Ganymede is the largest natural moon of Jupiter and also the largest moon in the solar system. It is even larger than Mercury, which was once considered a planet.
Jupiter’s moon Ganymede is almost a literal planet. This distant world is not only the largest moon in the Solar System, but its size is larger than the planet Mercury.
Ganymede has captured the imaginations of people on Earth for decades, making it a popular setting for many sci-fi works, from Robert A. Heinlein’s novels to the Cowboy animated series. Bebop. But the truth about Ganymede can be even more interesting than the fictional stories.
Ganymede was officially discovered in 1610 by the Italian astronomer Galileo. However, there is a possibility that it may have been first observed much earlier. The “Encyclopedia of the History of Science, Technology, and Medicine in Non-Western Cultures” refers to the ancient Chinese astronomer Gan De, whose records suggest he may have discovered this moon in 365 BC.
In his writings, he mentioned seeing a moon – the “small red star” next to Jupiter. Notably, Gan De made his observations without the use of a telescope; while, theoretically, Ganymede is bright enough to be seen with the naked eye, however it is often dimmed by the bright image of Jupiter.
In April 2023, ESA launched a spacecraft to Jupiter. Named the Explorer of Jupiter’s icy moons (Juice), Ganymede was both the primary target and the final destination on its 500-million-mile journey. When it arrives, it will give humanity a closer look at this fascinating mysterious world.
A moon the size of a planet
If Ganymede orbited the Sun instead of Jupiter, it would almost certainly be considered a real planet for several reasons. One of the main reasons is its size. It is 3,270 miles in diameter, about 8% larger than Mercury’s 3,032 miles. The planet that is getting the most attention right now as a destination for spacecraft and robotic explorers is Mars, which is also only 4,212 miles in diameter, not much larger than Ganymede.
However, the big difference between Ganymede and the planets is that it is significantly lighter than Mercury or Mars. Like the moon Europa, most of Ganymede’s mass is water and ice. As a result, its surface gravity is much lower than its large diameter ratio.
Therefore, when setting foot on Ganymede astronauts will certainly feel very similar to walking on Earth’s Moon.
One day in Ganymede is equal to 7 days on Earth
Ganymede rotates much slower than the Earth. A day on this moon lasts about 7 days on Earth. The reason why Ganymede’s day is so slow is because it is tightly locked to Jupiter.
Like Earth’s Moon, Ganymede is almost always locked on one side to its parent planet, while the other side is always facing away. In other words, a date on Ganymede coincides with its orbit. In Ganymede’s sky, Jupiter is a permanent fixture that never moves from its place.
Tidal locking occurs with any moon that orbits its planet close enough. When gravity on the moon is strong enough, the closer side will feel the pull stronger than the side facing out.
Ganymede possesses an iron heart
Inside, Ganymede is surprisingly Earth-like. Deep below the surface, our planet is divided into many distinct layers of matter. When the planet was still young and had not yet completely cooled, heavier matter sank deeper into the center, which is why Earth has an iron core.
According to an article published in the journal Nature, this feature also exists in Ganymede. Jupiter’s largest moon is also composed in almost the same way. If you can cut it in half, you’ll find several layers with their separate components.
Ganymede’s core is made of iron, like the cores of real planets like Earth. Some planetary scientists suspect that Ganymede’s core might even be molten. Apparently, Jupiter’s largest moon has more in common with Earth than we thought.
However, the only way to be sure of this is to take a closer look with specialized tools. This is one of the things ESA’s Juice spacecraft wants to learn more about. When it comes to Ganymede, learning about the moon’s core is one of its main scientific goals.
A magnetic moon
Earth’s iron core is what creates our planet’s magnetic field, protecting its surface — and us — from harmful cosmic radiation. Since Ganymede also has an iron core, it also has a magnetic field.
In fact, as NASA explains, it’s the only moon in the Solar System that has a planet-like magnetic field. Ganymede’s magnetic field is most likely generated by its core, but some observations suggest that something else on the moon is influencing its magnetism. Ganymede’s observations of the magnetic field suggest that it may harbor huge amounts of liquid water below the surface, affecting the shape of the moon’s magnetic field.
However, unlike Earth, Ganymede’s magnetic field is quite weak. Even at its strongest, it is weaker than Mars’s fragile magnetic field. Meanwhile, Mars has a magnetic field about 40 times weaker than Earth’s.
While Earth’s magnetism keeps its surface shielded, Ganymede’s magnetic field appears to be too weak to perform the same task. It probably won’t do much to protect any future astronauts who might land there.
Source: Earthlymission; Nature; NASA