A new technique that peers into our planet's deepest interior suggests that Earth's heart isn't all that it seems.

Key points: We know little about Earth's core structure and composition, and studying it is a tricky task

We know little about Earth's core structure and composition, and studying it is a tricky task Canberra researchers reanalysed earthquake data and calculated that the inner core is softer than previously thought

Canberra researchers reanalysed earthquake data and calculated that the inner core is softer than previously thought This means the core might contain pockets of melted material, or it might be a property of iron under high pressure and temperature

Hrvoje Tkalčić and Thanh-Son Phạm, both at the Australian National University, reanalysed the echoes of massive earthquakes bouncing around the inside of the planet.

They calculated that the solid inner core is softer than previously thought.

Their new method, published in Science today, raises its own questions, namely: what's going on down there?

"We originally thought it might be pure iron in the solid inner core, but that's not guaranteed," said Louis Moresi, a geophysicist at the University of Melbourne, who was not involved in the study.

"Maybe that's all now prised open by this new work."

Peeling back Earth's layers

To get a sense of the structure of what's inside our planet, seismologists analyse seismic waves unleashed by earthquakes.

"After large earthquakes, the earth oscillates, like a bell," Professor Tkalčić said.

A global network of instruments called seismometers picks up the faint shakes and shudders that travel around and through Earth.

It's not a new technique; seismometers have been around since 1880.

Until the 1930s, scientists thought Earth held a huge reservoir of liquid rock in its centre, which was enveloped by solid mantle and topped by the crust.

But after the 1929 magnitude-7.3 earthquake that struck New Zealand's South Island, instruments in Europe recorded waves that should not have been possible if the core was liquid.

Danish seismologist Inge Lehmann realised that there must be a solid component in there somewhere.

In 1936, she published her discovery: that Earth had an inner core.

A few years later, American mineral physicist Francis Birch postulated that the inner core was made of solid iron.

And we now think that the liquid outer core is arranged in rotating columns that generate Earth's magnetic field.

"Rotating fluids conduct electricity and generate magnetic fields. It's a sort of self-sustaining dynamo which we see in other places like the Sun and Jupiter," Professor Moresi said.

"A really important component of Earth's core is the fact that the inner core exists at all.

"It marshals a lot of the fluid flow into this quite stable set of cylinders.

"But if you want to study the inner core, you have to study through the outer core first, so that makes it even harder."

Seismic waves: Earthquakes generate two main types of seismic waves

Earthquakes generate two main types of seismic waves P waves, or primary waves, are rolling waves of compression travelling though material, like sound waves in air

P waves, or primary waves, are rolling waves of compression travelling though material, like sound waves in air They can travel through solids, liquids and gases

They can travel through solids, liquids and gases P waves are followed by the slower-moving S waves, or secondary waves

P waves are followed by the slower-moving S waves, or secondary waves These are the waves that shake the ground up and down or side to side

These are the waves that shake the ground up and down or side to side Unlike P waves, S waves can only travel through solid material

S waves and P waves

To probe the core, scientists analyse how S and P waves bounce around and through the planet.

For decades, seismologists have sought signs of S waves in the inner core.

Their speed can tell you about the stiffness, or rigidity, of the material.

And even though S waves can't travel through the liquid outer core, they do exist in the solid inner core.

This is because when a P wave travelling through the outer core hits the inner core, some of its energy is converted to S waves.

Those S waves can propagate through the inner core until they hit the liquid outer core, where they're converted again to P waves.

How to see the invisible

To tease out information about inner core S waves, Professor Tkalčić and his PhD student, Mr Phạm, reanalysed historical data of earthquakes of at least magnitude-6.8 that struck between 2010 and 2016.

But instead of examining the properties of individual signals, they analysed similarities between pairs of waveforms.

This produced mathematical constructs called "global correlograms", which the pair used to calculate the speed of S waves through the inner core.

It turned out that S waves were around 2.5 per cent slower than previously thought. And because S waves move fastest through stiffer material, it follows that the inner core is softer than previously thought, too.

What's causing this squishiness?

The new work doesn't say for sure, Professor Tkalčić said, and it's the subject of ongoing debate.

It might be an intrinsic property of iron at the hot, pressurised conditions in the centre of the Earth.

Or it could be that pockets of melted matter, enriched with lighter elements, are trapped inside the inner core.

A better map for the Earth's interior

Having a better idea of the centre of the Earth means geophysicists can sharpen up their reference models, Professor Moresi said.

"If you measure waves that go through the core or bounce off it, we can use those for imaging elsewhere," he said.

"But if we haven't fully understood the properties of the core, then there might be errors."

Understanding the core's properties will also help ascertain if it's rotating faster than the overlying mantle.

And we might get a more accurate picture in the next few years.

Professor Tkalčić and his crew will soon lay down a network of seismometers on and around Macquarie Island, halfway between New Zealand and Antarctica.

It's an area with plenty of earthquakes, but very little in the way of instruments.

The year-long experiment will not only help us understand what's going on under the surface in that part of the world, it will also add to our knowledge about the core.

Professor Tkalčić said it will be "like a large telescope pointing towards the centre of the earth".