What is the Biggest Moon in the Solar System?


Ganymede is the name of the biggest moon in the solar system. This moon, which belongs to Jupiter, is not just the biggest moon in the solar system; it is also bigger than some planets. Read on to explore what Ganymede is really like.

Key features


Interestingly, evidence has been found by NASA’s Hubble Space Telescope of an underground ocean on Ganymede. This ocean, made up of salt water, is believed to have more water than is on the entirety of Earth’s surface. This huge amount of water has a thickness of 60 miles (ca. 97 km). To put this into perspective, Earth’s ocean is only around 6 miles (ca. 10 km) thick, making Ganymede’s a whopping ten times thicker! Moreover, this massive ocean is believed to be buried deep under a crust around 95 miles (ca. 153 km) wide and made up mostly of ice. Identifying this ocean is fascinating and a huge step in researching whether there are habitable planets beyond Earth.

Magnetic field

Another key feature of Ganymede is that it has its very own magnetic field. While it is common for some planets to have their magnetic fields, it is not typical for moons to have their own. Ganymede is the only moon in our solar system that is known to have a magnetic field. Because Ganymede is close to Jupiter, its magnetic field lies within the planet’s magnetic field. This means that when Jupiter’s magnetic field changes, it causes Ganymede’s auroras, strips of electrified gas that circle the moon’s north and south poles, to change. Specifically, when the magnetic field on Jupiter changes, Ganymede’s auroras begin to rock back and forth.


There are two distinct terrain types on the solar system’s biggest moon. The first type of terrain is bright areas of ridges, and grooves, which cut across the second type of terrain, which is the older, darker areas.


Ganymede is the largest of Jupiter’s four moons, also known as the Galilean moons. This is because they were first discovered by the Italian astronomer Galileo Galilei in 1610. The discovery of the Galilean moons was significant, as it was the first time a moon was seen to be orbiting a planet besides Earth. This information was vital in leading scientists to conclude that the planets in the solar system orbit the Sun, not Earth.

Naming process

The biggest moon in the solar system gets its name from a figure in Greek mythology. Ganymede was supposedly the name of a beautiful young boy carried by Zeus to Olympus while disguised as an eagle. After this, Ganymede became the cupbearer for the Olympian gods.

Potential for life

As mentioned above, Ganymede has its ocean, so scientists believe the moon could support some life.

There has also been evidence found of a thin oxygen atmosphere on Ganymede.


Okay, we know that Ganymede is the biggest moon in the solar system, but how big is it? First, let’s get into some stats:

  • Ganymede has a radius of 1,635 miles (ca. 2,631 km), which makes it bigger than the planet, Mercury, and the dwarf planet, Pluto.
  • Ganymede has a circumference of around 10,272 miles (ca. 16,531 km).
  • Ganymede has a volume of around 47,413,422,486 miles3
  • Ganymede has a surface area of about 54,059,086 miles2

Orbit and rotation

Ganymede orbits around Jupiter at a distance of around 665,000 miles (ca. 1,070,214 km). The positioning of the Galilean moons in order of closeness are:

  • Io: 262,200 miles (ca. 421,970 km)
  • Europa: 417,000 mi (ca. 671,096 km)
  • Ganymede 665,000 miles (ca. 1,070,214 km)
  • Callisto: 1,170,000 mi (ca. 1,882,932 km)

One orbit of Jupiter takes Ganymede, the equivalent of around seven days on Earth. An orbit around the Sun takes much longer. Along with Jupiter, Ganymede completes one orbit around the Sun every 12 Earth years.

Ganymede, along with the moons, Io and Europa, is in a resonance pattern. This means that, for every time that Ganymede orbits Jupiter, Europa orbits twice, and Io orbits four times. This resonance is significant because it causes the orbit of the moons to take on a slightly elliptical shape, as opposed to the typical circular orbit other moons have.


Ganymede was formed from the same material as Jupiter’s other large moons. This material is believed to have been leftover from when Jupiter condensed out of the initial cloud of gas and dust around the Sun. This event was very early on in the history of the solar system. Ganymede is believed to be around the same age as the rest of the solar system, making it around 4.5 billion years old.


In terms of structure, Ganymede is split up into three main layers:

  1. A metallic iron core at the center of the moon
  2. A spherical shell of rock around the body (known as the mantle)
  3. A spherical surface largely made up of ice surrounds the body of the stone.

The moon’s surface is the top of the ice shell, where scientists have discovered a range of irregularly shaped bumps. These bumps are considered rock formations that have been around for billions of years.

