A reassessment of ancient rocks has led scientists to estimate that Earth’s inner core started to form earlier than was previously thought, around 1.3 billion years ago. As it started to freeze, the core began generating a bigger magnetic field, which continues to today.
Earth’s active core contrasts sharply with that of our neighbor Mars, whose strong early magnetic field died around four billion years ago. Our planet’s magnetic field is generated deep in the planet by the turbulent motion of the electrically conducting molten iron of the outer core. It aligns compass needles north-south, but also protects Earth from the solar storms that the Sun throws out relentlessly. At the poles, these storms produce Aurora northern or southern lights but they can also work destructively to strip away ozone in the upper atmosphere, an important shield against the Sun’s harmful ultraviolet radiation. It has been suggested that life on Earth has thrived because the magnetic field has allowed this protective atmosphere to persist over hundreds of millions of years.
The turbulent motion of iron in the liquid outer core is partly generated by excess heat in the centre of the Earth being transferred upwards and outwards by convection, and partly by the slow solidification of the solid inner core at the very heart of the planet. As the iron at the centre of the Earth freezes, forming the inner core, it expels light and buoyant impurities into the liquid outer core. They rise and boost convection in the outer core, amplifying the magnetic field. An increase in magnetic field is a signature that scientists have been searching for in the rocks of the deep geological past, a recording of the onset of core solidification. The lead author of the paper, Dr Andy Biggin of the University of Liverpool, UK, commented: “The timing of the first appearance of solid iron or ‘nucleation’ of the inner core is highly controversial, but is crucial for determining the properties and history of the Earth’s interior.”
The question of when molten iron in the heart of the planet started to freeze and form the inner core has, recently, been the topic of vigorous scientific discussion. Estimates and models of inner core formation rely on understanding the properties of iron under the extreme conditions at the centre of the Earth pressures of more than three million atmospheres, and temperatures of around 6,000C. Dr Biggin added: “The theoretical model which best fits our data indicates that the core is losing heat more slowly than at any point in the last 4.5 billion years and that this flow of energy should keep the Earth’s magnetic field going for another billion years or more.”
Dr Richard Harrison of the University of Cambridge, who was not involved in the study, told the BBC: “Studying the magnetism of ancient rocks is a huge scientific challenge, because old rocks can lose their magnetic memory, or the magnetic signals they carry can become overwritten and corrupted (just like the files on hard drive).”However, it is one of the best ways to look for concrete evidence of when the core started to solidify.”Although data are scarce, this study applied strict quality controls to decide which data were the most reliable and then used statistics to demonstrate that a boost to Earth’s the magnetic field occurred 1,300 million years ago. If this turns out to be the elusive signature of inner core growth, then we may have to revise our ideas about the core yet again!”
The inner core of our planet is notoriously difficult to investigate, being that it is over 6,300 kilometers (roughly 4,000 miles) away from us. The deepest humanity has ever drilled into the planet is a frankly minuscule 12 kilometers (7.5 miles). Seismologists long ago worked out that the physical properties of the core could be determined using the sound waves produced during earthquakes, but its age is less certain, with estimates ranging from 2 billion to a mere 0.5 billion years old. Today, a team of researchers led by the University of Liverpool has narrowed this down, revealing that the age of the inner core is somewhere between 1 and 1.5 billion years old.
The inner core is our planet’s deepest layer. By assessing the types of sound waves that do or do not travel through the core, scientists have worked out that it must be composed of iron and nickel. Not only that, but seismologists are confident that this sphere is slightly larger than Pluto, with a diameter of 2,440 kilometers (1,500 miles). The interaction of the static inner core with the swirling outer core generates the Earth’s magnetic field, which protects life from dangerous levels of solar radiation. Knowing when the inner core formed , in an event known as the “iron catastrophe” could enlighten scientists as to when this stable, protective magnetic field began to be generated. If indeed the inner core formed around 1 to 1.5 billion years ago as the authors suggest, then this would coincide with the rise of simple multicellular life on Earth, such as red algae, approximately 900 million years ago.
The magnetic field of the Earth changes frequently through time, and this record is preserved in specific igneous (volcanic) rocks as they cool down. This ancient magnetism referred to by scientists as palaeomagnetism was recorded in the immense oceanic crust as soon as it emerged and cooled from its respective tectonic plate boundaries. Scientists in the early 20th century used this magnetic record to prove that the planet’s continents used to be joined together over 200 million years ago before breaking apart. The authors of today’s study used the same science of palaeomagnetism to date the inner core. By painstakingly analyzing ancient igneous rocks, they discovered that the Earth experienced a sharp increase in the strength of its magnetic field between 1 and 1.5 billion years ago. They suggest that this occurred when the inner core began to “freeze out” and differentiate from the molten, turbulent outer core.
The inner core’s formation meant that it took many of the heavier, denser elements with it, removing them from the outer core. Consequently, the outer core was left with the less-dense elements, and the molten material began to rise and fall more efficiently than before. This boosted the Earth’s capacity to generate a magnetic field, leading to the spike detected by the research team. The team note that this is in sharp contrast to Mars, which once had a strong magnetic field. Today, Mars is unprotected against powerful solar radiation, its own magnetic field dying out after half a billion years. The debate as to why exactly this happened is still ongoing.
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