Jupiter’s moon Europa, slightly smaller than Earth’s moon, is a primary candidate in the search for life beyond our planet. Beneath its bright, icy shell, scientific consensus suggests a vast, globe-spanning ocean of liquid water exists. This hidden water world may hold more than twice the water of all Earth’s oceans combined. The potential for a habitable environment so far from the Sun has made Europa a focal point for planetary exploration.
Evidence for a Subsurface Ocean
The most compelling evidence for a Europan ocean comes from NASA’s Galileo spacecraft. As Galileo flew past Europa, its magnetometer detected a disruption in Jupiter’s magnetic field around the moon. This implies an electrically conductive layer exists within Europa, which scientists believe is a global ocean of salty water. The salt content makes the water conductive, allowing it to interact with Jupiter’s magnetic field and generate its own temporary magnetic field.
Surface geology provides additional clues. Images from the Voyager and Galileo missions revealed a young and smooth surface with very few impact craters. This suggests geological processes have recently resurfaced the moon, erasing older features. The surface is crisscrossed by extensive cracks, ridges, and jumbled blocks of ice known as “chaos terrain.” These formations suggest the ice shell has been broken from below by the movement of liquid water or warmer ice.
Tentative detections of water vapor plumes erupting from Europa’s surface also support the presence of liquid water. Observations by the Hubble Space Telescope have provided hints of these plumes, which could be cryogeysers blasting water from the subsurface ocean into space.
Characteristics of the Hidden Ocean
Based on available data, scientific models describe Europa’s hidden ocean and its overlying ice crust. The ice shell is estimated to be between 15 to 25 kilometers (10 to 15 miles) thick. Beneath this ice lies a body of liquid water thought to be approximately 60 to 150 kilometers (40 to 100 miles) deep.
This vast ocean is not just pure water; it is salty. The reddish-brown material seen streaking the cracks on Europa’s surface may be salts, like magnesium sulfate, mixed with water ice altered by Jupiter’s radiation. This suggests that material from the ocean is being transported to the surface, offering clues about its chemical makeup.
The ocean is believed to be in direct contact with a rocky silicate mantle. This allows for chemical reactions and the potential leaching of minerals from the seafloor into the water. This process could supply the chemical nutrients to support a habitable environment.
Potential for Extraterrestrial Life
The potential for life on Europa stems from the likely presence of three ingredients: liquid water, chemical elements, and an energy source. The subsurface ocean provides a stable liquid environment. The chemical building blocks for life, such as carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, are common in the solar system and were likely incorporated into Europa when it formed or delivered by later impacts from comets and asteroids.
The energy source is notable because it does not rely on sunlight. Europa’s ocean is kept liquid by heat generated through tidal flexing. As Europa follows its slightly elliptical orbit, Jupiter’s gravitational pull stretches and compresses the moon’s interior, creating friction and heat. This same process could power geological activity on the seafloor, similar to volcanic activity on Earth.
This leads to the possibility of hydrothermal vents on Europa’s ocean floor. On Earth, such vents support ecosystems that thrive in darkness, powered by chemical energy from the planet’s interior. If similar vents exist on Europa, they could provide chemical energy and nutrients to support microbial life.
Future Exploration of Europa
To investigate these possibilities, future robotic missions are planned to explore Europa. NASA’s Europa Clipper mission is designed to assess Europa’s habitability. Its objective is not to find life directly, but to determine if the conditions that could support life exist. Launched in October 2024, the spacecraft will arrive in the Jupiter system in 2030 and perform dozens of close flybys of the moon, some as low as 25 kilometers (16 miles) above the surface.
Europa Clipper carries nine science instruments. These include an ice-penetrating radar to measure the thickness of the ice shell, spectrometers to analyze the composition of surface materials, and a magnetometer to confirm the ocean’s existence and measure its salinity and depth. By scanning nearly the entire moon, the mission will create a detailed map of its geology and chemical makeup.
The European Space Agency’s (ESA) Jupiter Icy Moons Explorer (JUICE) will also study Europa. Launched in April 2023, JUICE will perform two flybys of Europa as part of its larger tour of the Jovian system before focusing on Ganymede. Together, these missions will provide a wealth of new data about this hidden water world and its potential to harbor life.