Subsurface oceans on moon-like satellites (e.g., Europa, Enceladus) and the search for biosignatures
A new approach to habitability
For many decades, planetary scientists searched for life-supporting conditions mainly on Earth-like solid surfaces, assuming this occurs in the so-called "habitable zone" where liquid water can exist. However, recent discoveries show that icy moons may have internal oceans sustained by tidal heat sources or radioactive materials, where liquid water lies beneath thick ice layers—out of reach of sunlight. This expands our understanding of where life might thrive: from those near the Sun (Earth) to distant, cold, but energetically and stably suitable environments around giant planets.
Of all examples, Europa (Jupiter's moon) and Enceladus (Saturn's moon) stand out particularly: both have strong evidence of salty sub-ice oceans, possible chemical or hydrothermal energy sources, and potential nutritional resources. Studying these, as well as Titan and Ganymede, shows that habitability can exist in various forms and not necessarily only in traditionally understood surface layers. Below we review how such environments were discovered, what conditions might support life, and how future missions plan to search for biosignatures.
2. Europa: an ocean beneath the ice surface
2.1 Geological clues from "Voyager" and "Galileo"
Europa, slightly smaller than Earth's moon, has a bright surface covered with water ice, marked by dark linear features (cracks, ridges, chaotic terrains). The first hints were found in "Voyager" photos (1979), more detailed "Galileo" data (1990s) showed a young, geologically active surface with few craters. This suggests that internal heat or tidal forces continually renew the crust's surface, and beneath the icy layer there may be an ocean maintaining both smooth and "chaotic" ice.
2.2 Tidal heat and the subsurface ocean
Europa moves in a Laplace resonance with Io and Ganymede, so tidal effects flex Europa on each orbit. This friction generates heat, preventing the ocean from freezing. Models estimate:
- Ice shell thickness: from a few to ~20 km, most often cited as ~10–15 km.
- Liquid water depth: 60–150 km, so Europa could have more water than all Earth's oceans combined.
- Salinity: the ocean is likely salty, containing chlorides (NaCl) or magnesium sulfates, as indicated by spectral analysis and geochemical calculations.
Tidal heat prevents the ocean from freezing, while the icy shell insulates and helps maintain a liquid layer below.
2.3 Possibilities for life existence
For life as we understand it, the most important are liquid water, a source of energy, and essential chemical elements. On Europa:
- Energy: tidal heat and possibly hydrothermal vents on the seafloor if the rocky mantle is active.
- Chemistry: oxidants formed by radiation in surface ice can enter the ocean through cracks and support redox reactions. There may also be salts and organic compounds.
- Biosignatures: their possible search includes looking for organic molecules in ejected surface materials or even chemical traces in the ocean (e.g., imbalances indicating biological reactions).
2.4 Missions and future research
NASA's mission "Europa Clipper" (planned for launch in mid-2020s) will perform several flybys, study the ice shell thickness, chemical composition, and search for possible geysers or surface composition anomalies. A proposed lander could collect material from the surface. If cracking ice fissures or geysers bring ocean material to the surface, such analysis could reveal traces of microbial life forms or complex organic compounds.
3. Enceladus: the geyser moon orbiting Saturn
3.1 "Cassini" discoveries
Enceladus, a small (~500 km diameter) moon of Saturn, became an unexpected surprise when the "Cassini" probe (since 2005) detected water vapor, ice particles, and organic geysers rising from the south pole (the so-called "tiger stripes"). This indicates that beneath the thin ice layer in this area there is liquid water.
3.2 Ocean characteristics
"Cassini" mass spectrometer data revealed:
- Salty water in geyser particles, with NaCl and other salts.
- Organic compounds, including complex hydrocarbons, enhancing the possibility of early chemical evolution.
- Thermal anomalies: Tidal heating concentrated at the south pole supports at least a regional subsurface ocean.
Data indicate Enceladus may have a global ocean covered by 5–35 km of ice, though thickness may vary in different locations. There are hints that water interacts with a rocky core, possibly creating hydrothermal energy sources.
3.3 Habitability potential
Enceladus exhibits high potential for habitability:
- Energy: tidal heating plus possible hydrothermal sources.
- Water: confirmed salty ocean.
- Chemistry: presence of organic compounds in geysers, various salts.
- Accessibility: active geysers eject water into space, so probes can collect samples directly without needing to drill through ice.
Proposed missions could include an orbiter or lander designed to analyze geyser particles in detail—searching for complex organic compounds or isotopes that could indicate biochemical processes.
4. Other icy moons and bodies with possible subsurface oceans
4.1 Ganymede
Ganymede, Jupiter's largest moon, may have a layered internal structure with a possible watery layer. "Galileo" magnetic field data indicate a conductive (likely salty water) layer beneath the surface. That ocean is thought to be trapped between several ice layers. Although Ganymede is farther from Jupiter, tidal heating there is less, but radioactive and residual heat sources may maintain a partial liquid layer.
