Ganymede is the largest satellite of Jupiter and the largest moon in the solar system. It was discovered by Galileo Galilei in 1610 and was named by Simon Marius after one of the lovers of the Roman god Jupiter. Ganymede was the first moon, other than Earth’s moon, to be discovered.

Ganymede is 5,280 kilometers in diameter, which is larger than Mercury. It is revolving just over 1 million kilometers from Jupiter and is the seventh of the planet’s sixteen satellites. Ganymede is large enough to generate its own magnetic field – an unusual characteristic for a moon.

Ganymede always faces the Jupiter with the same side. Such relationships are quite common and are called synchronous. Another good example of a synchronous relationship between a planet and its satellite is Earth’s moon. Ganymede revolves in the same direction as Jupiter. Its orbit is almost circular, meaning that its eccentricity (the measure of how close to a circular orbit the satellite travels) is small. A circular orbit has an eccentricity of zero. Ganymede’s angle of inclination is less than a degree, meaning that this moon revolves almost exactly in the plane of Jupiter’s equator.

Although Ganymede is now locked into the same position facing Jupiter all the time, there are indications that this may not have always been the case. One clue is that the number of meteor craters should be greater on the leading side of Ganymede, as is the case with Callisto. However, this is not true for Ganymede. Another fact pointing to a change in the part of the ice shell facing Jupiter is the catenae that are found on the back side of Ganymede. Catenae are caused by a string of fragments from a comet that was broken up by the intense magnetic field of Jupiter but escaped capture to hit one of the satellites. They should occur only on the Jupiter-facing side of Ganymede.

Ganymede’s surface is covered by ice mixed with carbon-rich soil, which reflects large amounts of sunlight. When the ice underneath the surface is heated and melts, it erupts to the surface. The soil, which is denser than water, sinks below the water. The water then freezes, causing a bright spot on the surface. The water is heated either by radioactive decay or by tidal flexing. Not only does the gravity of Jupiter and Callisto pull on Ganymede; the moon also has Laplace resonance, which occurs because of the forces from the satellites Io and Europa. Every time Ganymede revolves around Jupiter once, Europa, the satellite just inside Ganymede, goes around Jupiter twice, and Io, the moon inside Europa, goes around four times. Thus, during every orbit the three satellites are aligned, magnifying the gravitational effect. This increased gravitational pull and then relaxation not only cause the orbits to become elliptical, but also causes stresses within the satellites themselves. This tidal flexing generates heat that melts ice and causes the surface of Ganymede to be smoother than what it would be on a dense planet/moon.

The percentage of ice on Ganymede has been measured at 45-55 percent. The bulk density of the moon is between that of ice and that of carbonaceous silicates, indicating a mixture of the two materials.

Ganymede has an intrinsic magnetic field that is opposite to the field of Jupiter. It also displays an induced magnetic field caused by the strong rotating, angled field of Jupiter. The induced field is an indication of a conducting ocean deep under the icy surface. If the ocean has enough minerals dissolved in it to make it strongly conducting, it could generate the intrinsic magnetic field. Jupiter’s strong magnetic field causes Ganymede to be bombarded by charged particles. This bombardment is thought to cause the molecular oxygen, O2, and ozone, O3, found in the surface of Ganymede.

Since Ganymede’s orbit is in the same plane as Jupiter, it is thought that they were formed by the same process. Jupiter was formed in the very hot, dense region. Ganymede was formed in a cooler region, where water did not boil away but instead froze to form part of the satellite.

Related Jupiter Articles
Related articles about Solar System Moons

[Images: nasa.gov; solarsystem.nasa.gov/galileo/; Wikipedia]

Jupiter’s moon Europa is one of the most interesting bodies of the Solar System. It was discovered by Galileo Galilei and Simon Marius (Simon Marius did it a bit later, and it was him who has offered the names to moons discovered by Galilei) in 1610. According to Greek mythology, Europa was a Phoenician princess stolen by Zeus who had transformed her into a white bull.

