Summary in seconds: As part of investigating the potential for life in our solar system, I presented the question of habitability on two primary planets—Venus and Mars—and an asteroid, Ceres, in the previous article (Part Five) of this series. In this article, I will continue investigating the habitability of life in our backyard, this time focusing on seven ocean worlds—the moons of our gas giants: Europa, Ganymede, Callisto, Io, Titan, Enceladus, and Triton—and what to expect in our next article.
Looking for life in our solar system, I listed a number of celestial bodies that have the potential to harbor life either now or earlier in their history. This list includes two primary planets (Venus and Mars) and a dwarf planet (Ceres), which I presented in my previous article (Part Five). In this article, however, I intend to present the seven moons that revolve around three giant gaseous planets: Jupiter (Europa, Ganymede, Callisto, and Io), Saturn (Titan and Enceladus), and Neptune (Triton).
Seven Ocean Worlds: The Moons of Gas Giants
Seven moons orbiting four of our solar system’s gas giants—Jupiter, Saturn, Uranus, and Neptune—offer potential habitable worlds. The following is a list of these gas giants and their moons that may be hosting life either now or in the past:
First: Jupiter
1. Europa
Europa is one of Jupiter’s Galilean moons and is one of the most promising places in the Solar System for finding extraterrestrial life. Beneath its icy crust lies a vast subsurface ocean that might hold twice as much water as all of Earth’s oceans combined, kept liquid by heat from tidal forces caused by Jupiter’s strong gravity. This ocean may contain essential elements like carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur—key building blocks of life. Interaction between the ocean and its rocky seafloor could allow chemical processes similar to those around Earth’s hydrothermal vents, where life thrives without sunlight. Europa also has energy sources, such as tidal heating and possibly hydrothermal activity, while radiation from Jupiter might create oxidants on the surface that could mix into the ocean. Its thick ice shell likely shields the ocean from harmful radiation, creating a more stable environment for life.
In 2024, NASA launched the Europa Clipper mission1 to study the moon’s ice shell, surface composition, and potential water plumes, aiming to assess its habitability. While challenges remain, Europa’s subsurface ocean and potential energy sources make it one of the best places to search for life, with future missions expected to provide more answers.
2. Callisto
Callisto is one of Jupiter’s largest moons and is considered a possible candidate for habitability because of its unique features. Scientists believe it may have a subsurface ocean of liquid water beneath its icy crust, kept warm by heat from radioactive decay. Its distance from Jupiter also means it gets less exposure to the planet’s harmful radiation. However, Callisto faces major challenges for supporting life. Its surface is extremely cold, with an average temperature of about -139°C (-218°F), and its thin atmosphere, mostly made of carbon dioxide, offers little protection from space radiation or micrometeoroids.
3. Ganymede
Ganymede, the largest moon of Jupiter, is an exciting candidate in the search for life beyond Earth. While no life has been found there yet, it has features that make it interesting for scientists. For example, Ganymede has a massive underground ocean with more water than all of Earth’s oceans combined. It also gets energy from tidal forces and chemical reactions. Uniquely, Ganymede is the only moon with its own magnetic field, which could protect it from Jupiter’s strong radiation, creating a safer environment for life. Observations suggest its underground ocean contains salts, making it similar to Earth’s oceans and increasing the chances of chemical reactions that could support life. However, Ganymede faces big challenges: its surface is extremely cold, with temperatures around -160°C (-256°F), and it is constantly hit by radiation from Jupiter, making it unlikely for life to exist on the surface.
4. Io
Io is one of Jupiter’s largest moons and faces many challenges that make it unlikely to support life. Its close distance to Jupiter causes strong tidal forces, leading to intense volcanic activity that covers its surface with sulfur and silicate materials. Io’s thin atmosphere, mostly made of sulfur dioxide, is far too weak to support life as we know it. The moon is also bombarded with intense radiation from Jupiter’s magnetosphere, and without a magnetic field to protect it, the environment is extremely hostile. The surface is covered in molten lava, sulfur deposits, and lacks liquid water. While it is uncertain if there is any water beneath the surface, the intense heat and lack of insulating ice make it unlikely.
Although Io is not hospitable, extremophiles2 on Earth show how life can adapt to harsh conditions, so any potential life on Io would need to survive without water, endure high radiation, and thrive in a sulfur-rich and volcanically active environment.
