Sterilization of Martian surface by cosmic radiation

Ionizing Radiation and Life

“…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. This natural radioactivity of martian regolith is believed to be much lower than that of terrestrial rocks (Mileikowsky et al, 2000) but would still sterilize deep dormant bacteria in 40 million years (Pavlov et al, 2002;Dartnell et al, 2007a).…”

“…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).…”

“…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.…”

“…These include the destruction rate of amino acids (Kminek and Bada, 2006), racemization of an amino acid enantiomeric excess (Bonner, 2000, and references therein), alteration of biogenic organics (Court et al, 2006), nuclear reactions that lead to the loss of 13 C/ 12 C isotopic bias (Pavlov et al, 2002), and biopolymer fragmentation (Pavlov et al, 2002). The general issue is that after millennia of cosmic irradiation it may be difficult to distinguish between breakdown debris of unambiguous biomolecules, a valid signature of extinct life, and simple abiotic organic species created in situ by prebiotic chemistry or exogenously delivered by meteoritic or cometary infall.…”