Accretionary Wedge #9: Deep Impact

The topic for the latest accretionary wedge hosted at Harmonic Tremors is a significant geological event.

Source: Robin M. Canup, “Simulation of a Late Lunar-Forming Impact,” Icarus, vol. 168 (2004)

Julian at Harmonic Tremors has left the term ‘significant’ open to interpretation so here is mine. I think that the most significant event in Earth history was the impact of a Mars sized body about 4.53 Ga on planet Earth which lead to the formation of the Moon. Our Moon is much larger than moons of other terrestrial planets and has had a very significant affect.

This was significant in so many ways. To start on a ‘small scale’ the Moon’s gravitational attraction generates tides and has effects on ocean currents, global climate and life in the tidal zone.

The Moon has had a stabilising effect on the Earth’s rotation axis. Earth’s rotation has enough of a wobble to affect climate (via Milankovitch cycles) but Mars has had much more of a wobble resulting in large variation in ice caps across the surface. It can be argued that Earth’s (relatively) stable climate has promoted evolution of higher life forms.

However, I think that there are other important effects as a consequence of the Earth absorbing the impactor’s core, giving the Earth a larger than normal core. First there is the generation of a large magnetic field compared to other terrestrial plants. This has protected the Earth’s surface from harmful radiation which could have been seriously damaging to life on Earth. Second is the potential consequences for plate tectonics. There have been recent articles suggesting that if global warming were to increase, plate tectonics would cease a the thermal gradient driving convection would decrease. What is being ignored here is the contribution from below. There is a significant contribution of heat from below as the Earth cools overall and the lower part of the liquid outer core ‘freezes’ to become solid inner core. Without the Earth’s enlarged core we could have ended up like Venus. No convection in the liquid core, no magnetic field, limited mantle convection and possibly no plate tectonics either.

Without either a strong magnetic field or plate tectonics it would be unlikely that life (at least as we know it) would exist on Earth. I’d call that significant.