Life on Earth not only exists on the surface, but it also includes a subsurface biosphere extending several kilometres in depth. At such depths, the only reasonable source of energy to sustain life comes from the planet's own internal heat. Indeed, a planet that is located far from its host star, resulting in surface temperatures too cold to support life, can potentially harbour a thriving subsurface biosphere that is sustained solely by the planet's own internal heat.
Figure 1: Artist’s impression of a terrestrial planet.
A study by S. McMahon et al (2013) show that subsurface liquid water maintained by the internal heat of a planet can support an underground biosphere even if the planet is too far from its host star to support life on the planet's surface. The authors introduce a term known as the “subsurface-habitability zone” (SSHZ) to denote the range of distances from a star where a terrestrial planet (i.e. a rocky planet like the Earth) can sustain a subsurface biosphere at any depth below the surface down to a certain maximum habitable depth. This maximum depth depends on numerous factors, but in general, it is the depth where the enormous pressure starts to make the material too compact for life to infiltrate.
Based on the premises that the global average temperature of a terrestrial planet (1) decreases with increasing distance from its host star and (2) increases with depth beneath the planet's surface, the inner (i.e. closer to the host star) and outer (i.e. further from the host star) boundaries of the SSHZ can be determined. The outer edge of the SSHZ is where temperatures are below the freezing point of water at all depths down to the maximum habitable depth. The inner edge of the SSHZ is where the average surface temperature reaches the boiling point of water.
Figure 3: If the maximum habitable depth for an Earth-analogue planet is 5 km, the outer edge of the SSHZ would be at 3.2 AU. For a maximum habitable depth of 10 km, the outer edge of the SSHZ would be at 12.6 AU. At a maximum habitable depth of 15.4 km, the outer edge of the SSHZ tends towards infinity. Credit: S. McMahon et al (2013).
Figure 5: Subsurface habitability for three planetary masses of 0.1, 1.0 and 10 Earth-masses. Other than planet mass, the calculations assume a planet with the Earth’s current bulk density, heat production per unit mass, albedo and emissivity. Credit: S. McMahon et al (2013).
Results from the study show that for a planet with high albedo (high reflectivity), the SSHZ is narrower and closer to the star than for a planet with low albedo (low reflectivity) (Figure 4). Furthermore, planets with larger mass have subsurface biospheres that are thinner, shallower and less sensitive to the heat flux from the host star (Figure 5). This is because a more massive planet is expected to have a steeper geothermal gradient whereby the temperature rises more rapidly with increasing depth as compared to a less massive planet. In fact, a 10 Earth-mass planet can support a ~1.5 km thick subsurface biosphere less than ~6 km below its surface even if the planet is at an arbitrarily large distance from its host star.
The possibilities for subsurface biospheres mean that a planet whose surface is too cold for life can still support a deep biosphere that derives its energy and warmth from the planet's own internal heat. An advantage that life in a subsurface biosphere has is that it is well protected from ionizing stellar and cosmic radiation by the overlying rock layers. Since the SSHZ is vastly greater in extent than the traditional habitable zone, cold planets with subsurface biospheres may turn out to be much more common than planets with surface biospheres. Nevertheless, detecting the biosignature of a subsurface biosphere from remote sensing will be more challenging than for a surface biosphere.
McMahon et al., “Circumstellar habitable zones for deep terrestrial biospheres”, Planetary and Space Science 85 (2013) 312-318