Places in the Solar System where life might exist (except the Moon and Ceres, which are just shown for scale).

The puzzling, fascinating surface of Jupiter's icy moon Europa looms large in this image from NASA's Galileo spacecraft in the late 1990s.

How much of Earth is made of water? Very little, actually. Although oceans of water cover about 70 percent of Earth's surface, these oceans are shallow compared to the Earth's radius. This illustration shows what would happen if all of the water on or near the surface of the Earth (pictured at right) was bunched up into a ball. The radius of this ball would be only about 700 kilometers, less than half the radius of Earth's Moon, but slightly larger than Saturn's moon Rhea which, like many moons in our outer Solar System, is mostly water ice. How even this much water came to be on the Earth and whether any significant amount is still trapped far beneath Earth's surface remain topics of research.

How much of Jupiter's moon Europa (pictured at left) is made of water? A lot, actually. Based on the Galileo probe data acquired during its exploration of the Jovian system from 1995 to 2003, Europa possesses a deep, global ocean of liquid water beneath a layer of surface ice. The subsurface ocean plus ice layer could range from 80 to 170 kilometers in average depth. Using an estimate of 100 kilometers depth, if all the water on Europa were gathered into a ball it would have a radius of 877 kilometers. With a volume 2-3 times the volume of water in Earth's oceans, the global ocean on Europa holds out a tantalizing destination in the search for extraterrestrial life in our solar system.

Habitable zone of the Sun with the planets of the inner Solar System.

These artist's drawings depict a proposed model of the subsurface structure of the Jovian moon, Europa. Geologic features on the surface, imaged by NASA's Galileo spacecraft might be explained by the existence of a layer of liquid water with a possible depth of more than 100 kilometers. If a 100 kilometer (60 mile) deep ocean existed below a 15 kilometer (10 mile) thick Europan ice crust, it would be 10 times deeper than any ocean on Earth and would contain twice as much water as Earth's oceans and rivers combined. Unlike the Earth, magnesium sulfate might be a major salt component of Europa's water, while the Earth's oceans are salty due to sodium chloride (common salt).

This illustration of Europa (foreground), Jupiter (right) and Jupiter's moon Io (middle) is an artist's concept. Water from Europa's subsurface global ocean bubbles up to the surface through vents (cryovolcanoes) and cracks (chaotic terrain - see below).

Reddish spots and shallow pits pepper the enigmatic ridged surface of Europa in this image from NASA's Galileo spacecraft. The spots and pits visible in this region of Europa's northern hemisphere are each about 10 kilometers (6 miles) across. The dark spots are called "lenticulae," the Latin term for freckles. Ruddy ice erupting onto the surface to form the lenticulae may hold clues to the composition of the ocean and to whether it could support life.

This colorized image of Europa is a product of clear-filter grayscale data from one orbit of NASA's Galileo spacecraft, combined with lower-resolution color data taken on a different orbit. The blue-white terrains indicate relatively pure water ice, whereas the reddish areas contain water ice mixed with hydrated salts, potentially magnesium sulfate or sulfuric acid. The reddish material is associated with the broad band in the center of the image, as well as some of the narrower bands, ridges, and disrupted chaos-type features. It is possible that these surface features may have communicated with the global subsurface ocean layer during or after their formation.

Radiation (charged particles) from the Sun and Jupiter, and even sulfur ejected from volcanoes on the neighboring moon Io, can alter or destroy molecules on Europa's surface. Material from Europa's ocean that ends up on the surface of Europa will be bombarded by radiation. The radiation breaks apart molecules and changes the chemical composition of the material, possibly destroying any biosignatures, or chemical signs that could imply the presence of life. To interpret what future space missions find on the surface of Europa we must first understand how material has been modified by radiation.

A transit image obtained as Europa passed in front of Jupiter in 2014 (left) has dark fingers or patches of possible absorption (inside the green oval) in the same place that auroral emission from hydrogen and oxygen, the dissociation products of water, was found in 2012. This likely indicates an active cryovolcano at that location on Europa's surface.

These images of the surface of Europa, from NASA's Galileo spacecraft, focus on a "region of interest" on the icy moon. The image at left traces the location of the erupting plumes of material observed by NASA's Hubble Space Telescope in 2014 and again in 2016 (see above). The plumes are located inside the area surrounded by the green oval. The green oval also corresponds to a warm region on Europa's surface, as identified by the temperature map at right, which is also based on observations by the Galileo spacecraft. The warmest area is colored bright red. The dark circle just below center in both images is an impact crater and is not thought to be related to the warm spot or the plume activity.

The "chaotic terrain" seen in many regions on the surface if Europa is evidence of extreme mixing between surface ice and sub-surface water. The white and blue colors seen in this image outline areas that have been blanketed by a fine dust of ice particles ejected at the time of formation of the large (26 kilometer in diameter) crater Pwyll some 1000 kilometers to the south. A few small craters of less than 500 meters in diameter can be seen associated with these regions. These were probably formed by large, intact, blocks of ice thrown up in the impact explosion that formed Pwyll. The unblanketed surface has a reddish brown color that has been coated by mineral contaminants carried and spread by water vapor released from below the crust when it was disrupted. The original color of the icy surface was probably a deep blue color seen in large areas elsewhere on the moon. The colors in this picture have been enhanced for visibility.