Planetary collision that formed the moon ‘made life possible on Earth’

A planetary collision that formed the moon made life possible on Earth, suggests a new study.

Researchers say most of Earth’s essential elements for life – including most of the carbon and nitrogen in us – probably came from another planet.

Earth most likely received the bulk of its carbon, nitrogen and other life-essential volatile elements from the planetary collision that created the moon more than 4.4 billion years ago, according to the study published in the journal Science Advances.

Study co-author Professor Rajdeep Dasgupta, of Rice University in the United States, said: “From the study of primitive meteorites, scientists have long known that Earth and other rocky planets in the inner solar system are volatile-depleted.

“But the timing and mechanism of volatile delivery has been hotly debated.

“Ours is the first scenario that can explain the timing and delivery in a way that is consistent with all of the geochemical evidence.”

The evidence was compiled from a combination of high-temperature, high-pressure experiments in Prof Dasgupta’s lab, which specialises in studying geochemical reactions that take place deep within a planet under intense heat and pressure.

In a series of experiments, study lead author and graduate student Damanveer Grewal gathered evidence to test a long-standing theory that Earth’s volatiles arrived from a collision with an embryonic planet that had a sulphur-rich core.

He said the sulphur content of the donor planet’s core matters because of the “puzzling” array of experimental evidence about the carbon, nitrogen and sulphur that exist in all parts of the Earth other than the core.

Mr Grewal said: “The core doesn’t interact with the rest of Earth, but everything above it, the mantle, the crust, the hydrosphere and the atmosphere, are all connected.

“Material cycles between them.”

His experiments, which simulated the high pressures and temperatures during core formation, tested the idea that a sulphur-rich planetary core might exclude carbon or nitrogen, or both, leaving much larger fractions of those elements in the bulk silicate as compared to Earth.

In a series of tests at a range of temperatures and pressure, Mr Grewal examined how much carbon and nitrogen made it into the core in three scenarios: no sulphur, 10 per cent sulphur and 25 per cent sulphur.

He said: “Nitrogen was largely unaffected.

“It remained soluble in the alloys relative to silicates, and only began to be excluded from the core under the highest sulphur concentration.”

Carbon, by contrast, was considerably less soluble in alloys with intermediate sulphur concentrations, and sulphur-rich alloys took up about 10 times less carbon by weight than sulphur-free alloys.

Using the information, along with the known ratios and concentrations of elements both on Earth and in non-terrestrial bodies, the research team designed a computer simulation to find the most likely scenario that produced Earth’s volatiles.

Finding the answer involved varying the starting conditions, running around one billion scenarios and comparing them against the known conditions in the solar system today.

Mr Grewal said: “What we found is that all the evidence – isotopic signatures, the carbon-nitrogen ratio and the overall amounts of carbon, nitrogen and sulphur in the bulk silicate Earth – are consistent with a moon-forming impact involving a volatile-bearing, Mars-sized planet with a sulphur-rich core.”

Prof Dasgupta, principal investigator on a NASA-funded project called CLEVER Planets that is exploring how life-essential elements might come together on distant planets, said better understanding the origin of Earth’s life-essential elements has implications beyond our solar system.

He said: “This study suggests that a rocky, Earth-like planet gets more chances to acquire life-essential elements if it forms and grows from giant impacts with planets that have sampled different building blocks, perhaps from different parts of a protoplanetary disk.

“This removes some boundary conditions. It shows that life-essential volatiles can arrive at the surface layers of a planet, even if they were produced on planetary bodies that underwent core formation under very different conditions.”

Prof Dasgupta said it does not appear that Earth’s bulk silicate, on its own, could have attained the life-essential mix that produced our biosphere, atmosphere and hydrosphere.

He added: “That means we can broaden our search for pathways that lead to volatile elements coming together on a planet to support life as we know it.”


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