SPACE JAGAT: Ganymede, A Satellite That Owns The Largest Ocean in This Solar Family

In discussing the different satellites of the different planets, today we will discuss the ninth largest object in the solar system which has a distinct identity and which has given it a special place. Today we are talking about Ganymede, the largest satellite of Jupiter as well as the largest satellite in the entire solar system indeed.

The satellite was discovered by Galileo Galilei, one of the world's most famous astronomers, on 7th January 1610, so Ganymede is called Galilean Moon. Although Ganymede is seventh from Jupiter in terms of distance, but it is called Jupiter-III according to Galileo's law. Coincidentally, the German astronomer Simon Marius discovered Ganymede at the same time in 1610. According to Greek mythology, the newly discovered satellite was named Ganymede after the handsome son of King Tros and was named by Simon Marius himself. However, 1,975 years before this event, in 365 BC, the Chinese astronomer Gan De probably identified Ganymede and also made accurate observations of several other planetary satellites with his companion Shi Shen.

The large amount of dust and hydrogen gas that surrounds Jupiter after its formation is called sub-nebula. From this sub-nebula, Ganymede was born about 10,000 years ago from today. The satellite orbits Jupiter once every 7 days and 3 hours with an average orbital velocity of 10.88 km per hour at an average distance of 10,70,400 km from Jupiter. Like other satellites, Ganymede is tidally locked, meaning that the diurnal motion of Ganymede is almost equal to its annual motion, so that a part of Ganymede is always bound to Jupiter. One day of Ganymede is equal to 7 days and 3 hours as Earth.

With a diameter of 5,268 km, this satellite is the heaviest massive satellite in the solar system. Its diameter is 0.41 times the diameter of Earth and 1.08 times the diameter of Mercury. Ganymede is 26 percent larger in size than Mercury. Compared to satellites, Ganymede's diameter is 1.02 times the diameter of Titan, the second largest in the solar system and Saturn's largest moon, and 1.51 times the diameter of our Earth's satellite Moon. In terms of mass, this satellite is 2.02 times the mass of the moon. According to the mass, Ganymede's escape velocity is 2.74 km per second or 9,867 km per hour. (Escape velocity: The minimum speed which is required to reach space beyond the gravitational force of an astronomical object.)

The density of Ganymede suggests that the satellite is composed of equal amounts of rocky material and water-ice. Some amount of water has formed the underground sea of Ganymede. Forty-three percent of the total incident rays, on the surface of the Ganymede, are reflected back into space, proving that water-ice is everywhere in the Ganymede. In addition to ice, various elements such as carbon dioxide, sulfur dioxide, cyanogens, hydrogen sulfide, magnesium sulfate, sodium sulfate and a variety of organic compounds may be present. The average temperature of the illuminated part of Ganymede is -112.7⁰ Celsius and the average temperature of the dark part is -182.7⁰ Celsius.

The surface of the Ganymede consists of two parts, the dark part with ditches and old organic matter, which covers one-third of the area of the satellite, and the luminous part with the relatively new and less number of dents. The dark part contains a lot of cosmic collision craters. The luminous part has craters but they are very few in number instead, grooved terrain is seen here. The reason for the flat nature of Ganymede's craters compared to other satellites' craters is probably the weak nature of Ganymede's icy crust. Anat is a notable ditch of Ganymede. In the dark part of this satellite there is a plain-like area called Galileo Ragio. Numbers of grooves have been formed during ancient geological activities which have been observed here. Ganymede has the most ice presence in its polar regions. Scientists say a large portion of the satellite's water has been transferred to the polar region and has formed ice caps in the polar region due to the effect of the satellite's magnetic field and a few other factors.

The core, inside Ganymede, is made up of iron, nickel and sulfide. The core has a radius of 500 km and a temperature of 1,500 to 1,800 Kelvin. This phenomenon explains Ganymede's own magnetic field. Galileo spacecraft has discovered this magnetic field, separate from magnetic field of Jupiter, while orbiting the spacecraft Ganymede and reported that the satellite's magnetic field is at least three times stronger than Mercury's. The core of the Ganymede is surrounded by a mantle of silicate and this mantle is surrounded by the surface of sea and frozen ice (Icy Crust). This underground sea is between two ice layers. This sea is divided into different layers and types by ice walls. However, the oceans may be the largest of all the planets and satellites in the solar system. There has long been speculation among scientists that Ganymede may be a possible habitat for marine life.

