maybe 800 km (497 miles) thick.
The surface is the very top of the ice shell.
Though it is mostly ice, the ice shell might
contain some rock mixed in.
Scientists believe there must be a fair amount
of rock in the ice near the surface.
Ganymede's magnetic field is embedded inside
Jupiter's massive magnetosphere.
In 2004, scientists discovered irregular lumps
beneath the icy surface of Ganymede.
The irregular masses may be rock formations,
supported by Ganymede's icy shell for billions
of years.
This tells scientists that the ice is probably
strong enough, at least near the surface,
to support these possible rock masses from
sinking to the bottom of the ice.
However, this anomaly could also be caused
by piles of rock at the bottom of the ice.
Spacecraft images of Ganymede show the moon
has a complex geological history.
Ganymede's surface is a mixture of two types
of terrain.
Forty percent of the surface of Ganymede is
covered by highly cratered dark regions, and
the remaining sixty percent is covered by
a light grooved terrain, which forms intricate
patterns across Ganymede.
The term "sulcus," meaning a groove or burrow,
is often used to describe the grooved features.
This grooved terrain is probably formed by
tensional faulting or the release of water
from beneath the surface.
Groove ridges as high as 700 m (2,000 feet)
have been observed and the grooves run for
thousands of kilometers across Ganymede's
surface.
The grooves have relatively few craters and
probably developed at the expense of the darker
crust.
The dark regions on Ganymede are old and rough,
and the dark cratered terrain is believed
to be the original crust of the satellite.
Lighter regions are young and smooth (unlike
Earth's Moon).
The largest area on Ganymede is called Galileo
Regio.
The large craters on Ganymede have almost
no vertical relief and are quite flat.
They lack central depressions common to craters
often seen on the rocky surface of the Moon.
This is probably due to slow and gradual adjustment
to the soft icy surface.
These large phantom craters are called palimpsests,
a term originally applied to reused ancient
writing materials on which older writing was
still visible underneath newer writing.
Palimpsests range from 50 to 400 km in diameter.
Both bright and dark rays of ejecta exist
around Ganymede's craters -- rays tend to
be bright from craters in the grooved terrain
and dark from the dark cratered terrain.
Before we continue to deep dive into Ganymede
and what's so interesting about it.
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5.
Size
Ganymede is the largest and most massive moon
in the Solar System.
Its diameter of 5,268 km is 0.41 times that
of Earth, 0.77 times that of Mars, 1.02 times
that of Saturn's Titan (the second-largest
moon), 1.08 times Mercury's, 1.09 times Callisto's,
1.45 times Io's and 1.51 times the Moon's.
Its mass is 10% greater than Titan's, 38%
greater than Callisto's, 66% greater than
Io's and 2.02 times that of the Moon that
orbits our own planet.
This size has led to much research about the
moon, especially about its composition as
we've documented.
But it also opens up questions about habitation,
as well as what might be under the surface...
4.
The Oceans Below
In the 1970s, NASA scientists first suspected
that Ganymede has a thick ocean between two
layers of ice, one on the surface and one
beneath a liquid ocean and atop the rocky
mantle.
In the 1990s, NASA's Galileo mission flew
by Ganymede, confirming the moon's sub-surface
ocean.
An analysis published in 2014, taking into
account the realistic thermodynamics for water
and effects of salt, suggests that Ganymede
might have a stack of several ocean layers
separated by different phases of ice, with
the lowest liquid layer adjacent to the rocky
mantle.
Water–rock contact may be an important factor
in the origin of life.
The analysis also notes that the extreme depths
involved (~800 km to the rocky "seafloor")
mean that temperatures at the bottom of a
convective (adiabatic) ocean can be up to
40 K higher than those at the ice–water
interface.
In March 2015, scientists reported that measurements
with the Hubble Space Telescope of how the
aurora moved over Ganymede's surface suggest
it has a subsurface ocean.
A large salt-water ocean affects Ganymede's
magnetic field, and consequently, its aurora.
The evidence suggests that Ganymede's oceans
might be the largest in the entire Solar System.
There is some speculation on the potential
habitability of Ganymede's ocean.
3.
Life In The Oceans
The prospect of life being in the oceans under
the surface of Ganymede is intriguing and
potentially life-altering in terms of how
we view the solar system.
