The largest moon in the solar system (Ganymede) in a photo taken by the Galileo space probe 🛰️
In 1610, Galileo Galilei looked up at the night sky using a telescope of his own design to observe Jupiter. He then noticed that there were many what looked like “luminous elements” surrounding the planet, and at first he thought they were just stars. Over time, he noticed that these luminous, star-like elements revolved around the planet, only to discover that they were in fact moons of Jupiter and not stars. They were then called Io, Europa, Ganymede, and Callisto. Among these moons, Ganymede is considered the largest in size, and it has many unique features, as it is the largest moon in the solar system, in addition to being larger than the planet Mercury itself. This is the only moon in the solar system that has a magnetic field, in addition to a thin atmosphere, and it is believed that there is an ocean inside it (like its companion moons: Io, Callisto, and Europa).
In the 1970s, NASA scientists first suspected that Ganymede had a thick ocean between two layers of ice, one at the surface and the other beneath the liquid ocean and above the mantle. In the 1990s, the Galileo spacecraft flew by the Moon and found evidence of such a subsurface ocean. An analysis published in 2014, taking into account realistic water thermodynamics and salt effects, suggests that Ganymede may contain several oceanic layers separated by different states of ice, with the lowest layer of water adjacent to the lithospheric mantle. The contact of rocks and water may have been an important factor in the emergence of life. The analysis also indicates that the extreme depths found (about 800 km to the rocky “sea floor”) mean that temperatures at the bottom of the convective ocean can reach 40 K hotter than those of the ice-water interface. Other evidence from the Hubble Space Telescope indicates that Ganymede’s oceans, if they exist, would be the largest in the entire solar system.
