The Oceanic Moon Ganymede Of Jupiter: Real Images

Science & Technology

When we talk about space, we often think about giant planets, stars, black holes, 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 secrets to be revealed.

Just to say, did you know that comet impacts on a planet's surface could enrich the planet with water? Don't you think this is amazing? And what about moons?
Credit to NASA / JPL-Caltech.

We don't have to go far to find a fantastic moon with interesting properties: we just have to point our telescopes toward Jupiter. There's no need to look for exotic exomoons 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 post, 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 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.
Hubble images of Ganymede's auroral belts (colored blue for illustration) are overlaid on a Galileo orbiter image of the moon.Credit to NASA/ESA

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 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 moon. To describe celestial objects, astronomers give some crucial parameters which provide us with important information.
The amount of rocking of the moon's magnetic field suggests that the moon has a subsurface saltwater ocean.Credit to NASA/ESA

Since it is a Jupiter moon, we want to know how far from the giant gaseous planet it is from the center. Accurate measurements show that it orbits around Jupiter at a distance of about 1 million kilometers, 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 behavior 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 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's 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 percent!).

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 us the 43% of the total solar radiation hitting on its surface. Also, as they were suspected usin of 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.
In 1998, Hubble's Space Telescope Imaging Spectrograph took these first ultraviolet images of Ganymede.Credit to NASA, ESA, Lorenz Roth (KTH)

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 a tenuous atmosphere, made of oxygen. We know this because HST observed the 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!
A denser rock mantle lies deeper in the moon, and finally an iron core beneath that.Credit to NASA, ESA, and A. Feild (STScI)

Also, the existence of such an oxygen atmosphere implies that an ionosphere should exist. We expect high-energy electrons coming from both solar radiation and the magnetosphere of the moon to ionize 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.
NASA's Pioneer 10 snapped this image of Ganymede during its flyby in Dec. 1973.Credit to NASA / JPL-Caltech.

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 color photo of Ganymede, the largest Galilean satellite, was taken on July 7, 1979, from a range of 1.2 million kilometers. 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 kilometers 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.
The north pole of Jupiter's giant moon Ganymede can be seen in this composite image of infrared data.Credit to NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

Most of the features seen on the surface of Ganymede are probably both internal and external responses to 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.
This montage compares New Horizons' best views of Ganymede, Jupiter's largest moon.Credit to NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The most striking features are the bright ray craters, which have a bluer color (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.
Credit to NASA / JPL-Caltech.

However, the record for the closest flyby is owned by Galileo Spacecraft. It passed just 264 km from the surface of Ganymede. It 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.

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