Astronomy
The Biological Potential of Subsurface Oceans on Icy Moons
Quick fact
Jupiter's moon Europa has a subsurface ocean with more liquid water than all of Earth's oceans combined, and it is kept warm by the gravitational pull of its planet—a process called tidal heating.
Why this is interesting
Imagine an ocean larger than all of Earth's oceans combined, locked beneath a shell of ice and cut off from sunlight. Could life be thriving there, unseen and powered by an energy source we hardly think about?
Read the full explanation
Understanding The Biological Potential of Subsurface Oceans on Icy Moons
Think of an icy moon as a frozen shell, like a giant snowball. But deep inside, beneath that shell, lies a hidden ocean of liquid water. How can water be liquid so far from the Sun? The answer lies in the moon's orbit. As the moon travels around its planet, the planet's gravity stretches and squeezes the moon, much like kneading a ball of dough. This constant flexing generates friction and heat, a process called tidal heating. This heat melts the ice from below, creating a vast ocean. On Europa, this ocean is estimated to be 60-150 kilometers deep, containing more water than all of Earth's oceans combined. On Enceladus, a moon of Saturn, geysers of water vapor and ice particles erupt through cracks in the ice, proving that the ocean is still active. These subsurface oceans are not just wet—they are in contact with a rocky seafloor, which is crucial because it provides minerals and chemical nutrients.
A deeper explanation
The biological potential of these oceans hinges on two essential ingredients for life as we know it: liquid water and a source of energy. While sunlight cannot penetrate the thick ice, another energy source exists: chemical energy. Hydrothermal vents on the seafloor of these oceans, akin to Earth's deep-sea vents, could release hydrogen, methane, and other chemicals. Microorganisms on Earth thrive in such environments, using these chemicals for energy in a process called chemosynthesis. On Enceladus, the Cassini spacecraft detected molecular hydrogen in the water plumes, which could be produced by reactions between water and rock—a potential food source for microbes. This is analogous to Earth's 'Lost City' hydrothermal field, where serpentinization reactions create hydrogen. These observations suggest that subsurface oceans meet the basic requirements for life: liquid water, energy, and the building blocks of life (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur). However, the actual presence of life remains unproven. These environments also pose challenges, such as high pressure and potential acidity. Yet, they expand the definition of a habitable zone beyond the traditional 'Goldilocks' region around a star, showing that planetary bodies can create their own habitable niches. Understanding these mechanisms not only guides the search for life beyond Earth but also informs the design of missions like the Europa Clipper, which will investigate the moon's ocean and its potential for habitability.