On the Role of 40K in the Origin of Terrestrial Life

Abstract

The abundance and biological role of potassium suggest that its unstable nuclide was present in all stages of terrestrial biogenesis. With its enhanced isotopic ratio in the Archean eon, 40K may have contributed to the special, perhaps unique, biogenetic conditions that were present in the primitive Earth. Compared to the U and Th radionuclides, 40K has a less disruptive radiochemical impact, which may drive a moderate, but persistent evolution of the structural and functional properties of proto-biological molecules. In the main β-decay route of 40K, the radiation dose generated by an Archean solution with potassium ions can be larger than the present background radiation on Earth by one to two orders of magnitude. Estimates of the rates of organic molecules indirectly affected by β decays are provided for two schematic models of the propagation of secondary events in the solvent of prebiotic solutions. The left-handed β- particles emitted by 40K are the best candidates to trigger an enantiomeric excess of L-type amino acids via weak nuclear forces in the primitive Earth. The concentration-dependent radiation dose of 40K fits well in dry-wet scenarios of life's origins and should be considered in realistic simulations of prebiotic chemical pathways.

Keywords: molecular chirality; origins of life; potassium; radiation chemistry.

Figure 1

Dose of ${\beta }^{-}$ radiation calculated with Equation (3) for aqueous solutions of potassium ions with molar concentration, $C_K$, indicated in the legend. The evolution of the ${}^{40}$K/K isotopic ratio from the time of Earth’s formation to the present time is calculated with Equation (1). The vertical band indicates the interval during which life emerged on Earth.

Figure 2

Effects of ${\beta }^{-}$ decays of ${}^{40}$K on prebiotic molecules dissolved in an aqueous solution with potassium ions ($C_K=1$ M) at $t_{\mathrm{BP}}=4$ Ga. Vertical axis: number of prebiotic molecules affected by secondary events per unit time within a sphere of radius 1 $\mathsf{\mu }$m (representative size of a protocell). Horizontal axis: effective radius of influence of secondary events, $r_s$. Each curve was obtained at a constant molar concentration of prebiotic molecules, $C_m$, indicated in the legend. Solid lines: string of spurs model of the propagation of secondary events (Section 3.2.1). Dashed lines: model of the propagation of secondary events via hydrogen bonding (Section 3.2.2).