On the possibility of galactic cosmic ray-induced radiolysis-powered life in subsurface environments in the Universe

Abstract

Photosynthesis is a mechanism developed by terrestrial life to utilize the energy from photons of solar origin for biological use. Subsurface regions are isolated from the photosphere, and consequently are incapable of utilizing this energy. This opens up the opportunity for life to evolve alternative mechanisms for harvesting available energy. Bacterium Candidatus Desulforudis audaxviator, found 2.8 km deep in a South African mine, harvests energy from radiolysis, induced by particles emitted from radioactive U, Th and K present in surrounding rock. Another radiation source in the subsurface environments is secondary particles generated by galactic cosmic rays (GCRs). Using Monte Carlo simulations, it is shown that it is a steady source of energy comparable to that produced by radioactive substances, and the possibility of a slow metabolizing life flourishing on it cannot be ruled out. Two mechanisms are proposed through which GCR-induced secondary particles can be utilized for biological use in subsurface environments: (i) GCRs injecting energy in the environment through particle-induced radiolysis and (ii) organic synthesis from GCR secondaries interacting with the medium. Laboratory experiments to test these hypotheses are also proposed. Implications of these mechanisms on finding life in the Solar System and elsewhere in the Universe are discussed.

Keywords: astrobiology; radiolysis; subsurface life.

Figure 1.

(a) A colony of Ca. D. audaxviator, discovered in a 2.8 km deep gold mine near Johannesburg, South Africa. https://upload.wikimedia.org/wikipedia/commons/1/1e/Desulforudis_audaxviator.jpg (public domain), via Wikimedia Commons. (b) Transmission electron micrograph of D. radiodurans acquired in the laboratory of Michael Daly, Uniformed Services University, Bethesda, MD, USA. http://www.usuhs.mil/pat/deinococcus/index_20.htm (public domain), via Wikimedia Commons. (Online version in colour.)

Figure 2.

Simulation results of the subsurface energy deposition rate in different cases. Vertical axis is the energy deposition rate in units of eV g⁻¹ s⁻¹ and the horizontal axis is the depth in metres. Long dashes represent energy deposition in ice of a planet with no atmosphere (Europa/Enceladus), short dashes represent deposition for a planet with no atmosphere in rock with density 2.65 g cm⁻³ (Pluto), dots represent deposition in ice for Mars and solid line is for deposition in rock on Mars.

Figure 3.

Subsurface energy deposition rate as a function of depth in rock of density 2.65 g cm⁻³ (solid) and water/ice (dots). Vertical axis has energy deposition rate in eV g⁻¹ s⁻¹ and horizontal axis has depth in kilometres.