Europa and Ganymede: Which is More Life-Friendly?

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley Ph.D., Johns Hopkins University

Europa and Ganymede are two of the four main moons of Jupiter. They were discovered in 1610. Despite being a moon, Europa has shown some life-friendly aspects. Ganymede, on the other hand, is the largest moon of the whole solar system, and even though Ganymede also has water and ice, Europa could be a better choice to travel to.

3d illustration showing Jupiter with its four main moons, Ganymede, Europa, Io, and Callista.
Europa and Ganymede stand out among all the moons of Jupiter as two of the four main moons with distinctive and bold features. (Image: alexaldo/Shutterstock)

Along with the most volcanic celestial body, Io, Jupiter also has the biggest moon of the solar system, Ganymede. Io, Europa, and Ganymede are the three innermost moons. They have a 4:2:1 orbital resonance, which significantly affects Io. How about Europa and Ganymede?

Liquid and Ice Water on Europa

Between Europa and Ganymede, Europa immediately wins scientists’ attention to look for life and is also among the top choices in the solar system. The ice moon has no volcanoes but a heavily-cracked water ice surface that varies in depth from a few miles to 30 miles. There are also sulfur-rich compounds giving a reddish-brown color to the ice at places. The sulfur might come from Europa’s interior or Io’s constantly-erupting volcanoes.

There are not many craters on Europa, which means it is relatively young. The trapped ice rafts indicate that there might be liquid water under the ice shell. Another hint to liquid water is the magnetic signal in Europa fitting a salty liquid water ocean. The salt is most likely to generate the electrical currents that shape the magnetic field.

The Hubble Space Telescope showed that this subsurface ocean sends plumes of water vapor into space. Traces of water vapor were also recognizable in the Galileo mission’s magnetic data.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.

Earth and Europa: Similarities and Differences

On Earth, the rocky lithosphere is broken up into plates, and they float on the asthenosphere layer in the Earth. We also know that the plates on Earth move around and are constantly recycled into the Earth through plate tectonics. It turns out that something similar happens on Europa, too. In 2014, a 12,000-square-mile piece of the surface ice went missing on Europa: ice volcanoes (cryovolcanoes) showed the piece had gone under the surface. Europa is similar to a terrestrial planet—notwithstanding the ice—with a rocky mantle and iron core.

Planet Jupiter with some of the 79 known moons with the Milky Way galaxy in the distance.
Among all the moons orbiting Jupiter, Europa is the most life-friendly and has some earth-like characteristics. (Image: Dotted Yeti/Shutterstock)

The surface temperature of Europa is lower than −260°F at the equator and −370°F at the poles. The subsurface liquid ocean is present due to tidal heating, created by gravitational interactions with the other Galilean moons and ocean tides breaking against the ice and rock.

Despite the 1593-kilometer radius, which makes Europa a small moon, the mass of all smaller moons in the solar system combined cannot exceed Europa’s. Europa has many essential life elements.

Learn more about how our Sun defines our solar system.

Life Elements on Europa

Europa has five key elements of life:

  1. It has lots of water: two or three times more than the earth.
  2. There are essential elements like carbon, hydrogen, oxygen, and nitrogen on Europa.
  3. Europa has a source of energy despite the subsurface ocean receiving no solar energy. Deep-ocean life can form under the hydrothermal vents.
  4. The environment can remain stable enough for life to form. Tidal heating from Jupiter is a stable heat source that keeps the interior warm for long periods.
  5. Europa’s liquid ocean is well-protected by the surface ice from solar radiation.

Maybe Europa is the place to look for life. How about Ganymede?

Ganymede, the Largest Moon

This image represents the size comparison between Neptune and Jupiter's moons with Earth.
Ganymede is significantly bigger than Europa, Io, and any other moon around in the solar system, but it lacks one of the critical elements of life. (Image: Diego Barucco/Shutterstock)

Ganymede is the largest moon in the solar system, even bigger than Mercury. Even though Ganymede is also covered with ice and perhaps a subsurface liquid ocean, there are no plate tectonics like in Europa. This moon is farther from Jupiter, and mountains and grooves are signs of tidal flexing of the surface.

The impact craters show Ganymede is much older than Europa: around four billion years. The older and darker spots are dusted with carbon and silicon compounds from debris hitting the surface over millennia.

The structure of the planet is layered: ice surface, liquid water ocean, another ice layer, a rocky mantle, and an iron core. All of Europa’s life criteria are present on Ganymede, except for one.

Learn more about mighty Jupiter, the ruling gas giant.

Is Ganymede Also Life-Friendly?

As the rocky layer is not immediately under the ocean, life might have a much more difficult time to form above and under the ice. Another problem is that even if life somehow manages to form in Ganymede, it will be extremely hard to detect under the thick layer of ice.

Ganymede has its own magnetic field, as the only moon in the solar system with an active dynamo. Still, the life elements do not seem sufficient for life to form in the subsurface ocean of this giant moon.

Common Questions about Europa and Ganymede

Q: Can humans breathe on Europa?

Even though Europa is covered with ice and there is some oxygen in its atmosphere, humans cannot breathe there.

Q: Is Ganymede volcanically active?

No. Europa and Ganymede are both ice moons. Unlike Io with its 400 active volcanoes, the closest thing to volcanoes on the ice moons is an ice volcano that emits no sulfur or particles into space.

Q: Is Europa Jupiter’s biggest moon?

No. Europa and Ganymede are significantly different in size. Ganymede is Jupiter’s largest moon, and not Europa.

Q: What is special about Ganymede?

Ganymede is the largest moon of not only Jupiter but also in the whole solar system.

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