Date and Time
Bath Royal Literary and Scientific Institution, 16-18 Queen Square, Bath BA1 2HN
Our solar system was formed about 4.56 billion years ago by the gravitational collapse of a proto-stellar cloud. The first 500 million years of the solar system were a decisive time for the geological evolution of the Earth into the planet we know today and how it became a habitable place. Understanding the chemical and physical processes by which the Sun and the planets were formed is of central interest to Earth and planetary sciences. These processes ultimately delivered water and the essential organic compounds to Earth from which life originated and are, therefore, key to the environmental and biological evolution of our planet.
Current numerical and chemical models suggest that the Earth’s formative years were tough. It grew by colliding with its neighbouring planetary bodies, ultimately emerging the victor having incorporated the opposition. The energies involved in these violent impacts were sufficient to melt much of the entire growing planet, allowing dense iron metal melts to sink to the centre to form the Earth’s core. The release of immense amounts of potential energy by the sinking of the metal melts, together with the internal heating of the Earth by the decay of short-lived radioactive elements may be responsible for the existence of a super-heated liquid outer core. This turbulently spinning liquid outer core is responsible for maintaining a magnetic field around the Earth shielding us, to this day, from the solar wind, a stream of high energy charged particles emanating from the sun. The infant Earth was constantly shattered by catastrophic collisions with other proto-planets that probably re-melted the whole planet several times and must have left the Earth an inhospitable place for life. The accretionary phase culminated in the collision of the proto-Earth with a giant impactor, most likely the size of Mars, less than 100 million years after the start of the Solar system. This massive impact event ejected vast amounts of material from the proto-Earth and flung the impactor into an orbit around our planet. Numerical simulations suggest that, after cooling, this material finally coalesced to form the Moon.
What followed was a much quieter time in the solar system, marked only by the comparatively low-energy impacts of meteors on the young Earth and Moon. During this time, the decline of giant impact events and the progressive cooling of the Earth’s surface may have allowed the formation of an initial planetary crust. Later tectonic processes destroyed all remnants of both, the crust and the meteoritic impact sites on Earth. However, the final traces of this ‘terminal bombardment’ can still be seen today by the cratering of the lunar surface. Yet, it was this terminal bombardment that may prove to have been crucial for the formation of life on Earth. It has been suggested that this late meteoritic shower delivered most of the ingredients essential for life, such as water, carbon and other volatile compounds. This rather ‘mild’ meteoritic bombardment of Earth - also called ‘late veneer’ - terminated about 3.9 billion years ago, shortly before the emergence of first life. However, the energies released during the waning stages of this bombardment, may have sustained widespread hydrothermal activity within the Earth’s crust and may thus prove to be conducive to life’s emergence and early diversification.
The formation of the core also depleted the silicate portion of the Earth in elements that have a high affinity for metal phases, so-called highly-siderophile elements, like precious metals. According to this model, the Earth’s mantle and crust should be devoid of any highly-siderophile elements, which is at odds with the fact that we can still mine them today. I will explore, to what extent the ‘terminal meteorite bombardment’ replenished the precious metal content of the Earth’s mantle during the ‘late veneer’ epoch more than 3.9 billion years ago.
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