Friday, October 16, 2015

How we discovered that the Earth's inner core is older than previously thought:

How we discovered that the Earth's inner core is older than previously thought

According to recent estimates, the Earth's solid inner core started forming between half a billion and one billion years ago. However, our new measurements of ancient rocks as they cool from magma have indicated that it may actually have started forming more than half a billion years earlier.

While this is still relatively late in the Earth's four-and-a-half billion year history, the implication is that the Earth's deep interior may not have been as hot in the deep past as some have argued. That means the core is transferring heat to the surface more slowly than previously thought, and is less likely to play a big role in shaping the Earth's surface through tectonic movements and volcanoes.

Just after the Earth formed from collisions in a huge cloud of material that also formed the Sun, it was molten. This was because of the heat generated by the formation process and the fact that it constantly collided with other bodies. But after a while, as the bombardment slowed, the outer layer cooled to form a solid crust.

The Earth's inner core is, today, a Pluto-sized ball of solid iron at the centre of our planet surrounded by an outer core of molten iron alloyed to some, as yet unknown, lighter element. Despite the Earth being hottest at its centre (about 6,000°C), liquid iron freezes into a solid because of the very high pressures there. As the Earth continues to cool down, the inner core grows at a rate of about 1mm per year by this freezing process.

Knowing the point in time at which the Earth's centre cooled down sufficiently to first freeze iron gives us a fundamental reference point for the entire thermal history of the planet.

The magnetic field of the Earth is generated by the movement of electrically conducting molten iron in the outer core. This movement is generated by light elements released at the inner core boundary as it grows. Therefore, the time when iron was first frozen also represents a point in time when the outer core received a strong additional source of power.

It is the signature of this boost of the magnetic field – the largest long-term increase in its entire history – that we think we have observed in the magnetic records recovered from igneous rocks formed at this time. Magnetic particles in these rocks "lock-in" the properties of the Earth's magnetic field at the time and place that they cool down from magma.

The signal can then be recovered in the laboratory by measuring how the magnetisation of the rock changes as it progressively heated up in a controlled magnetic field. Hunting for this signature is not a new idea but has only just become viable – a combination of having increased amounts of measurement data available and new approaches to analysing them.

The Earth has maintained a magnetic field for most of its history through a "dynamo" process. This is similar in principle to a wind-up radio or a bicycle-powered light bulb in that mechanical energy is converted to electromagnetic energy. Before the inner core first started to solidify, this "geodynamo" is thought to have been powered by another entirely different and inefficient "thermal convection" process.

Once iron started to freeze out of the liquid at the base of the core, the remainder became less dense, providing an additional source of buoyancy and leading to much more efficient "compositional convection". Our results suggest that this efficiency saving happened earlier in the Earth's history than previously thought, meaning that the magnetic field would have been sustained for longer with less energy overall. Since the energy is mostly thermal, this implies that the core as a whole is likely cooler than it would have been if the inner part formed later.
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