The question of whether life exists elsewhere in the universe has fascinated humanity for centuries. While most early speculation about alien life focused on the Moon and Mars, our understanding of the outer solar system has expanded dramatically in modern times thanks to advances in astronomy and space exploration. Several moons of the gas giant planets, as well as some Kuiper Belt objects beyond Neptune, are now considered possible habitats for extraterrestrial life.
Evidence for Oceans
The discovery of subsurface oceans on some outer solar system moons has been a major breakthrough in assessing their potential for hosting life.
On Jupiter’s moon Europa, observations from the Galileo spacecraft in the 1990s provided strong evidence for a global ocean underneath the moon’s icy crust. More recent studies using the Hubble Space Telescope have revealed water plumes erupting from its surface.
Saturn’s moon Enceladus also has a subsurface ocean evidenced by plumes spewing from its south pole that contain water vapor, ice particles, and organic molecules. Observations by the Cassini spacecraft determined this ocean is likely 10 km deep around the south pole.
Another of Saturn’s moons, Titan, has lakes and seas of liquid hydrocarbons like methane and ethane on its surface. While Titan’s surface temperature of -180°C makes water oceans implausible, the interior may harbor a subsurface ocean of water.
The presence of so much liquid water on these moons boosts their habitability potential and makes them high-priority targets in the search for alien life. Liquid water is essential for all life on Earth, so it is logical to focus on environments beyond Earth that also have access to water.
Potential Energy Sources
In addition to liquid water, life requires an energy source to sustain metabolism. Several moons have mechanisms that could provide the necessary energy:
- Tidal Heating: The gravitational pull from Jupiter and Saturn flexes and heats the interiors of Europa and Enceladus, similar to the tidal heating of Io. This energy could help sustain life on these ocean moons.
- Radiogenic Heating: Radioactive decay of elements in rocky cores can serve as an internal heat source, as is the case on Earth. Models suggest Titan may have enough radiogenic heating to drive biological processes.
- Hydrothermal Vents: On Earth’s ocean floors, microbial communities thrive around hydrothermal vents where energy-rich chemicals emerge from the crust. Similar processes could occur on ocean moons like Europa and Enceladus.
Potential for Chemical Complexity
A third requirement for life is access to the chemical building blocks necessary to assemble biomolecules like proteins, lipids, and nucleic acids. Again, some outer solar system moons show promise:
- Europa’s subsurface ocean likely contains a mixture of water, salts, and organic compounds based on data from the Galileo mission. Organics may originate from meteorite impacts on the surface.
- Enceladus’ plumes spew organic-rich water into space, containing simple organics like methane as well as more complex nitrogen-bearing molecules.
- Titan’s hydrocarbon lakes and atmosphere are chemically complex environments believed to be rich in prebiotic compounds. Energy from meteorite impacts could drive chemical synthesis.
While the specific molecules required for life remain elusive on these worlds, the presence of organic compounds is an encouraging sign. Further study of the chemical composition and processes on these moons could reveal environments capable of supporting life.
Possible Abodes for Life
Given what we know right now, a few specific environments seem like particularly promising places to search for life in the outer solar system:
- Europa’s ocean: The contact between Europa’s ocean and rocky seafloor could create hydrothermal vents that provide energy and nutrients to sustain ecosystems, just as such vents do on Earth.
- Enceladus’ ocean floor: Around the southern polar region, hydrothermal processes could support life in Enceladus’ ocean depths like they do on Earth.
- Titan’s hydrocarbon lakes: The liquid ethane and methane lakes of Titan’s frigid surface may provide a habitat for exotic microbial biochemistries.
- Icy crusts: Microbes on Earth thrive inside ice sheets and glaciers. By analogy, life might inhabit small pockets of liquid water within the thick icy crusts of Europa, Enceladus, and other moons.
- Kuiper Belt objects: Dwarf planets like Pluto, Eris, Makemake and Haumea could have internal oceans due to radiogenic heating. These oceans would likely be sandwiched between icy mantles and rocky cores.
Future Exploration Plans
Upcoming missions will provide more insight into the potential for life in the outer solar system:
- The Europa Clipper mission, set to launch in 2024, will make dozens of close flybys of Europa to map the ice shell and ocean and sample plumes for evidence of life.
- NASA’s Dragonfly mission, launching in 2027, will send a rotorcraft to explore Titan’s complex chemistry and seek signs of prebiotic processes starting.
- Proposed missions to Enceladus would directly sample the plumes and analyze their chemical and biological composition.
Summary
The outer solar system presents several encouraging possibilities for alien life. Moons like Europa, Enceladus, and Titan have liquid water oceans, energy sources, and complex chemistry that make them potentially habitable. Upcoming missions will provide more concrete evidence as to whether these are merely ocean worlds or actual life-bearing worlds. Even if no life is found on this first round of exploration, the prospects for life elsewhere in the galaxy seem greater knowing that worlds with the basic ingredients for life exist right here in our own solar system’s backyard. The search for life only promises to get more exciting as we explore farther out into the solar system and beyond in the coming decades.