Planet smash delivered carbon to Earth 4.4 billion years ago: study


If carbon sizzled away during the planet's early days, where did life come from? Belinda Smith reports.


An artist's impression of a collision between a carbon- and silicon-rich planetary embryo and Earth 4.4 billion years ago. Earth absorbed the proto-planet – and its elements – to end up with a carbon-rich mantle.
A. Passwaters / Rice University / NASA / JPL-Caltech

A Mercury-sized planetary embryo that slammed into Earth around 4.4 billion years ago delivered virtually all of the planet's carbon, new research suggests.

Experiments simulating the dense, hot conditions of the deep Earth by Yuan Li from the Chinese Academy of Sciences, Rajdeep Dasgupta from Rice University in the US, and colleagues provide an explanation to the mystery of how Earth seeded life when the lion’s share of carbon should have boiled away into space or been trapped deep in the core.

The work was published in Nature Geosciences.

Unravelling the early Earth’s composition is no easy feat. Billions of years ago, the solar system was a whirling mess of comets, asteroids and protoplanets.

Back then, Earth was a hot ball of rock. As you might expect, volatile molecules such as water and carbon boiled and hissed out into space. Modelling has shown that any carbon that didn’t escape the planet at this time was hooked by iron-rich alloys and dragged into the core.

But today, Earth is carbon rich. Without it, we wouldn’t have life as we know it. So where did it come from?

Water, planetary scientists think, was ferried to Earth on comets and asteroids. The same could well be the case for carbon, not to mention other volatiles such as sulfur and nitrogen.

The problem with that is there are no known meteorites that could produce the right ratio of these elements in the Earth’s mantle. The mantle is a 2,900-kilometre-thick layer sandwiched between the core and crust.

It’s huge – accounting for around two-thirds the planet’s mass – and constantly exchanges elements with the crust and atmosphere above.

So Li, Dasgupta and their crew decided to see how the addition of other elements, such as sulfur and silicon, might change the core’s affinity for carbon. If they made the core less likely to hang on to carbon, then more carbon might remain in the mantle.

"We thought we definitely needed to break away from the conventional core composition of just iron and nickel and carbon," Dasgupta says.

"So we began exploring very sulfur-rich and silicon-rich alloys, in part because the core of Mars is thought to be sulfur-rich and the core of Mercury is thought to be relatively silicon-rich.”

They squeezed rocks in hydraulic presses that emulate the high pressure and high temperature conditions of the deep Earth.

They found carbon wasn’t automatically locked away if the iron-rich alloys in the core were also rich in silicon or sulfur.

Rajdeep Dasgupta

One scenario that could yield today’s volatile concentrations was if a carbon-rich Mercury-sized planet with a core full of silicon or sulfur smashed into and was absorbed by Earth.

The dynamics of the collision meant the silicon sank straight to Earth’s core while the carbon was mixed with the mantle, ready to be cycled up to the crust and eventually, develop life.

Belinda smith 2016 2.jpg?ixlib=rails 2.1
Belinda Smith is a science and technology journalist in Melbourne, Australia.