Galilean Moons: The Four Largest Moons Of Jupiter

~4 min

Being the largest planet in the Solar System, Jupiter has the second-largest number of moons. The first four of them – the Galilean moons – were discovered more than 400 years ago. Which of them is the biggest in the Solar System, and which is potentially habitable? Let’s find out.


How many moons does Jupiter have?

So far, 79 Jupiter moons have been found; astronomers may find more in the future. Only 53 of the moons are named. Most of them are small, less than 10 kilometers (6.2 miles) in diameter, and were discovered between the 1970s and 1990s by modern automated spacecraft. The first four discovered Jovian satellites, however, were spotted back in the 17th century using a homemade telescope.

Who discovered the four largest Jupiter moons?

The four largest moons of Jupiter were discovered in 1610 by Italian astronomer Galileo Galilei – that’s why they are also called the Galilean moons. At first, he couldn’t separate Io and Europa and saw three objects instead of four. Moreover, he mistook them for fixed stars. Only later he noticed that they were not standing still but orbiting Jupiter.

Galileo’s discovery was revolutionary. It revealed that not all celestial bodies revolve around the Earth, contrary to the geocentric theory that was in use at that time. Moreover, it showed the importance of a telescope for astronomical observances: Galileo could see the Jovian satellites only when he improved the design of his homemade telescope.

Initially, Galileo named the moons “Cosimo’s stars” (later – “Medician stars”) after his patron Cosimo Medici. In his notebooks, he referred to the moons as Jupiter I (Io), Jupiter II (Europa), Jupiter III (Ganymede), and Jupiter IV (Callisto). The names we use now were chosen by the astronomer Simon Marius, who discovered the moons nearly at the same time as Galileo and decided to name them after the Greek mythological characters. Let's see what moons were first seen near Jupiter.

Io: the “moldy pizza”

  • Mass: 0.015 Earths
  • Diameter: 3,660 km (2,274 miles)
  • Equatorial circumference: 11,445.5 km (7,111.9 miles)
  • Orbital speed: 17.334 km/s (10.770 miles per second)
  • Surface temperature: -183 °C to -143 °C (-297 °F to -225 °F)
  • Apparent magnitude: 5.02
  • Named after: priestess of Hera, Zeus’ lover

When the Voyager scientists saw Io for the first time, they described it as “moldy pizza”. What causes the moon’s unusual look? Io is the most geologically active object in the Solar System – it hosts more than 400 volcanoes. Io’s silicate surface is covered with sulfur dioxide frost, dark dots of erupting volcanoes, and lava flows. That’s why we see black, white, orange, yellow, and green spots over Io.

Europa: potentially habitable world

  • Mass: 0.008 Earths
  • Diameter: 3,122 km (1,939.9 miles)
  • Equatorial circumference: 9,807 km (6,093.8 miles)
  • Orbital speed: 13.743 km/s (8.5 miles per second)
  • Surface temperature: -223 °C to -148 °C (-369 °F to -234 °F)
  • Apparent magnitude: 5.29
  • Named after: Phoenician noblewoman, Zeus’ lover

Europa is the smallest and smoothest of the Galilean moons. It is notable for potentially having an ocean of water underneath its 15-kilometer-thick icy cover. If it’s true, this ocean may contain twice as much liquid water as all the Earth’s oceans combined. More importantly, it might have suitable conditions for extraterrestrial life to evolve, making Europa a good study object for astrobiologists.

Ganymede: the largest moon in the Solar System

  • Mass: 0.025 Earths
  • Diameter: 5,268 km (3,273.4 miles)
  • Equatorial circumference: 16,532 km (10,272.5 miles)
  • Orbital speed: 10.880 km/s (6,8 miles per second)
  • Surface temperature: -203 °C to -121 °C (-333 °F to -186 °F)
  • Apparent magnitude: 4.61
  • Named after: Zeus’ cupbearer

Ganymede looks similar to the Earth’s Moon but is 1.5 times larger (this is how it would look if Ganymede orbited Earth). It is big enough to be the only moon with its own magnetic field. Ganymede is also thought to have an underground salty ocean, which might mean that the moon harbors living organisms.

Callisto: Valhalla is here

  • Mass: 0.018 Earths
  • Diameter: 4,821 km (2,995.6 miles)
  • Equatorial circumference: 15,144 km (9,410 miles)
  • Orbital speed: 8.204 km/s (5.09 miles per second)
  • Surface temperature: -193 °C to -108 °C (-315 °F to -162 °F)
  • Apparent magnitude: 5.65
  • Named after: a nymph, Zeus’ lover

Callisto is the most heavily cratered object in the Solar System. One of the largest craters is named after Valhalla – the majestic hall for the dead warriors to rest in the afterlife. Callisto has been considered “dull”, as it didn’t show any volcanic or tectonic activity. But in the 1990s, data collected by NASA’s spacecraft revealed that there might be an ocean underneath its surface. Since then, Callisto is also on the list of potentially habitable worlds.


Why does Jupiter have so many moons?

Jupiter is big – it’s twice as massive as all the other Solar System planets combined. Therefore, its gravitational field is strong enough to attract as many as 79 satellites.

Can you see Jupiter’s moons?

Yes, but they are almost impossible to spot with the naked eye. Better use modern binoculars that are optically superior to Galileo’s homemade telescope. To find them via the Sky Tonight app, you just need to zoom in on Jupiter. If the satellites are not visible, open the quick settings panel at the bottom of the screen and adjust the apparent magnitude filter.

Do Jupiter’s moons have an atmosphere?

Io has an atmosphere consisting primarily of sulfur dioxide. Callisto’s atmosphere is composed of carbon dioxide. Both Europa and Ganymede have an oxygen atmosphere, but it’s too tenuous for us to breathe.

Can we colonize Jupiter’s moons?

The Jovian system, in general, is not ideal for colonization, mostly because of its severe radiation environment: for example, after spending a few days on Europa, you can get a lethal dose of radiation. Of all the moons of Jupiter, Callisto is the least affected by radiation and is considered the most likely place for building a human base.

Is the water on Europa drinkable?

No one has drunk it yet, but due to chemical reactions with the rock, it most likely tastes like water in the Earth's oceans. This means it is too salty to drink.