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.
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Figure 1: Artist’s impression of a terrestrial planet.
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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.
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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).
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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.
Reference:
McMahon et al., “Circumstellar habitable zones for deep
terrestrial biospheres”, Planetary and Space Science 85 (2013) 312-318