Second: Saturn
1. Enceladus
In 2005, NASA’s Cassini spacecraft3 flew close to Enceladus, one of Saturn’s moons, and discovered clues about its potential to support life. Cassini detected water vapor, molecular hydrogen, methane, silicates, and complex organic molecules in plumes erupting from the moon’s surface, suggesting there might be hydrothermal vents on the ocean floor, similar to those on Earth that support ecosystems without sunlight. Enceladus is also outside Saturn’s radiation belts, making it less exposed to harmful radiation than other moons like Europa. However, its surface temperature is extremely cold at around -198°C (-324°F), and the exact conditions in its subsurface ocean remain uncertain.
Future missions, like NASA’s proposed Enceladus Orbilander4, aim to study these plumes and the surface to search for signs of life and assess the habitability of its hidden ocean. While Enceladus shows promise as a place to look for life, no definitive evidence has been found yet.
2. Titan
Titan’s atmosphere is similar to early Earth’s but thicker. Although Titan is smaller than Earth with weaker gravity, its atmosphere is mostly nitrogen, and it has a hidden ocean of liquid water beneath its icy surface. The surface is extremely cold, averaging around -179°C (-290°F), but the subsurface ocean, possibly made of water and ammonia, might have conditions that could support life. Titan has complex organic molecules, the building blocks of life, in its atmosphere and on its surface. The Huygens probe5, which landed there in 2005, provided lots of information, and future missions like NASA’s Dragonfly will explore its chemistry and the possibility of life. While Titan lacks surface liquid water for life as we know it, there is a small chance it could support exotic methane-based life forms.
3. Dione
Dione is Saturn’s fourth-largest moon, but it has not been studied as much as Enceladus or Titan. Scientists believe there might be an ocean of liquid water beneath its icy surface, about 100 kilometers deep, based on gravitational data from the Cassini spacecraft. This ocean, if it exists, could support microbial life, especially if hydrothermal vents are present on its ocean floor, similar to those hypothesized for Enceladus6.
However, Dione has many challenges for habitability. Its surface is extremely cold, averaging -186°C (-302°F), and it lacks a thick atmosphere to protect against radiation or allow liquid water on the surface. Unlike Enceladus, Dione does not have active plumes or surface activity that could bring subsurface material up for study. Even though Dione is not a top candidate for finding life, its hidden ocean makes it an interesting target for future missions.
Third: Neptune
- Triton
Triton7, Neptune’s largest moon, is unique because it orbits in the opposite direction of Neptune’s rotation, called a retrograde orbit8. Scientists think Triton was once a dwarf planet, like Pluto, and was captured from the Kuiper Belt. Triton is geologically active, with a rocky and metallic core surrounded by water. Heat from radioactive decay and tidal forces suggests there could be a layer of liquid water or a subsurface ocean, which might make it possible for life to exist.
Triton has a thin atmosphere mostly made of nitrogen, with traces of methane, and its surface shows signs of cryovolcanism, where icy material from inside the moon erupts onto the surface. Despite this, Triton is extremely cold—about -235°C (-391°F)—and its thin atmosphere makes it hard for life to survive. Energy sources like hydrothermal vents or chemical reactions, which are needed for life, are uncertain but important for further study.
In my next article (Part Seven) of this series, I will present the potential for life on exoplanets, or planets outside the Solar System.
References and Sources
1. NASA, “Europa Clipper Mission.”
https://europa.nasa.gov/mission/about
2. Interdisciplinary Center for Biotechnology Research. “Extremophiles and Extreme Environment.” National Library of Medicine, 7 August, 2013. https://pmc.ncbi.nlm.nih.gov/articles/PMC4187170/
3. Barnett, Amanda. “Cassini and Enceladus.” National Aeronautics and Space Administration, 03, November, 2024. https://science.nasa.gov/mission/cassini/science/enceladus/
4. Spencer, John. “Enceladus Orbiter.” Solar System Explorer, 09 January, 2025. https://solarsystem.nasa.gov/studies/222/enceladus-orbiter/
5. Barnett, Amanda. “NASA, Cassini, Huygens Probe.” Science NASA.gov, 03 November, 2024.
6. Beuthe, Mikael; Rivolini, Attilio; Trinh, Anthony. “Enceladus’s and Dione’s Floating Ice Shells Supported by Minimum Stress Isostasy.” Advanced Earth and Space Sciences, 28 September, 2016. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL070650
7. Creative Commons Attribution. “Triton-moon.” Wikipedia, 10 February, 2025. https://en.wikipedia.org/wiki/Triton_(moon)
8. Retrograde Orbit: an orbit in the direction opposite to its planet’s rotation.