Overview

When Galileo Galilei discovered Europa and Jupiter’s three other large moons — Io, Ganymede and Callisto — more than 400 years ago, he revolutionized humanity’s view of the universe. The discovery disproved the common belief that Earth was the center of all motion in the heavens.

Today Europa continues to challenge how we think about the universe and our place in it.

Scientists are almost certain that hidden beneath the icy surface of Europa is a saltwater ocean thought to contain about twice as much water as Earth’s global ocean. It may be the most promising place in our solar system to find present-day environments suitable for some form of life beyond Earth.

Slightly smaller than Earth's Moon, Europa’s water-ice surface is crisscrossed by long, linear fractures, cracks, ridges and bands. The moon’s ice shell is probably 10 to 15 miles (15 to 25 kilometers) thick, beneath which the ocean is estimated to be 40 to 100 miles (60 to 150 kilometers) deep. Like Earth, Europa is thought to also contain a rocky mantle and iron core.

Namesake

In Greek mythology, Europa was a mortal who became a princess of Crete after Cupid hit Zeus (the Greek version of Jupiter) with an arrow and he fell under Europa’s spell. The moon of Jupiter and the continent of Europe are named after her.

Europa Compared to Earth

Europa's global ocean may have been in existence for billions of years. It's an ocean that is probably more vast in depth and volume than our own ocean.

Earth Europa
Diameter 7,981 miles (12,742 kilometers) 1,939 miles (3,120 kilometers)
Average Ocean Depth ~ 2.5 miles (~ 4 kilometers) ~ 60 miles (~ 100 kilometers)
Volume ~ 870 million cubic miles (~ 1.4 billion cubic kilometers) ~ 2 billion cubic miles (~ 3 billion cubic kilometers)
Surface 29 percent land and 71 percent liquid water Global water ice shell. Thickness estimates range from 2 to 20 miles (3 to 30 kilometers).

Why does this matter? On Earth, nearly everywhere we find water, we find life. Earth’s oceans are home to 50 to 80 percent of all life on Earth, with at least 224,000 named species — plants, bacteria, fungi, reptiles, mammals, coral reefs, algae, fish, mollusks and many more. Earth's oceans also are home to extremophiles — organisms that survive everywhere from the freezing temperatures below arctic sea ice to the boiling temperatures and high pressures near deep sea hydrothermal vents.

Graphic showing how Europa may have more than twice as much water as Earth.
Europa: Water World infographic. Credit: NASA/JPL-Caltech

So, if Europa has twice the water of Earth, could it be habitable? That is, could it have environments suitable for some form of life?

Scientists used to think that in order for a world to be habitable, it had to be at just the right distance from the Sun so its surface could have liquid water. Europa is a real game changer. It is far from the Sun and yet has a liquid water ocean. The reason Europa has liquid water is because tides — similar to the tidal interactions between Earth and its moon — cause Europa's ice shell and interior to flex during the course of its orbit around Jupiter. The heating is not enough to permit water to exist in liquid form on the frigid surface, but it should be sufficient to maintain a liquid water ocean beneath an outer ice shell.

Along with liquid water, life as we know it needs two other keystones. The first is the right chemistry to use as the building blocks of life, and the second is a source of energy. Along with helping maintain liquid water, Europa’s tidal energy may also allow the ocean to interact with rocks on Europa's sea floor. Chemical reactions between water and rock could help provide not just the building blocks for life, but also the energy for life.

Exploration

Six robotic spacecraft have explored Europa, but most of what we know comes from three of them: NASA’s Voyager 2 spacecraft, the Galileo orbiter and the Hubble Space Telescope. The Voyager 2 spacecraft provided the first images close enough to reveal that Europa's surface is crossed by ridges and cracks, and hinted that Europa might be geologically active today. The Galileo mission provided even closer views of the icy moon and found the strongest evidence of an ocean there. Galileo made 12 close passes of Europa, providing the most detailed surface images to date.

Europa and Its Shadow
Hubble Space Telescope captured a rare look at three of Jupiter's largest moons parading across Jupiter in 2015. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

In 2018, scientists revisited old Galileo data and found evidence Europa may be venting plumes of water vapor into space. The Hubble Space Telescope has also found intriguing evidence of plumes at Europa. Hubble, a powerful Earth-orbiting space telescope studying far off destinations, made its first detection of possible plumes at Europa in 2012.

NASA’s Pioneer 10 and 11 spacecraft passed through the Jovian system in the early 1970s, providing only distant images of Europa. But they were still the first images of Europa taken by a spacecraft, and they helped scientists refine calculations of Europa’s size and mass.

In 1979, the twin Voyager spacecraft flew through the Jovian system, mapping the surfaces of the Galilean moons and providing more precise measurements of the moons’ sizes. Of the Galilean moons, Europa was the most poorly observed because of where it was in its orbit during the Voyager flybys. Still, images from Voyager 2 revealed a smooth surface marked with fracture-like linear features, fuzzy-looking “mottled terrain” and relatively few craters, suggesting that something was resurfacing Europa’s icy crust. A few scientists had previously speculated that Europa (as well as Ganymede and Callisto) could be hiding a liquid ocean beneath its surface ice, and the Voyagers provided exciting hints that this might indeed be the case, paving the way for Galileo’s later exploration.

Future missions include NASA’s upcoming Europa Clipper and ESA’s JUICE mission. The JUICE mission will explore Jupiter and its three largest icy moons, Europa, Ganymede and Callisto. Europa Clipper will be the first spacecraft designed to observe Europa exclusively and thoroughly.

Europa Travel Poster
A whimsical poster imagining Europa as a future tourist spot. Credit: NASA/JPL-Caltech

Pop Culture

Europa has been featured in short stories, comics and novels, with perhaps the best-known being the Arthur C. Clarke novel “2010: Odyssey Two,” which was also adapted for film.

More recently, Europa was the setting for the 2013 film “Europa Report,” and it was featured in an episode of the animated television show “Futurama.” The moon has also been the setting or subject of several video games, including “Call of Duty: Infinite Warfare” and “Galaga: Destination Earth.”

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