Recent discoveries suggest that moons, rather than planets, may be the most promising places in the solar system to find extraterrestrial life.

Several significant space missions are planned within the next decade to search for signs of life on these celestial bodies.

Unlike neighboring planets, some moons contain vast amounts of liquid water. Jupiter's moon Europa, for example, is believed to have more liquid water than all of Earth's oceans combined. A thick layer of ice, stretching several kilometers deep, shields this water from space radiation and asteroid impacts, providing a stable environment for potential life.

On Saturn's moon Enceladus and Jupiter's Europa, plumes of water vapor suggest the presence of liquid oceans beneath their icy crusts. These oceans are not heated by the Sun but by radioactive decay in their cores or tidal heating caused by their host planets' gravitational forces. Evidence points to liquid oceans on other moons as well, including Jupiter's Ganymede and Callisto. A study published in June estimates Enceladus's ocean to be around one billion years old, possibly even older—enough time for life to evolve.

These alien oceans are thought to be salty, containing sodium chloride, much like Earth's seas. This resemblance boosts optimism about finding life similar to that on Earth. Moreover, the interface between liquid water and the rocky mantle beneath these oceans could drive fascinating chemical reactions, potentially providing the key ingredients for life. NASA's Cassini spacecraft has detected complex organic molecules in Enceladus's plumes, suggesting the presence of hydrothermal vents on its seafloor.

On Earth, deep-sea hydrothermal vents, where magma meets saltwater, produce heat, chemicals, and substrates that foster complex chemical reactions. Many scientists believe these vents played a critical role in the origin of life. Despite the lack of sunlight, these vents teem with life, a scenario that may mirror conditions on moons like Europa and Enceladus.

Around 20 years ago, the BBC documentary Natural History of an Alien speculated about an entire ecosystem based on deep-sea vents on Europa. The proposed food chain starts with bacteria extracting energy via chemosynthesis from vent emissions. These bacteria could form tall sedimentary tubes miles above the seabed, which larger organisms, akin to fish, might pierce to consume the bacteria. Predators resembling sharks could then hunt these creatures, employing streamlined bodies and echolocation to detect prey.

However, this vision of advanced life is far more complex than most scientists expect. Harvard professor Andrew Knoll emphasizes that for 90% of Earth's history, life was exclusively microbial. If extraterrestrial life exists, it is likely to be microorganisms, particularly in environments like Europa or Enceladus, which rely entirely on chemosynthesis for energy and could support only minimal biomass.

Harvard astronomer Dimitar Sasselov, head of the Origins of Life Initiative, agrees that such ecosystems are plausible. While Europa's oceans are cold and energy-poor, they might still support small-scale, complex ecosystems. Sasselov speculates that evolutionary innovations could lead to small, multicellular predators that are more advanced than single-celled organisms.

Saturn's moon Titan presents a completely different possibility. Unlike any other known world, Titan has stable liquid on its surface, but instead of water, its rivers, lakes, and seas are composed of liquid methane and ethane. With surface temperatures around -180°C (-292°F), any water on Titan exists as solid rock and mountains.

If life exists on Titan, it would depend on methane rather than water, representing a form of life entirely alien to Earth’s biochemistry. Sasselov describes this possibility as a "completely independent biochemistry." Research by Cornell University has shown that small molecules of nitrogen, carbon, and hydrogen could form cell membranes under Titan’s conditions. NASA has further confirmed the presence of vinyl cyanide in Titan’s atmosphere, a compound that could theoretically enable the formation of such cells in Titan’s methane-rich oceans.

Detecting alien life will be challenging. Microbial life forms, the most likely candidates, are difficult to identify, while the existence of multicellular organisms remains speculative. NASA’s Dragonfly mission, set to launch in 2026 and arrive on Titan by 2034, aims to explore dozens of promising sites for signs of life. Plans for a robotic submarine to explore Titan's largest sea, Kraken Mare, are also under consideration, though such a mission remains decades away.

Europa and Enceladus are also targets for exploration. NASA’s Europa Clipper mission, launching in 2023, will investigate Europa’s potential habitability through multiple flybys. Meanwhile, a privately funded mission to Enceladus could launch as early as 2025 to search for signs of life in its plumes.

The ultimate goal is to send a submarine capable of drilling through kilometers of ice to explore these hidden oceans directly. Researchers have proposed nuclear-powered tunnel robots to achieve this, sampling the ice and water during descent. However, such missions face immense technical challenges and are still in the conceptual phase.

Even if life is not found in these alien oceans, these moons may hold significance in humanity's distant future. In about 5 billion years, when the Sun expands into a red giant, it could melt the ice on these moons, making them more Earth-like and potentially habitable. For now, these icy worlds offer tantalizing glimpses of what alien life might be and raise profound questions about humanity's place in the universe.

As Earth's habitability diminishes over time, exploring and understanding these potential refuges may become essential. The quest for alien oceans and the life they may harbor underscores the necessity of continued space exploration.