4.2 Titan
Saturn's largest moon Titan has a dense nitrogen atmosphere, methane/ethane lakes on the surface, and possibly a subsurface water/ammonia ocean. "Cassini" data show gravitational anomalies consistent with a liquid layer deep inside. Although surface liquids are mostly hydrocarbons, Titan's internal ocean (if confirmed) would likely be water, which could be another habitat for life.
4.3 Triton, Pluto, and others
Triton (Neptune's moon, likely "captured" from the Kuiper Belt) may have maintained a subsurface ocean beneath the ice due to tidal heating caused by capture. Pluto (studied by "New Horizons") may also have a partially liquid interior. Many trans-Neptunian objects (TNOs) may have transient or frozen oceans, although this is difficult to confirm directly. Thus, water may not only lie near Mars' orbit: watery layers and potential life incubators may exist in more distant regions.
5. The Search for Biosignatures
5.1 Examples of Biosignatures
Possible signs of life in icy oceans may include:
- Chemical disequilibrium: For example, concentrations of incompatible oxidants and reductants that are difficult to explain by non-biological processes.
- Complex organic compounds: Amino acids, lipids, or polymeric compounds ejected in geysers or surface ice.
- Isotopic ratios: Carbon or sulfur isotope compositions that deviate from abiotic fractionation patterns.
Since these oceans lie beneath several or even dozens of kilometers of ice, obtaining direct samples is difficult. However, Enceladus geysers or possibly Europa's eruptions allow the ocean's contents to be studied directly in space. Future instruments could detect even trace amounts of organics, cellular structures, or isotopic signatures.
5.2 Direct Exploration Missions and Drilling Concepts
Planned projects such as the "Europa Lander" or "Enceladus Lander" propose drilling at least a few centimeters or meters into fresh ice or collecting material ejected from geysers with advanced instruments (e.g., gas chromatograph-mass spectrometry equipment, microscope-level imaging). Despite technological challenges (risk of contamination, radiation environment, limited power source), such missions could decisively confirm or refute the existence of microbial life.
6. The Overall Role of Icy Ocean Worlds
6.1 Expansion of the Concept of the "Habitable Zone"
Typically, the habitable zone means the region around a star where liquid water can form on the surface of rocky planets. However, with the discovery of internal oceans sustained by tidal or radioactive heat, we see that habitability does not necessarily depend directly on stellar heat. Therefore, the moons of giant planets—even far from the "classical habitable zone"—may have conditions vital for life. Thus, the habitability of moons orbiting in the outer regions of exoplanetary systems is also a real possibility.
6.2 Astrobiology and the Origin of Life
Studies of these ocean worlds illuminate alternative evolutionary pathways. If life can arise or persist beneath ice, without sunlight, then its distribution in the Universe could be much broader. In Earth's ocean depths near hydrothermal vents, there is often the possibility that the first living organisms could have formed here; analogous conditions on the seafloor of Europa or Enceladus could create chemical gradients for life.
"6.3 The significance of future research"
If obvious biosignatures were found on an icy satellite, it would be a major scientific breakthrough, indicating a "second genesis of life" in our Solar System. This would change our perception of the commonality of life in space and encourage more targeted exomoon searches in distant star systems. Missions like NASA's "Europa Clipper", proposed Enceladus orbiters, or advanced drilling technologies are essential steps for this astrobiology breakthrough.
7. Conclusion
"Subsurface oceans" in icy satellites, e.g., Europa and Enceladus, are among the most promising habitability sites beyond Earth. Tidal heating, geological processes, and possible hydrothermal systems indicate that even far from the Sun's warmth, these hidden oceans could harbor microbial ecosystems. A few other bodies – Ganymede, Titan, possibly Triton or Pluto – may also have similar layers, each with unique chemistry and geology.
"The search for biosignatures" in these locations is based on the study of ejecta (material erupting) or in the future – the collection of deep samples. Any discovery of life (or at least an advanced chemical system) here would cause a scientific revolution, revealing a "second" origin of life in the same Solar System. This would expand the understanding of how widely life may exist in the Universe and what its conditions might be. Continuing research, the concept that "habitability" is only possible in the traditional surface context in the nearest star zone, is constantly expanding – confirming that the Universe may hide habitats of life in the most unexpected and remote corners.
Links and further reading
- Kivelson, M. G., et al. (2000). "Galileo magnetometer measurements: A stronger case for a subsurface ocean at Europa." Science, 289, 1340–1343.
- Porco, C. C., et al. (2006). "Cassini observes the active south pole of Enceladus." Science, 311, 1393–1401.
- Spohn, T., & Schubert, G. (2003). "Oceans in the icy Galilean satellites of Jupiter?" Icarus, 161, 456–467.
- Parkinson, C. D., et al. (2007). "Enceladus: Cassini observations and implications for the search for life." Astrobiology, 7, 252–274.
- Hand, K. P., & Chyba, C. F. (2007). "Empirical constraints on the salinity of the Europan ocean and implications for a thin ice shell." Icarus, 189, 424–438.