Planet-sized Moon: Ganymede

Ganymede is the largest of the 63 known moons of the gas giant Jupiter and the largest moon known in the solar system.

The moon was discovered by Galileo Galilei in 1610, along with the other 3 large moons of Jupiter. These 4 moons, called Galileo Moons, can be observed smoothly even with a small telescope or binoculars. These Galileo Moons constitute 4 of the 6 largest known moons of the Solar System.

Ganymede is even larger than the planet Mercury with a diameter of 5,262 km (Mercury's diameter is 4,879 km). Therefore along with Titan (Saturn's moon) is among the rare celestial bodies the size of the planet that could become the moons of other planets.

If we had taken Ganymede from the orbit of Jupiter and placed it somewhere in the orbit of the Sun, it could easily be regarded as a planet without being unobtrusive with its dimensions. However, although it has a larger diameter than Mercury, Ganymede is quite light. Its mass is about 2.8 times less than Mercury's mass. This shows that the moon's structure is largely made of ice.

There is a thick layer of ice on the surface of the Ganymede moon, but unlike that, there are plenty of impact marks, grooves, and craters on this layer. Some of the craters on its surface can reach 4 Billion years old. The reason why the craters have been preserved for such a long time is dependent on the lack of tectonic movements in the moon's crust. So, Ganymede is not very active geologically.

In addition to being the largest, Ganymede is the only moon to have its own magnetosphere. The moon's internal structure is still hot and has a molten layer. Although it is located within the magnetosphere of Jupiter, its magnetosphere is effective in an area twice its diameter. This magnetosphere protects the equatorial regions of the moon from high energy flow with Jupiter.

However, this magnetosphere cannot prevent the surface of the moon from being exposed to 8 rem (0.08 sievert) ionizing radiation per day. Considering that our daily normal in the world is 0.14 rem, it is obvious that in the future manned colonies should be strengthened against radiation or be established underground to the Ganymede moon.

Ganymede, where we recently learned about the underground saltwater ocean, is an important place about extraterrestrial life as well as our life.

Prepared and Translated by Emir Bolukbas.


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