Mineral Radioactivity Promotes Organic Complexity on Rocky Planets

The Role of Irradiation: Irradiation is widely invoked as a mechanism by which simple carbonbearing molecules may be polymerised or otherwise reacted to form progressively more complex molecules that could ultimately synthesize life [1]. In our solar system, cosmic irradiation has been invoked as a source of complex molecules on the Galilean satellites, and on Titan, Pluto, Uranus and Neptune. These are icy bodies, where irradiation of a water/carbon dioxide ice mixture may be enough to create a chemistry that could support life. Cosmic irradiation has also been suggested as the mechanism for organic synthesis on early Earth, in which irradiated atmospheric components could have yielded amino acids and other prebiotic organic molecules [2]. Rocky planets present an alternative energy source. Like cosmic irradiation, energetic particles from heavy radioelements (uranium, thorium) can cause polymerisation of organic molecules and enhance prebiotic synthesis and the evolution of DNA. Where uranium/thorium are concentrated in minerals, they can be templates for complexation and precipitation of organic compounds, evidenced by carbonaceous coatings on grains of uraninite, Th-rich monazite, U-rich zircon, U-rich apatite, and other minerals (see below). Recent evidence for zircon crystals, and water at the planetary surface at a time prior to the record of life (Fig. 1) [3,4], invite a reassessment of the potential of mineral radioactivity to promote organic complexation on the early Earth. Radioactive Mineral Sands: Radioelements were readily concentrated at the Earths surface during the Hadean and Archean. The oldest zircons indicate the production of granitic magma at 4.4Ga [3,4] (Fig. 1). Isotope data from these zircons indicate that there were surface waters at this time [3,4]. Zircons, monazites and other heavy minerals were liberated into surface sediments by erosion of plutonic hinterlands. Their high density means that the minerals are particularly susceptible to hydrodynamic sorting by moving water (rivers, tidal flow, wave action). The sorting of heavy minerals was enhanced by tidal motion induced by the presence of a moon, and they form concentrations in sediments throughout geological history. Hence, for example, they form accumulations at the margins of oceans, shallow seas and lakes. At the present planetary surface, radioactive monazite sands are concentrated at the margins of each of the large oceans, e.g. in Brazil, Vietnam, Australia, and India. They represent the most radioactive sedimentary deposits at the Earth’s surface today, with effective dose rates up to 270 mSv/yr, about 650 times the world average value [5], and are a globally significant source of high-dose irradiation. The strand-lines of heavy mineral sands extend for up to hundreds of kilometres: Indian beaches alone contain 8 million tonnes of monazite [6]. Early in Earths history before the atmosphere became significantly oxygenated, uraninite-rich sands were also stable at the surface, emitting exceptionally high irradiation. The closer proximity of the moon during early Earth history would have made tidal sorting of grains even more effective than today.