Mainly because at present, the Earth is the
only place in the solar system (and universe
at large) that has confirmed life on it.
As a result, many astronomers and scientists
have observed the moons, planets and even
stars for any traces of life in any form.
Even looking to other moons like Europa and
Titan because they have potential for life
in certain assets, but the question of whether
they have the "whole picture" is a major question.
In searching for worlds (or even moons) to
live on, the question of water (or even water
vapor) is brought up.
Because most agree that water is a key component
to have life, as proven on Earth.
So the proof that there are potential oceans
of great size underneath Ganymede means that
there is a chance that it could be there.
If you think this is a stretch, look back
at Earth instead of near Jupiter.
When life was formed, it started in the oceans
and THEN made its way to land, not the other
way around.
So for all we know there could be life in
those oceans.
But what kind of life?
That's the mystery that still needs to be
solved outside of the potential for it.
It would be a stretch to say that it would
be complex life "without a doubt" because
we don't know the contents of the oceans and
the various effects the planet itself has
on the oceans.
However, potentially, microbial life is possible
as well as other simple forms of life.
It's a question that's going to be asked for
a while, and as such, many are going to debate
whether it's possible or not.
2.
Atmosphere
Believe it or not, there has been a lot of
conflicting data and beliefs about what the
atmosphere of Ganymede is.
For example, in 1972, a team of astronomers
believed they found proof of a thin atmosphere
around the moon.
But then, when Voyager 1 came to the area
and looked at the moon, it found very different
results than them.
Despite the Voyager data, evidence for a tenuous
oxygen atmosphere (exosphere) on Ganymede,
very similar to the one found on Europa, was
found by the Hubble Space Telescope (HST)
in 1995.
Additional evidence of the oxygen atmosphere
comes from spectral detection of gases trapped
in the ice at the surface of Ganymede.
The detection of ozone (O3) bands was announced
in 1996.
In 1997 spectroscopic analysis revealed the
dimer (or diatomic) absorption features of
molecular oxygen.
Such an absorption can arise only if the oxygen
is in a dense phase.
The best candidate is molecular oxygen trapped
in ice.
1.
Formation
The formation of our solar system is something
that is still debated to this day due to the
varying beliefs on the Big Bang, the Solar
Nebula theory, and more.
Ganymede probably formed by an accretion in
Jupiter's subnebula, a disk of gas and dust
surrounding Jupiter after its formation.
The accretion of Ganymede probably took about
10,000 years, much shorter than the 100,000
years estimated for Callisto.
The Jovian subnebula may have been relatively
"gas-starved" when the Galilean satellites
formed; this would have allowed for the lengthy
accretion times required for Callisto.
In contrast Ganymede formed closer to Jupiter,
where the subnebula was denser, which explains
its shorter formation timescale.
This relatively fast formation prevented the
escape of accretional heat, which may have
led to ice melt and differentiation: the separation
of the rocks and ice.
The rocks settled to the center, forming the
core.
In this respect, Ganymede is different from
Callisto, which apparently failed to melt
and differentiate early due to loss of the
accretional heat during its slower formation.
This hypothesis explains why the two Jovian
moons look so dissimilar, despite their similar
mass and composition.
Alternative theories explain Ganymede's greater
internal heating on the basis of tidal flexing
or more intense pummeling by impactors during
the Late Heavy Bombardment.
In the latter case, modeling suggests that
differentiation would become a runaway process
at Ganymede but not Callisto.
After formation, Ganymede's core largely retained
the heat accumulated during accretion and
differentiation, only slowly releasing it
to the ice mantle.
The mantle, in turn, transported it to the
surface by convection.
The decay of radioactive elements within rocks
further heated the core, causing increased
differentiation: an inner, iron-sulfide core
and a silicate mantle formed.
With this, Ganymede became a fully differentiated
body.
Today, Ganymede continues to cool slowly.
The heat being released from its core and
silicate mantle enables the subsurface ocean
to exist, whereas the slow cooling of the
liquid Fe–FeS core causes convection and
supports magnetic field generation.
Thanks for watching!
What did you think of this look at Ganymede
and the history and facts about it?
Did you learn something special about the
moon and why it might be incredibly special?
What do you think humanity will learn about
the moon next?
Let us know in the comments below, be sure
to subscribe, and I'll see you next time on
the channel!