The biggest moon in the solar system

After Ganymede, these are the largest moons in the solar system:


Titan, which orbits around Saturn, is the second-largest moon in the solar system. This moon was discovered in 1655 by a Dutch astronomer, Christiaan Huygens, and it has a radius of about 1,600 miles (ca. 2,575 km). The atmosphere on this massive planet is super dense, much like the atmosphere on Earth. It is composed largely of nitrogen, with the remainder being made up of methane and small amounts of ammonia, argon, and ethane. In terms of its structure, Titan primarily consists of ice and rocky materials.

For Titan, one orbit of Saturn takes the equivalent of around 16 Earth days. This moon also has a range of seas and lakes on its surface, all of which are filled with what’s known as liquid hydrocarbons (unprocessed oily liquids extracted from natural gas). A fun fact about this massive moon is that it is the only body in our solar system, aside from Earth, to contain bodies of water.

Like many other planets and moons, Titan gets its name from Greek mythology. Specifically, this moon is named after a group of ancient gods called Titans.


Callisto is another of Jupiter’s huge moons. It is the second-largest moon in Jupiter’s orbit and the third-largest moon in the entire solar system. This moon’s radius was around 1,497 miles (ca. 2,409 km), and its surface was covered in craters. In terms of its composition, the moon is primarily made up of water ice and contains materials like magnesium and hydrated silicates. The surface of Callisto has a dark appearance and is believed to have a saltwater sea lying beneath it.

Callisto is believed to be around 4.5 billion years old, the same age as the rest of the solar system, and was discovered by Galileo Galilei on the 7th of January, 1610. After its discovery, the moon was named after a nymph in Greek mythology called Callisto.

Callisto orbits around Jupiter at a rough distance of 1,169,855 miles (ca. 1,882,699 km), making it the furthest moon from the planet. This distance means that Callisto is not affected by the magnetic field around Jupiter.

The moon takes around 16 Earth days to complete one rotation on its axis and to orbit around Jupiter.


It is the fourth-largest moon in the solar system, with a radius of about 1,131 miles (ca. 1,820 km). Galileo Galilei also discovered this moon in 1610, making it one of the Galilean moons. The surface of this moon is composed largely of floodplains of liquid rock and lava lakes, which gives it a blotchy appearance of white, red, yellow, black, and orange. Under this smooth surface, the moon has an iron core and an outer layer of brown silicate. Interestingly, Io is the body with the most volcanic activity after Earth.

This moon orbits around Jupiter at a distance of around 262,218 miles (ca. 421,999 km), and each orbit takes about 1.77 Earth days. So it will come as no shock that this moon also got its name from a figure in Greek mythology. Io is named after a nymph of the same name, who Zeus famously seduced.


The fifth-biggest Moon in our solar system belongs to our very own planet. The Earth’s moon is believed to be the youngest large object in the solar system. Scientists and astronomers have theorized that the Moon was formed out of the debris caused by an ancient giant impact. This impact is believed to have happened 50 million years after the other planets and their satellites were formed.

Like all other moons in the solar system, our moon is tidally locked to Earth. When a moon is tidally locked to a planet, it rotates simultaneously as it takes to orbit the earth. That is why we only see one side of the Moon. The basic structure and composition of the Moon are pretty similar to that of Earth rocks. This makes it stand out from all other large non-planetary objects in the solar system.


Europa is the smallest of Jupiter’s four moons. This moon is covered in a shell of ice that floats on an ocean around 40 to 100 miles (ca. 161 km) deep. While this may not seem overly inviting, Europa is the most hospitable of Jupiter’s moons. Much like the biggest moon in the solar system, Ganymede, Europa has a thin atmosphere composed primarily of oxygen.


Triton is the seventh-biggest moon in the solar system, with a diameter of around 1,682 miles (ca. 2,707 km). This is the largest of Neptune’s moons.

In terms of appearance, Triton has a partially-bumpy surface, with a selection of ice volcanoes across it. These volcanoes spew a mixture of liquid nitrogen, methane, and dust into the atmosphere, which freezes and then snows back down to the surface. Interestingly, Triton’s character is so icy that it reflects most of the sunlight that hits it, making it one of the coldest objects in the solar system (around -240 degrees Celsius).

Moreover, Triton is the only large moon in our solar system that rotates around its planet in a direction opposite to its rotation. This leads scientists to believe that, at one point, Triton was an independent object that Neptune captured. Neptune’s gravity pulls on Triton as it orbits around the planet. This causes Triton to slow down and fall closer and closer to the earth. As a result, it is estimated that millions of years from now, Triton will come so close to Neptune’s gravitational force that it will break apart.

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