Tidal forces inside Europa are much smaller compared to another Jupiter’s moon – Io. First readings from spacecrafts orbiting Jupiter did not show any signs of eruptions on Io. But further examinations found a sultan over the moon’s limb, which included water, ammonia and other products. Nevertheless, gas eruptions that are so typical for Io seem to be very rare on Europa.

Photos of Europa show that the entire surface of the moon is covered by ice. It looks very unusual. A thrilling idea of artificial origin of channels on Mars was proposed around 100 years ago. However, these lines turned out to be an optical illusion caused by hardly distinguishable details on the distant planet. But here on surface of orange-brown Europa scientists found a really dense network of crossed lines. Pictures of Europa’s surface look very similar to the pictures of the Arctic Ocean made from the orbit. In the beginning scientists were very careful about arising analogy. But spectral measurements did not leave a place for doubts — Europa’s surface is covered by ice.

The size and average density of Europa allowed scientists to calculate the amount of ice in the total mass of the moon. With a diameter of 3138 km and average density of 3,04 g/sm3, Europa should have much more water than Io or the Moon. Because of this, first calculations predicted that Europa’s ice cover should be about 100 km thick. Further research, however, has led to smaller figures.

According to the latest research Europa’s ocean could be tens of kilometers deep, and its ice cover is probably just a few kilometers thick. This cover is fragile and sometimes breaks under tidal pressure. These breaks give liquid water access to moon’s surface, which has no atmosphere.

It is likely that a global network of lines visible on Europa’s surface is cracks in the thick ice cover, caused by tectonic processes. These breaks are not accompanied by any movements of the ice cover, and cracks are quickly filled with orange liquid. Breaks can be tens of kilometers to 100s of kilometers wide and more than 3000 km long. Water instantly begins to boil and simultaneously freezes. Evaporated water returns to the moon’s surface in the form of snow and frost. The boiling carries away a lot of heat, and it only takes few minutes for water to form a half-meter layer of ice.

Europa is a very flat moon. Its highest ‘mountains’ are less than 50 meters high.  All of this can be explained in two ways: either Europa is a very young moon or there is a ‘mechanism’, which smoothens its surface. Few facts speak in favor of the second option – relatively high temperature (ocean of liquid water) and ability of ice to move.

Interest to the ocean lying under the ice of Europa had been stimulated by the assumption that life could exist in it. Even if this life is in elementary form. If Europa’s ocean is 50—60 km deep, its volume should be close to the terrestrial oceans. Free fall acceleration on the surface of the moon is 1.32 m/s2. This means that the pressure at the bottom of Europa’s ocean is the same, as it is at the 4-kilometer depth on Earth. It is well known that life on Earth has started in oceans. But there is one fundamental difference – Europa’s ocean does not have a constant energy source. Sunlight is such source on Earth. Life and photosynthesis are inseparable. However, there is one exception – sulfur compounds formed at rather high temperatures of underwater volcano eruptions are used by some microorganisms in chemosynthesis (chemical synthesis under the influence of heat). There are other, equally speculative ideas, such as light absorption by microorganisms while new cracks are still free of ice.

Existence of an ice cover on Europa had been proven and there are no doubts about it.  As for the ocean and the assumptions associated with it – very little is known about what is hidden under the ice cover and most hypotheses are only speculative at this stage.

Related Jupiter Articles
Related Articles about Solar System Moons

[images: nasa.org]

Jupiter has a number of moons. Four of these were known since Galileo days – Io, Europa, Ganymede and Callisto. They stand out form the rest of the moons due to their size (varies from the size of the Moon to the size of Mercury) and their proximity to the planet. There are few other satellites that rotate even closer to Jupiter: three very small ones and Amalthea, which has an irregular shape (its size is approximately 130 x 80 km). These small satellites together with the Galilean satellites form a so-called accurate system, which has a distinctive coplanarity (location of satellite orbits in the equatorial plane of the planet) and almost circular shape of the orbits.

