A Different Habitable Zone for Subsurface Biospheres

Circumstellar habitable zones for deep terrestrial biospheres

Authors:

1. Sean McMahon (a)
2. Jack O'Malley-James (b)
3. John Parnell (a)

Affiliations:

a. School of Geosciences, University of Aberdeen, Meston Building, King's College, Aberdeen, AB24 3UE

b. School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS

Abstract:

The habitable zone (HZ) is conventionally the thin shell of space around a star within which liquid water is thermally stable on the surface of an Earth-like planet (Kasting et al., 1993). However, life on Earth is not restricted to the surface and includes a “deep biosphere” reaching several km in depth. Similarly, subsurface liquid water maintained by internal planetary heat could potentially support life well outside conventional HZs. We introduce a new term,subsurface-habitability zone (SSHZ) to denote the range of distances from a star within which rocky planets are habitable at any depth below their surfaces up to a stipulated maximum, and show how SSHZs can be estimated from a model relating temperature, depth and orbital distance. We present results for Earth-like, Mars-like and selected extrasolar terrestrial planets, and conclude that SSHZs are several times wider and include many more planets than conventional surface-based habitable zones.