The question of how life-essential elements such as carbon, nitrogen and sulfur came to our planet has been long debated and is a clearly important and slippery scientific subject.
Did these volatile elements accrete onto the proto-Earth from the sun’s planetary disk as the planet was being formed? Did they arrive substantially later via meteorite or comet? Or was it the cataclysmic moon-forming impact of the proto-Earth and another Mars-sized planet that brought in those essential elements?
Piecing this story together is definitely challenging, but now there is vigorous support for one hypothesis — that the giant impact brought us the elements would later be used to enable life.
Based on high pressure-temperature experiments, modeling and simulations, a team at Rice University’s Department of Earth, Environmental and Planetary Sciences makes that case in Science Advances for the central role of the proto-planet called Theia.
“From the study of primitive meteorites, scientists have long known that Earth and other rocky planets in the inner solar system are volatile-depleted,” said study co-author Rajdeep Dasgupta. “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.”
“What we are saying is that the impactor definitely brought the majority supply of life-essential elements that we see at the mantle and surface today,” Dasgupta wrote in an email.
Some of their conclusions are based on the finding of a similarity between the isotopic compositions of nitrogen and hydrogen in lunar glasses and in the bulk silicate portions of the Earth. The Earth and moon volatiles, they conclude, “have a common origin.”
Carbon, nitrogen and sulfur are deemed “volatile” elements because they have a relatively low boiling point and can easily fly off into space from planets and moons in their early growing stages. A number of other life-important chemicals, including water, are volatiles as well.
The recent findings are grounded in a series of experiments by study lead author and graduate student Damanveer Grewal, who works in the Dasgupta lab. Grewal gathered evidence to test the theory that Earth’s volatiles arrived when the embryonic planet Theia — that had a sulfur-rich core — crashed into very early Earth.
The sulfur content of the donor planet’s core matters because of the puzzling evidence about the carbon, nitrogen and sulfur that exist in all parts of the Earth — other than the core. The team needed to test the conditions under which a core with sulfur could, in effect, exclude other volatiles, thus making them more common in the planet’s mantle and above — and as a result more available to a planet it might crash into.
The high temperature and pressure tests led to a computer simulation to find the most likely scenario that produced Earth’s volatiles. Finding the answer involved varying the starting conditions, running approximately 1 billion scenarios and comparing them against the known conditions in the solar system today.
“What we found is that all the evidence — isotopic signatures, the carbon-nitrogen ratio and the overall amounts of carbon, nitrogen and sulfur in the bulk silicate Earth — are consistent with a moon-forming impact involving a volatile-bearing, Mars-sized planet with a sulfur-rich core,” Grewal said.
Another often-cited explanation about how Earth received its volatiles is the “late veneer” theory, which holds that volatile-rich meteorites, leftover chunks of primordial matter from the outer solar system, arrived after Earth’s core formed.
And while the isotopic signatures of Earth’s volatiles match these primordial objects, known as carbonaceous chondrites, the elemental ratio of carbon to nitrogen is off. Earth’s non-core material, which geologists call the bulk silicate Earth, has about 40 parts carbon to each part nitrogen, approximately twice the 20-1 ratio seen in carbonaceous chondrites.
This led to their conclusion that the late veneer theory could not explain the conditions they had found.
Although the Rice team’s paper does not go in depth into the question of how water got to Earth, Dasgupta wrote that the team’s conclusion that the moon-forming impact brought with it other volatiles, “it is likely that the impactor would contain and bring some water too. This is especially likely because this impactor needs to form in part from oxidized carbonaceous chondritic materials (that is the condition our experiments simulated as well).
“So although we did not factor in matching the water budget in our model, it is entirely possible that this impactor brought Earth’s water budget too, if the proto-Earth was water-poor.”
Dasgupta, the principal investigator on a NASA-funded effort called CLEVER Planets that is exploring how life-essential elements might come together on distant rocky planets, said better understanding the origin of Earth’s life-essential elements has implications beyond our solar system.
“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,” Dasgupta said.
“This removes some boundary conditions,” he said. “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.”
Dasgupta said it does not appear that Earth’s initial composition of bulk silicate, on its own, could have attained the concentrations of those life-essential volatiles needed to produce our atmosphere and hydrosphere and biosphere.
This all has great implications for exoplanet studies, he said. It means that “we can broaden our search for pathways that lead to volatile elements coming together on a planet to support life as we know it.”
CLEVER Planets is part of the Nexus for Exoplanet System Science, or NExSS, a NASA astrobiology research coordination network that is dedicated to the study of planetary habitability. CLEVER Planets involves more than a dozen research groups from Rice, NASA’s Johnson Space Center, UCLA, the University of Colorado Boulder and the University of California, Davis.