The Galilean Moons of Jupiter

Of the estimated 63 moons that orbit the great planet of Jupiter, the most dominant to backyard observers are the four largest, which are Io, Europa, Ganymede, and Callisto. It is my intent with this article to give the reader some insite into the utter magnifiscence that these jovian satillites offer to the amatuer observer fortunent enough to behold them in the eyepiece of a good telescope. We will begin with the moon Io which is the closest of the four to Jupiters surface.

Although views of these jovian moons, only show up in the scope as bright white dots, it is fantastic to watch these dots dance around Jupiter from night to night as they constantly switch positions like jockeys in a horse race, and if you are real luckey, on certain nights when the seeing conditions are good, you can even witness the transiting of moons across the surface of Jupiter, which appear as black dots, these black dots are shadows traversing the face of this giant planet. Now, if you are ready, I am ready. As I mentioned earlier the first jovian moon up for discussion is Io.


As Io passes across Jupiter surface it moves at an incredible speed of 38,000 miles per hour, or 17 kilometers a second. She is roughly 2,262 miles in diameter (3,640 kilometers). Io is dotted with volcanoes, spacecraft images show a surface splattered with colors of yellow, red, and orange. The voyager mission found in all 9 erupting volcanoes, before it was all over scientist had discovered potentially 120 possible volcanoes.

The volcanic forces in play on Io’s surface produce more lava than all of Earth’s volcanoes together; in fact Io is the most active volcanic region in the solar system. This is due to the gravitational tug of war that Io endures from its close dance with not only Jupiter but its sister moons. These moons pull Io’s body one way while the great gas giant pulls the other, this causes vast amounts of frictional heat to build up in the core of Io causing her to twist in different directions at the same time, resulting in the pangs of labor that she is forced to endure.

Looking at the surface of Io is like gazing at a cancer ridden body where the volcanoes appear as great sores that infect the surface. All of this crustal twisting and turning causes long mountains to rise up suddenly from the surface of Io to heights in some areas as tall as 26,000 feet, only to break apart and ooze lava across the scarred surface. The tallest mountain summit measured on Io so far is roughly 52,000 feet.

To give you an idea of how massive these volcanoes are, one was imaged forcing a plume of hot lava about 250 miles into space. The high altitudes achieved by these eruptions are possible, because of the low gravity of Io that provides only a slight resistance to the eruptions. The molten lava erupted by these volcanoes, chiefly consists of sulphur compounds and basaltic molten rock mixed with gas comprised of sulphur dioxide. These are propelled by hot water vapor and carbon dioxide.


Unlike the hot volcanic ridden surface of Io, the surface of Europa is as different as night and day. This is the smallest of the four main moons of Jupiter, measuring in at 1,950 miles in diameter, and appears as an ice covered snowball hanging above Jupiter at a distance of 416,754 miles from the surface.

It is strongly believed by scientist that Europa harbors one of the solar systems greatest discovery’s, a vast ocean several miles deep covered over by a thin layer of ice coated with dust that forms a shrouded crust. Sheets of ice have broken up and drift around on the sometimes unfrozen surface. These broken ice sheets are reminiscent of the ice packs in Earth’s artic regions. The surface of Europa, just like the surface of Io, is twisted and stretched by the combined gravitational forces of both the sister moons and the gas giant Jupiter. These forces have cracked the frozen surface into discolored crevices that carry colorful Celtic names such as Conamara Chaos and Conmac, not to mention a mammoth impact crater called Pwyll that appears in orbital images like a baseball has hit a pane of glass.

These crevices bisect and cross the icy surface making it reminiscent of a stained glass panel separated by lead fillings. The tidal forces that rip and tear at the surface cause the moon to heat up and liquefy the ice just under the surface enough to allow the formation of the alleged ocean that lurks just below, which by the way might be marine in nature due to the evidence of salt deposits spotted by spacecraft near the banks of cracks.

The surface of Europa with the exception of the fractured icy surface, and a few large impact craters, appears devoid of the many impact scars that litter other moons and planets in our solar system, and I assume this is due to the global ocean that apparently covers the surface of Europa. However, there is evidence of a couple of large impact regions. One of these is about 600 miles due north of Pwyll, at approximately 274 degrees west longitude by 9 degrees north latitude. This area is marked by a large deformation in the ice, measuring about 1,640 feet in diameter, discolored by the mineral deposits laid out on the surface by the impact. The other named Tyre, which is about 86 miles in diameter, and is located at 34 degrees north latitude by 146 degrees west longitude.

Although life has not been found as of yet on Europa there is however clues that life may yet be discovered. For example there is the strange red markings along the Agenor Linea region, which is of unknown origin (could be some type of bacterial formation along the edge of the crack, that’s my guess anyway). There has been a recent discovery of sulfuric acid on the surface of Europa’s dark side, which is reminiscent of the area found around underwater volcanic vents on Earth, where small creatures called extremophiles exist in an environment rich in sulfuric acid compounds.

Just like on Earth life can exist with out the presence of light to produce chlorophylls, as long as there is the existence of an alternate energy source, such as the sulfuric acid which is a known form of energy (oxidant). Creatures that thrive on a highly acidic environment such as Sufolobus shibatae and the Deinococcus radiodurans on Earth just happen to be the same color as the deposits seen on Europa (now, imagine that, what a coincidence).


