Seminar – Prof. Jon Wade

Speaker: Prof. Jon Wade, Department of Earth Sciences, University of Oxford

Title: The irony of planetary habitability

Abstract: Planetary habitability – how conducive a planet is to both support and initiate life – has often been interpreted as whether a planet can support liquid water on its surface, and hence its distance from the parent star.  However, we show that the abundance of iron in the planet’s rocky portion, and its bioavailability through geological time, is crucial to planetary habitability. Synthesised at the end of a star’s life, most of our planet’s iron resides in its metallic core, which in turn generates the magnetic field which protects surface life from harmful solar radiation. Importantly, iron is the only major elemental component of rocks that undergoes valence changes on planetary surfaces, a fact extensively exploited by life.  The quantity of iron available to life therefore places a significant constraint on potential habitability. This is determined in the very earliest phases of planetary building, by the conditions under which a planet’s metallic core segregates from its rocky silicate host.  The subsequent rise of atmospheric oxygen over 2 billion years ago –  the first great ‘pollution’ event – resulted in significant reduction in oceanic iron, placing severe stress on nascent life.  However, this catastrophic loss of iron from the ancient seas provided the impetus for life to develop a variety of coping strategies, including multicellularity and more aggressive mechanisms of iron acquisition.  We suggest that the evolution of complex life is intrinsically linked to changes in iron bioavailability and, given the cosmochemical abundance of iron, any complex life in the Universe is likely to be ‘red in tooth and claw’.

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