If you compare them to the position of Earth’s Moon, Io is 10% further, and Callisto – 4.9 times further from the Jupiter than Moon is from Earth. But because of Jupiter’s huge mass it only takes them 1.8 and 16.7 days to orbit the planet.

Since their discovery, those four moons remain one of the most popular objects of astronomical observations. But if only astronomers knew then what wonders are hidden in these celestial bodies!

Io

Murphy’s Law

Relatively short history of space exploration is full of accidents and unexpected discoveries. Gradually experts have developed a specific slang, which is only known to people related to space exploration. No wonder space experts came up with their own version of Murphy’s law: “Everything that can go wrong – will go wrong. Anything that can not go wrong – will go wrong too.” But, fortunately, this law can also work the opposite way. The discovery outlined below is a prime example of such amazing luck.

In order to accurately determine spacecraft’s position on the orbit of a distant planet, specialists do not rely on navigation information only. They also use pictures of visible satellites, which are transmitted by the spacecraft. Relative positions of celestial bodies are then entered into a computer, which determines the coordinates of the spacecraft. One legend says that when Voyager 1 was approaching Jupiter, a computer reported an error in the image of Io. The reason for the error was unclear, but in the end scientists found that Io’s image transmitted by the spacecraft did not match the expected image of a round celestial body. Something was protruding on the side of Io. That “something” turned out to be a huge gas cloud, which rose about 250-300 km above the active volcano.

An emission of oxygen, sodium and sulfur was discovered along the orbit of Io several years ago. The question is – How does such cloud remain in space? At first, scientists thought that images of Io explained this phenomenon: 7-8 active volcanoes on its surface constantly emit gaseous fountains that rise hundreds of kilometers above its surface. If we assume that only part of the eruption products is dissipated into space, the origin of the gas cloud along Io’s orbit can be explained.

But the fact is that Io is rather massive celestial body – its mass is 20% greater than the Moon. The average density of Io is 3.53 g/cm3. Its diameter is 3620 kilometers (Moon is 3476 kilometers). Calculations show that the free fall acceleration at its surface is also large enough – 1.81 m/s2. Heavy sulfur dioxide and sulfur vapor ejected from the volcanic caldera quickly condense in low temperatures, and fall on Ios` surface in the form of frost and snow. This process is faster than the destruction of gas molecules by ultraviolet radiation from the Sun. At the same time, the free-fall acceleration is not enough to retain an atmosphere, as it is on Mars, although Io has some traces of the atmosphere. In order for gas clouds to be several hundred kilometers high, it requires gas outflow of around about 1 km / sec from the volcano. Io’s atmosphere has very small density – 10 to 100 million times smaller than Earth. This is another factor contributing to the high gas outflow.

But let’s leave a question of how do sulfur and sodium get into space for now, and look at Io’s magnificent volcanos.

Energy for Io’s volcanoes

Io is not large enough for the radioactive decay of elements in its core to cause strong heating of the crust as it does on Earth. Energy for heating is drawn from an entirely different source: from tidal effects of the second Galilean satellite – Europe, Jupiter itself, and, to a lesser extent, the third satellite – Ganymede.

These tidal effects force Io’s lithosphere to bend, and it heats up – just like a wire does when you bend it. Enormous energy is released because of the tidal effects – 60-80 million MW. This energy is most likely distributed unevenly – the majority of it is probably released in the surface layers of the moon.

Volcanoes and hot spots on Io

Around 2% of Io’s surface is covered by active hot spots. There are more than 10 of these known to scientists. Temperatures in these spots are believed to be between 310 and 600 K, with spot sizes varying from 75 to 250 km. “Voyager 1″ found 8 active giant eruptions. When 4 months later “Voyager-2” reached Io, it found that 7 of them still continued to erupt. The only one that stopped erupting was one of the largest volcanoes called Pele (in honor of the god of volcanoes).

