Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology

Abstract

[1] The damaging effect of ionising radiation on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats. Here we model the propagation of solar energetic protons and galactic cosmic ray particles through the Martian atmosphere and three different surface scenarios: dry regolith, water ice, and regolith with layered permafrost. Particle energy spectra and absorbed radiation dose are determined for the surface and at regular depths underground, allowing the calculation of microbial survival times. Bacteria or spores held dormant by freezing conditions cannot metabolise and become inactivated by accumulating radiation damage. We find that at 2 m depth, the reach of the ExoMars drill, a population of radioresistant cells would need to have reanimated within the last 450,000 years to still be viable. Recovery of viable cells cryopreserved within the putative Cerberus pack-ice requires a drill depth of at least 7.5 m.

Ionizing Radiation and Life

“…While there are uncertainties in both the physics of high-energy particle transport and the biological response to irradiation, these studies broadly agree and have found that the top 20 cm of the martian surface is sterilized of even the most radiation-resistant microorganisms (based on terrestrial models such as D. radiodurans) within a million years or so, with greater survival found at increasing depths due to shielding, especially in ice deposits (Dartnell et al, 2007a). In material with a density of around 1 g/cm 3 , the more energetic GCR cascades begin to dominate over the average SEP flux within around 10 cm depth (Dartnell et al, 2007a), and radiation from the GCR cascades peaks at (the Pfotzer maximum) 25-50 cm deep and at a third of that depth within solid rock or regolith (Mileikowsky et al, 2000;Pavlov et al, 2002;Dartnell et al, 2007b). Beneath the penetration of GCR cascades, at 3-4 m depth, depending on surface properties, the remaining source of ionizing radiation is the decay of radionuclides in the surrounding regolith.…”

“…More recently, however, evidence has been mounting that other factors such as high intracellular manganese concentrations and protection of proteins from oxidation (Daly et al, 2004;Ghosal et al, 2005;Daly, 2009) are key determinants of D. radiodurans' radiation survival. Due to this extreme radioresistance, as well as resilience to desiccation, hydrogen peroxide, and UV radiation, D. radiodurans is commonly used as a model strain for microorganisms able to survive on the martian surface (e.g., Richmond et al, 1999;Horneck, 2000;Pavlov et al, 2002;Dartnell et al, 2007a;de La Vega et al, 2007). More recently, strains of a hyperthermophilic archaeon Thermococcus, isolated from a submarine hydrothermal vent environment, have been found with a radiation resistance approaching that of D. radiodurans ( Jolivet et al, 2003( Jolivet et al, , 2004.…”

“…More specifically in the interests of astrobiology, several modeling studies have involved calculation of the radiation dose profile generated by CR through the martian regolith to predict the likely survival times of spore-forming or radiation-resistant bacterial strains such as D. radiodurans in the subsurface (Mileikowsky et al, 2000;Pavlov et al, 2002;Kminek et al, 2003;Dartnell et al, 2007aDartnell et al, , 2007bDartnell et al, , 2010. While there are uncertainties in both the physics of high-energy particle transport and the biological response to irradiation, these studies broadly agree and have found that the top 20 cm of the martian surface is sterilized of even the most radiation-resistant microorganisms (based on terrestrial models such as D. radiodurans) within a million years or so, with greater survival found at increasing depths due to shielding, especially in ice deposits (Dartnell et al, 2007a). In material with a density of around 1 g/cm 3 , the more energetic GCR cascades begin to dominate over the average SEP flux within around 10 cm depth (Dartnell et al, 2007a), and radiation from the GCR cascades peaks at (the Pfotzer maximum) 25-50 cm deep and at a third of that depth within solid rock or regolith (Mileikowsky et al, 2000;Pavlov et al, 2002;Dartnell et al, 2007b).…”

What characterizes planetary space weather?

“…The Exo-Mars rover will, therefore, drill up to 2 m under the surface to search for signs of past life. Recent simulations of the subsurface Mars surface radiation environment (Dartnell et al 2007) indicate that microbial survival times increase significantly with depth. Similar studies have started to model the environment in the clouds of Venus (Nordheim et al 2014).…”

Surface mineral crusts: a potential strategy for sampling for evidence of life on Mars

“…Although other forms of irradiation (solar energetic particles, galactic cosmic radiation, mineral radiation) penetrate rocks further, living organisms may have developed genetic damage repair systems that can cope with it, however in the shallow subsurface (<2 m depth) the biomolecules in fossil organic matter in crusts would experience progressive degradation (Kminek & Bada 2006;Dartnell et al 2007). Fossil organic matter is best sought where shielded from long-term irradiation (recently excavated channels or craters, or sampled by coring).…”