The quest for life beyond Earth has expanded beyond Mars to include distant, icy worlds. Jupiter’s moons, with their unique characteristics, have emerged as compelling candidates for hosting life, driving extensive scientific inquiry.
Prime Candidates for Life
Among Jupiter’s numerous moons, three stand out as prime candidates for potentially harboring life due to compelling evidence of subsurface liquid water oceans.
Europa, the smallest of the four Galilean moons, is perhaps the most famous. Its smooth, fractured icy shell strongly suggests a global ocean beneath, estimated to be up to 100 kilometers deep and potentially holding more than twice the volume of all Earth’s oceans combined. Data from past missions indicates dynamic geological activity on Europa, which could facilitate interaction between the ocean and a rocky seafloor.
Ganymede, the solar system’s largest moon, also shows strong indications of a vast saltwater ocean. Evidence from the Galileo spacecraft and Hubble Space Telescope, particularly observations of its magnetic field and auroras, points to an ocean layered between ice and a rocky interior. This ocean is thought to be approximately 100 kilometers deep and could contain more water than all of Earth’s oceans combined. Its metallic core also generates its own magnetic field, a unique feature among solar system moons.
Callisto, the outermost of the Galilean moons, presents a more ancient, heavily cratered surface, yet it too may conceal a subsurface ocean. Re-analysis of Galileo mission data provides stronger support for a deep, salty liquid water ocean. The presence of an electrically conductive layer, best explained by a salty ocean, helps account for its induced magnetic field.
Essential Ingredients for Life
Life as we understand it requires specific conditions: liquid water, an energy source, and chemical building blocks. Jupiter’s icy moons are thought to potentially meet these requirements.
The presence of liquid water beneath their frozen surfaces is largely attributed to tidal heating. Jupiter’s immense gravitational pull, combined with gravitational interactions among the moons, causes them to flex and stretch. This continuous kneading generates internal friction, warming their interiors and preventing the subsurface oceans from freezing solid.
An energy source is also needed to drive biological processes. Scientists hypothesize that hydrothermal vents on the ocean floors could provide such energy, releasing chemically rich fluids heated by interactions with the moon’s rocky core. Water-rock interactions at the seafloor could also provide chemical energy, as cool seawater seeps into the rock, heats up, and resurfaces with altered chemistry.
The building blocks for life, including elements like carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur (CHONPS), are believed to be present. On Europa, carbon dioxide has been detected in specific regions, hinting at a connection between the subsurface ocean and the surface. These chemical ingredients, combined with liquid water and a source of energy, could create environments conducive to the emergence of life.
Hypothetical Life Forms
If life exists within the subsurface oceans of Jupiter’s moons, it would likely be microbial and adapted to extreme conditions. These potential organisms would resemble Earth’s extremophiles, which thrive in environments considered hostile to most life. Psychrophiles can endure extreme cold, while thermophiles flourish near heat sources. Chemotrophs, organisms that derive energy from chemical reactions rather than sunlight, offer a compelling model for potential life in these dark, deep oceans.
Such life forms would probably rely on chemosynthesis, utilizing chemical energy from inorganic compounds released by water-rock interactions. This differs from photosynthesis, which requires sunlight. Life on these moons would need to tolerate high pressures and the absence of light, adapting to environments vastly different from Earth’s surface. The development of complex, multicellular life is considered highly improbable. Any life present would most likely exist as simple, single-celled organisms or microbial communities.
The Search for Evidence
The scientific community actively pursues evidence of habitable conditions and potential life on Jupiter’s moons through various missions and observational techniques. Past missions like NASA’s Galileo spacecraft provided foundational data, revealing strong indications of subsurface oceans on Europa and Ganymede, and hints on Callisto. Galileo also provided early insights into potential plumes on Europa, observed as a localized magnetic field bend in 1997 data. NASA’s Juno mission, while primarily studying Jupiter itself, has also conducted close flybys of Ganymede, Europa, and Io, gathering valuable data on their environments.
Future missions are specifically designed to investigate these icy worlds in greater detail. NASA’s Europa Clipper, launched in October 2024 and arriving in 2030, will orbit Jupiter and perform nearly 50 close flybys of Europa. Its nine instruments include an ice-penetrating radar to map the ice shell and locate subsurface water, and magnetometers to confirm the ocean’s existence, depth, and salinity. Spectrometers will analyze the composition of Europa’s surface and atmosphere to infer ocean chemistry, while cameras and a thermal imager will search for and characterize plumes, which could be sampled by the spacecraft.
The European Space Agency’s (ESA) Jupiter Icy Moons Explorer (JUICE) mission, launched in April 2023, will perform multiple flybys of Europa and Callisto before entering orbit around Ganymede in 2034. JUICE will carry instruments to characterize the moons’ oceans, ice shells, and potential habitability. These missions will collect data on magnetic fields, surface composition, and gravity, providing crucial insights into the internal structures and processes of these moons. Beyond these orbiters, long-term concepts include landers to analyze surface materials and even submersibles to directly explore the subsurface oceans.