It is interesting to note that the center of the eruption is for some reason dark, while products of eruptions are orange. Scientists believe that these products accumulate in a molten state in deep reservoirs under Io’s surface.

Io’s volcanoes can be divided into two main categories. Volcanoes of the first group have temperatures of around 350 – 400 K and release gaseous products at a rate of about 500 m/sec. Their gas clouds are usually around 100 km high, and precipitations are white in colour. The majority of Io’s volcanos are of the first type. Volcanos in the second group have very high caldera temperatures; have emission rate of about 1 km/s and heights of their gas clouds can reach around 300 km. Abovementioned Pele, and later found Surt and Aten are examples of volcanos in the second group.

One of the volcanic objects, known as Patera Ra, has a very unusual form and deserves special mentioning. The serpentine lava flows that start at Patera Ra extend over distances of up to 200 km. Their colours vary from shades of brown to light orange and snow-white tones. The nature of these volcanic flows remains unclear, as is nature of even more mysterious objects – lava lakes.

The strongest signal recorded by Voyager’s thermal radiometers, came from a strange object, which was later called Loki. On high-resolution images it appears as a slightly truncated dark circular formation. This formation is around 250 km in diameter and has a bright yellow object of an angular shape in its center. It is believed that the yellow object is a 100-kilometer “iceberg” of solid sulfur, which floats in the center of a lake of molten sulfur! More of these small floating fragments can be seen around the main one.

Topography of Io

Few words about Io’s relief. The moon is mostly flat. There are several large mountainy areas and the mountains in the center of the Pele area. There are also high mountains at the South Pole, covering an area of ​​about 150×80 km. An interesting object was detected on one of the photos – 2.5 km high conical shape mountain with a base diameter of about 85 km.

Interaction with Jupiter 

Io’s orbit is located in in the center of Jupiter’s radiation belt – a part of magnetosphere, where the streams of charged particles are especially dense.

Electric and magnetic phenomena are very intense in Jupiter’s magnetosphere. Io is a part of one of the nature’s wonders – a powerful natural electrical generator. Perhaps “powerful” is not the right word though. A current of around 5 million Amperes flies between Jupiter and Io. This is more than 20 times more than the total energy produced by all Earth power plants. The mechanism by which this fantastic power is produced, is probably associated with a very peculiar structure of the so-called current shells in the Jupiter’s plasmasphere. Some scientists argue that strong electrical currents on Io’s surface can focus on small areas. And potentially volcanic eruptions could be related to these electric activities. So, to summarize, Io “works” as a part of a giant natural particle accelerator.
Related Jupiter Articles
Related Articles about Solar System Moons

[Images: www.nasa.org]

• Diameter of Jupiter: 85,788 miles. Jupiter is the largest planet of the solar system – more than 12 Earths could line up across it
• Temperature Range: -163° C to -121° C
• Distance from Sun: about 466 million miles
• Atmosphere of Jupiter: Mostly hydrogen and helium
• Rotation of its axis: 9 hours, 55 minutes in Earth time (the length of one rotation).  Jupiter spins faster than any other planet of the solar system.
• Rotation around the Sun: 12 Earth years
• Magnetic Field: Yes
• Number of Moons: at least 63
• Ganymede, Jupiter’s moon is the largest moon in our solar system. It is even larger than Mercury and Pluto. Ganymede is the only moon that has its own magnetic field.
• Four of Jupiter’s moons are known since Galileo days – Io, Europa, Ganymede and Callisto. They are all in synchronous rotation with Jupiter – always faces Jupiter with one side.
• On Jupiter you will weigh more than you weigh on the Earth, because of the fact that Jupiter is larger planet and has stronger gravity field than Earth.  If you weigh 70 pounds on Earth, on Jupiter you would weigh 185 pounds.

Related Jupiter Articles
Related Articles about Solar System Planets