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.

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

“One small step for man, one giant leap for mankind” – this is what Neil Armstrong, an American and the first man to step on the surface of the Moon, had to say. Truly, the first steps of mankind on the Moon have been immortalized, literally and figuratively (since there’s no atmosphere on the Moon, the footprints of all the astronauts will be preserved for millions of years due to absence of erosion). This little satellite of our planet has been very popular among artists and scientists alike. Humans have found out a lot of information about the Moon and we are still sending unmanned voyages to the Moon to find out more – the latest being the ‘Chandrayaan’ by India.

Moon Formation

The Moon is estimated to have come into formation about 30-50 million years after the Solar System came into being. That makes the Moon more than 4.527 billion years old – the Earth itself is 4.54 billion years old. Some researchers and computer simulations suggest that in the initial days of the Earth, its spin was so great that the immense centrifugal force caused the fission of the Moon from the Earth’s crust. However, in order for the Earth to gravitionally capture the pre-formed Moon, its atmosphere would have had to be unfeasably extended from what exists today.

Another hypothesis takes into consideration the fact that in the early days of the Solar System, giant impacts between planets and huge asteroids or meteorites were very common, and this theory suggests that a similar giant impact was responsible for the formation of the Moon. Possibilities exists that a Mars-sized planet collided with the newly formed Earth, thus blasting material from the Earth into the orbit resulting in the formation of the Earth-Moon system.

Moon Atmosphere and Temperatures

It would be incorrect to say that the Moon has no atmosphere at all, though it is so insignificant that it is easier to ignore its presence. Total weight of the Moon’s atmosphere is about 10 metric tons; compare that with the Earth’s atmpsphere which is about 5 quadrillion tons. This causes the temperature ranges to be at extremes on the Moon and the Sun is the only factor which influences it. The temperature on the side of the Moon facing the Sun can go upto a maximum of 123°C (253°F) while the side in darkness can record temperatures as low as -153°C (-243°F). The poles and the perpetually dark craters can sometimes experience temperatures close to the absolute zero.

Moon Size and Speed

The Earth’s Moon is the second largest moon of the Solar Syste, when the relative size is taken into consideration. In other words, the relative size of the Earth and the Moon is quite large when compared to the relative sizes of planets like Jupiter and Saturn and their moons. The diameter of the Moon is 2,000 miles ( 3,476 kilometres). To give an idea of the surface area of the Moon, it is comparable to the size of the continent of Africa. The speed at which the Moon orbits the Earth differs according to its distance from the Earth at different points in its orbit. On an average, the Moon revolves around the Earth at 2,288 miles per hour (3,683 kilometres per hour).

Moon Distance to Earth and Lunar Month

The center of the Moon is at a distance of 250,000 miles (384,400 kilometres) from the center of the Earth. A beam of light from the surface of the Earth to the surface of the Moon would take about 1.26 seconds to travel this distance. The Moon takes 27 days, 7 hours, 43 minutes, 11.6 seconds to complete one orbit around the Earth. It should be noted that the Lunar month (time between two new moons) is considered to be 29 days, 12 hours, 43 minutes, 11.6 seconds long. Though the two are basically based on the same phenomenon – i.e. the completion of one orbit around the Earth – the difference in time arises since the Earth is constantly moving around the Sun and thus the Moon has to cover more than 360 degrees to complete one revolution around the Earth.

The Moon has quite a few influences on the Earth and a lot of changes would affect the Earth if the Moon was to be taken out of the picture. For example, the gravitational pull of the Moon causes the water of the oceans to move away from the polar regions. Without the Moon, the water would redistribute itself near the polar regions and cause the sea levels to change a little. The spin of the Earth is slowed down by the Moon and without the Moon we would experience shorter days. Also, the axis of rotation of the Earth is said to be stabilized due to the Moon.

Though only two countries have so far reached the Moon, an international treaty has been signed which declares the Moon as a property of the whole of mankind. This restricts countries from using the Moon for anything other than peaceful purposes.

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[images: nasa.gov]

Scientists have found oxygen in the exosphere (the upper atmosphere) of Saturn’s moon Dione. The article about this discovery was published in the Geophysical Research Letters magazine. Dione is a relatively large satellite. Its diameter is 1,123.4 kilometers and it consists mainly of water ice and rock.

Researchers have used data collected by the Cassini probe in April 2010. During that period Cassini came close to the moon and was able to gather data about its atmosphere. The amount of oxygen in the atmosphere is extremely low - 0.01 to 0.09 oxygen ions per cubic centimetre of exosphere.

Scientists believe that oxygen ions are formed as a result of the bombing of Dione’s surface by charged particles trapped by Saturn’s magnetic field. Prior to Dione oxygen was found on Saturn’s moon Rhea, as well as Jupiter’s moons Ganymede, Europa and Callisto.

“Cassini” is a joint project of ESA, NASA and the Italian Space Agency. It was launched in 1997.  The probe’s mission will continue until 2017.

[image: nasa.org]

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.

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[images: nasa.org]