Ganymede is the largest moon in our solar system. This moon measures in at 3,270 miles in diameter. The crustal surface of this moon is riddled with craters. The surface features show the strain of the same tidal forces that were exerted on the sister moons. Evidence of this shows up in the spacecraft images, which show a vast light colored region, which resembles a scar. It is apparent that this was caused by either tectonic plate movement, or cryovolcanic action. What ever the cause the land mass has come together, and slid across each other like a great fault resembling Earth’s San Andreas Fault region.

Ganymede bears a strong resemblance to Saturnian moons such as Enceladus, when it comes to the cryovolcanic appearance of the lava flows in some regions. On the other hand there are also areas with a strong volcanic appearance, such as very rugged terrain with buckled and fractured surfaces indicative of tectonic plate movement. So, the true story seems to be that what has happened here is that both forces are at fault. With the first event being the cryovolcanic eruptions in the form of massive geysers, that were followed by the subsequent tectonic movements.

These bright regions termed Harpagia Sulcus by scientist have revealed to orbiting spacecraft a terrain of confused topography, a mixture of ancient terrain right along side the younger flat regions, but investigations of this area reveal that although the terrain is different it was still a result of volcanic activity, and when compared to the cryovolcanic Sippar Sulcus region the difference is immediately obvious.

A few scientists have offered an alternative explanation for the extreme differences in terrain between cryovolcanism and volcanic activity being so closely oriented to each other. The scientist postulate that at some time in the distant past Ganymede for some reason or another (maybe by impact) left its orbit to fall into a temporary erratic elliptical orbit that would take it close to Jupiter with enough proximity to allow to much tidal stress to be applied to Ganymede causing the core to heat the frozen surface to a point that it would melt the surface ice with volcanic activity that would later be covered over by cryovolcanic activity. This would explain why we see two different types of terrain so close to each other.

After the gravitational fields stabilized over a period of time Ganymede returned to its original more stable orbit. Now, with all that said, let us examine this for a moment.

If Ganymede was moved out of its orbit it would not move on its own, so some outside force must have moved it. Now, it seems to me that proof of some type of a gravitational unbalance should be apparent on the other moons. If not, then some proof of a large impact should be evident on the surface of Ganymede. We have seen, or I should say we have discussed, impact scars on the surface of Ganymede, but none of these are large enough to have caused Ganymede to have left its orbit. Unless Ganymede still resides in that new orbit, by that I mean that is it possible that Ganymede used to be in a closer orbit to Jupiter then slowly migrated to an orbit further out, and while in the closer orbit it was a lot warmer with a more pliable surface, but once it moved out further it froze over (maybe).

Another strange anomaly reported by passing spacecraft was the discovery of a double magnetosphere. Even though this was the first time a magnetosphere has been discovered on a moon sized orb other than Mercury. It seemed strange to me to find out that the probe discovered two instead of one magnetic field. Now, is it possible that this might be the discovery of the remains of an impacter large enough to have caused Ganymede to have moved out of its orbit? Now, what made me think of this is the discovery of magnetic anomalies in the Aitken Basin of our Moon. Now before you ask, I know the first question that comes to mind is where is the rim of the crater basin?

Going back to the theory of Ganymede being in an elliptical orbit closer to Jupiter, the surface of Ganymede would have been in a more pliable state due to the massive tidal forces of the combined moons and Jupiter. In this state it might have absorbed the impact but retained the remains of the impacter below the surface, without showing any surface signs of its entry. This would be like throwing a piece of iron in quicksand the marks of entry would not be present, but the piece of iron would still be there just under the surface. With all subsequent impacter strikes, being to a solid surface, that’s why you can still see them.

Callisto is quite different in appearance than all the rest of the moons, its crater pocked face looks like the surface of a giant golf ball. Callisto measures in at 2,985 miles in diameter, and holds the record as the most cratered celestial body in our solar system. One of the largest impact basins on Callisto is Valhalla. This region is 370 miles in diameter, with multiple parameter rims extending out as far as an astonishing 2,500 miles. Many younger craters reside within the basins boundaries. The impact was so large; the scientist’s were amazed to not find more damage than was present.

Some researchers believe the multiple rims are the result of the surface upheaval as a response to the impacters contact with the surface, and the resulting rings were the outcome of the raised surface attempting to return to its original position, but the excavation made by the impacter only allowed the ground to collapse into a pit, resulting in the numerous apparent rings.

The most astounding discovery made by orbiting spacecraft flying over the surface of Callisto, was the one made while taking spectral images of an area inside a crater located 55degrees south latitude by 30 degrees west longitude. It was while collecting this data that the craft came across some rather strange data concerning absorption bands particularly at 4.57 micrometers. This was a reading indicating a triple bond between carbon and nitrogen, but of a higher nature such as some type of tholin, which was a type of organic compound that resemble those types found in primitive life forms, the same type of compounds that were thought to have seeded the Earth when comets or meteors fell to Earth billions of years ago.

The rest of the surface of Callisto is covered with various craters between 6 to a little over 30 miles in diameter. There are so many craters that a lot of them overlap each other in fact in some places it even appears that the impacters must have been multiple hits to the same area, indicative to a whole air strike of meteors simultaneously. This was a little odd when compared to other planets and moons where most crater counts reveal a majority of small hits, but when examining craters on Callisto we find the opposite to be true there are more large hits than smaller ones. I am sure that this occurred from many collisions with other smaller moons through the ages.

Closing Remarks

I hope that you have enjoyed the article, and have found it educational. Now remember keep your eye’s to the skies, for you never no what you are missing.