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Jupiter's Ganymede Moon

Galileo's journal discovering the 4 moons.
Galileo's journal

Ganymede is one of planet Jupiter’s moons. It was discovered on January 7, 1610 by Galileo Galilei. This discovery along with Jupiter’s other Galilean moons (IO, Europa, Callisto) had great significance because it was the first time it was observed a moon orbiting another planet than Earth. This helped understand that planets in our solar system orbit the Sun, rather than everything orbits the Earth. Galileo originally named Ganymede “Jupiter lll” (number 3) since he just used numerical values 1 to 4 for the moons. It wasn’t until about 250 years later that the moon “Jupiter lll” was named Ganymede, which was named from Greek mythology after a boy who was carried to Olympus by Zeus to become the cupbearer of the Olympian Gods.

Jupiter's moon Ganymede.
Jupiter's moon Ganymede

Ganymede is the largest moon in our solar system with a diameter of 5,262.4 km (3,270 mi) which is 1.51 times larger than Earth’s moon. The volume is 76,304,506,998 km3 (18,306,424,688 mi3) which is 3.47 times more than Earth’s moon. The surface area is 86,999,665.93 km2 (33,590,758.81 mi2) which is 2.29 times more than Earth’s moon. The mass is 1.4819 x 1023 kg which is 2.02 times more than Earth’s moon. Ganymede is so large that if it orbited the Sun it would be classified as a planet, a matter of fact it is larger than the planet Mercury.

Ganymede is the 7th moon from Jupiter with an average orbit distance of 1,070,400 km (665,116 miles) which is 2.78 times farther than Earth and its moon. The closest (Perigee) point during Ganymede’s orbit is 1,069,008 km (664,251 miles) and farthest (Apogee) is 1,071,792 km (665,981 miles). The orbit inclination to ecliptic (orbit angle relative to Jupiter’s equator) is 0.177° with an orbit eccentricity (amount of deviation from a perfect circle) of 0.0013. Ganymede’s orbital period (time for 1 trip around Jupiter) is 7.155 Earth days travelling at a velocity of 39,165.6 km/h (24,336 mph) and has a rotation period (time for 1 rotation) of 171.7 hours.

Ganymede has a surface gravity of 1.428 m/s2 (4.68 ft/s2) which is slightly less than Earth’s Moon surface gravity. That means it would take about 6.5 seconds to drop a solid ball from a 10 story building, where on Earth it would be about 2.5 seconds. Ganymede’s escape velocity is 9,869 km/h (6,133 mph) which is a little higher than Earth’s moon escape velocity.

Ganymede’s surface temperature ranges from -297°F (-183°C) to -171°F (-112°C), which is much colder than any place on Earth. The reason why it is so cold is because it receives such little sunlight and has no thick atmosphere (very thin composed of Oxygen) to hold in heat. The night time temperatures which can drop down to -315°F (-193deg;C). The coldest parts on Ganymede are near the North and South Poles.

Ganymede is the only moon in our solar system that has a magnetosphere. Some have argued this is just from Jupiter’s magnetic field; however Ganymede’s magnetic fields are much stronger than Jupiter’s magnetic fields from its distance, a matter of fact it is 3 times stronger than Mercury’s magnetic field with a moment value of 1.3 × 1013 T·m3.

Ganymede is generally made up of 3 layers. The first layer is the core which is made of metallic iron. The second layer is mantle (rock). The third layer is mainly ice which can be up to 800 km (497 miles) thick. About 60% of Ganymede’s surface is covered by a light grooved terrain and remaining 40% is covered by a dark grooved terrain. The dark regions are generally old and may be the original crust of Ganymede. The groove ridges can run for thousands of kilometers and can be up to 700 m (2,297 feet) high.