In 1972, a group of Indian, English and American scientists claimed that Ganymede had a thin layer of oxygen around it. Although the Voyager spacecraft's subsequent search for information led to some confusion, in 1995 the Hubble Space Telescope has confirmed that the speculation and claim of scientists is absolutely right and reported that the oxygen could be dissolved in ice on the surface of the Ganymede. However, having oxygen in Ganymede does not mean the existence of life. Water ice on the surface of the Ganymede disintegrate into hydrogen and oxygen by radiation, and due to very light weight of hydrogen atom, it forms molecular hydrogen and accumulate at an altitude of about 3,000 km above the surface of the Ganymede. The point to note here is that the type of radiation that occurs in Ganymede is not only extremely dangerous to any organism but can also cause rapid death. The Hubble Space Telescope has found two white-spotted areas in the thin atmosphere of Ganymede's polar region. Whether there is ionosphere in the atmosphere of this satellite is still a mystery. To date, no definite conclusion has been reached on this issue. The ozonosphere was discovered in Ganymede's atmosphere in 1996, meaning that Ganymede has ozone gas in its atmosphere, and it tries to create obstacle for at least a few harmful cosmic rays, coming from distant space.

From the seventies of the last century to the first two decades of the twenty-first century, Ganymede has had multiple space missions. This route was started by Pioneer-X spacecraft. On 3rd December 1973, the spacecraft made its flyby at a distance of 4,46,250 km from Ganymede. (Flyby: According to the terminology of astronomy, the incident of observation by an observer spacecraft at the shortest distance on a star, planet or satellite.) Its twin spacecraft, Pioneer-XI Spacecraft, made its flyby 6,92,300 km from Ganymede on 2nd December 1974, exactly one year later. Now the Voyager spacecrafts came forward to explain in more detail the information sent by these two spacecraft. The Voyager-I spacecraft made its flyby at a distance of 1,14,710 km from the satellite on 7th March 1979, and the Voyager-II spacecraft made its flyby only at a distance of 62,130 km from Ganymede on 7th July of that year. After making flyby and observing some more planets and satellites, its predecessors like Pioneer-10, Pioneer-11 and Voyager-1 left this solar family forever and went for a more distant space mission. Both of these Voyager spaceships eliminate errors in size and various measurements of Ganymede. (Voyager's link is mentioned here for more details about these two spacecrafts https://spacejagat.blogspot.com/2020/02/voyeger.html )

The orbit of Galileo Spacecraft was based on the information they received after the expedition of these two legendary spacecraft. On 27th June 1996, the spacecraft made its first flyby over Ganymede at a distance of only 835 km. The spacecraft then made flyby over Ganymede five more times, the last time this happened on 28th December 2000, when the distance between the spacecraft and the satellite was 2,321 km. The Galileo spacecraft made closest flyby over the satellite on 6th September 1996, and the farthest from the satellite on April 5, 1997, out of a total of six flyby incidents. At that time the distance between them was 260 km and 3,102 km respectively. The Galileo spacecraft reported the existence of the magnetic field of Ganymede during the first flyby and the existence of the ocean below the surface during the last flyby. The New Horizons Spacecraft sent high-quality images to the Earth on 27th February 2007, while making flyby from long distance (35,000,000 km) to Ganymede on its way to Pluto. On 25th December 2019, the Juno Spacecraft sent a lot of pictures to Earth while making flyby over Ganymede from different distances.

The National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) have launched a joint venture project called The Europa-Jupiter System Mission (EJSM) in a few days. This will observe Ganymede better. The Japanese Aerospace Exploration Agency (JEXA) is expected to take part in the mission. Jupiter Icy Moon Explorer (JUICE) of the European Space Agency will be launched in 2022 which will make flyby over Ganymede. Russian astro-biologists are currently building a spacecraft called Ganymede Lander for Ganymede research. Either this spacecraft will go along with JUICE or will launch a single space mission in 2024.

Maybe human settlements will never be built in Ganymede. But the largest satellite, even larger than planet Mercury, in the solar system must have an importance. Ganymede will certainly play an important role in the future in understanding the structure of this solar family and its long history. There is no question of doubt or disagreement on this point.