Life on Earth may have begun with a bang – as planet-sized bodies crashed into each other and scattered the chemical seeds of life over Earth.
According to a new study, a Mars-sized planetary body that slammed into a young Earth and formed the Moon may have also delivered the ingredients necessary for life to begin.
Some elements were critical for life to have first arisen on Earth. They include oxygen, hydrogen, sulphur, nitrogen and carbon. Scientists generally believe that Earth didn’t have these elements available in ways conducive to the formation of life when it first formed. Instead, meteorites, comets and/or other interplanetary bodies rich in volatile materials brought them along when they collided with a nascent Earth.
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“Yes, there was the idea that the key elements for life came from planetary building blocks such as chondritic meteorites,” Damanveer Grewal, an author of the study and a PhD student at Rice University, Houston, told The Wire. “But what was thought is that the impacting materials were small, undifferentiated, primitive objects and not a planet-scale object.”
Another thing that bothered scientists, apart from the source of these materials, was the abundance of carbon relative to nitrogen or hydrogen. It was out of line with the amount of carbon found within the bodies of the earliest known lifeforms. In fact, the carbon-nitrogen ratio on the surface of early Earth – 40:1 – was much higher than that in the meteorites, known as carbonaceous chondrites, thought to have brought the volatiles to Earth. The ratio is closer to 20:1 on the chondrites.
Where was the excessive carbon coming from?
According to Grewal, this could’ve been possible if Earth’s core had been rich in sulphur. The ensuing geochemical reactions would then have ensured that more carbon was expelled from the core than nitrogen, and the carbon would then travel to the surface.
However, early Earth was rich in silicates, not sulphur, so Grewal and his colleagues wondered if there could have been another source.
And that’s when they began to suspect if two sulphur-rich planetary bodies may have crashed into each other somewhere near Earth.
To test their idea, the researchers hit the lab. Using metal and silicate mixtures that contained carbon, nitrogen, and sulphur, among other elements, they simulated the high pressure and high temperature conditions that would prevail in a cataclysmic collision.
The researchers were interested to find out where these elements ended up getting trapped: within metals or within silicates. If most of them ended up trapped within metallic compounds, then they would be sequestered into a planet’s core. If most of them ended up within silicates, then they would eventually be available on the planet’s surface.
Their experiments yielded a definite answer: that if a planet had a metallic core with a lot of sulphur dissolved in it, the planet’s outer layers would have a carbon-nitrogen, or carbon-sulphur, ratio similar to that on Earth.
Ergo, Earth’s life-giving elements, and the compounds they went on to form, need not have come predominantly from carbonaceous chondrites. They could’ve come from a whole other rocky planet.
And now that their premise seemed to hold up, the researchers set out to determine the size of the planetary body that could’ve bring the right dose of volatile materials to Earth. This they did using computer simulations built using the results of the previous experiment.
This led them to a body about the size of Mars.
Researchers have already reported strong evidence of a Mars-sized body having slammed into an inchoate Earth about 4.5 billion years ago. The remnants of this impact are thought to have formed the Moon – borne out by its geochemical similarities to our planet.
Grewal and co. speculated that this event also delivered the ingredients for the recipe of life on Earth.
The results are not surprising, according to Simone Marchi, who studies the origins of terrestrial planets at the Southwest Research Institute, Colorado. “But the study provides a compelling evidence for this scenario, as opposite to other different scenarios proposed in the past.”
At the same time, he’s not completely convinced that the Moon-forming impact was the only collision that delivered the elements. “This aspect requires further testing, as it seems possible that even smaller collisions than what is required to form the Moon could deliver significant amount of volatiles to Earth.”
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But even more interesting to him is the possibility that large collisions involving a young planet could drastically alter its budget of volatile elements. This has implications for the long-term evolution of the planet’s atmosphere and, of course, whether or not it can sustain life.
For example, a rocky planet needn’t possess all the ‘life-forming’ elements. It could simply collide with other bodies, and emerge in the aftermath with a newfound ability to support life.
And considering such unlikely origins of life is important in our search for alien life, especially now that we’re discovering exoplanets in the thousands.
“The exploration of other habitable worlds should recognise that there is not a single mechanism to establish conditions for habitability,” said Grewal.
Lakshmi Supriya is a freelance science writer based in Bengaluru.