The land probably received most of its carbon, nitrogen and other volatile elements essential for life, froma planetary collision that created the moon 4,400 million years agos.
"From the study of primitive meteorites, scientists have known for a long time that the Earth and other rocky planets in the inner solar system are depleted by volatility," Rajdeep Dasgupta, of the Rice University of Texas (USA) and co-author of an investigation that supports that hypothesis.
"But the timing and the mechanism of the volatile delivery have been hotly debated," he adds, "ours is the first scenario that can explain the moment and deliver it consistently with all the geochemical tests."
The evidence was gathered from a combination of high temperature and high pressure experiments in the Dasgupta laboratory, which specializes in the study of geochemical reactions that take place inside a planet under intense heat and pressure.
In a series of experiments, the study's lead author and graduate student Damanveer Grewal gathered evidence to test a long-term theory that the volatiles of the Earth came from a collision with an embryonic planet that had a nucleus rich in sulfur.
The sulfur content of the donor planet's core is important because of the bewildering variety of experimental evidence about carbon, nitrogen and sulfur that exists in all parts of the Earth other than the nucleus.
"The core does not interact with the rest of the Earth, but everything above it, the mantle, the crust, the hydrosphere and the atmosphere, are all connected," says Grewal.
A long-term idea about how the Earth received its volatiles was the theory of 'late plating', that the volatile-rich meteorites, leftover pieces of primordial matter from the outer solar system, arrived after the Earth's core was formed. And while the isotopic signatures of Earth's volatiles coincide with these primordial objects, known as carbonaceous chondrites, the elemental ratio of carbon to nitrogen is deactivated. The non-central material of the Earth, which geologists call 'the bulk silicate Earth', has approximately 40 parts of carbon for each part of nitrogen, approximately double the 20-1 ratio seen in carbonaceous chondrites.
Grewal's experiments, which simulated high pressures and temperatures during core formation, proved the idea that a planetary core rich in sulfur could exclude carbon or nitrogen, or both, leaving much larger fractions of those elements in the silicate. in bulk compared to Earth. In a series of tests over a range of temperatures and pressures, Grewal examined the amount of carbon and nitrogen that became the nucleus in three scenarios: no sulfur, 10 percent sulfur, and 25 percent sulfur.
"The nitrogen was not greatly affected," he explains, "it remained soluble in the alloys in relation to the silicates, and only began to be excluded from the nucleus under the highest concentration of sulfur."
By contrast, the carbon was considerably less soluble in alloys with intermediate concentrations of sulfur, and the sulfur-rich alloys absorbed approximately 10 times less carbon by weight than the sulfur-free alloys.
Using this information, along with known relationships and concentrations of elements on both Earth and non-terrestrial bodies, postdoctoral researcher Chenguang Sun designed a computer simulation to find the most likely scenario to produce Earth's volatiles. Finding the answer involved varying the start conditions, running approximately 1,000 million scenarios and compare them with the known conditions in the current solar system.
"What we found is that all the evidence (isotopic signatures, the carbon-nitrogen ratio and the general amounts of carbon, nitrogen and sulfur in Earth's silicate) is consistent with a Luna formation impact that involves a volatile support, a planet the size of Mars with a nucleus rich in sulfur, "says Grewal.
According to Dasgupta, understanding the origin of the essential elements of life on Earth has implications beyond the solar system. "This study suggests that a rocky planet similar to Earth has more possibilities of acquiring essential elements for life if it is formed and grows from giant impacts with planets that have sampled different building blocks, perhaps from different parts of a protoplanetary disk, "he says.
"This eliminates some boundary conditions," he says, "and shows that the volatiles essential for life can reach the surface layers of a planet, even if they occurred in planetary bodies that underwent the formation of nuclei under very different conditions."
Dasgupta argues that it does not seem that the bulk silicate of the Earth, by itself, could have reached the volatile budgets essential for life that produced the biosphere, atmosphere and hydrosphere. "That means we can expand our search for paths that lead to volatile elements that come together on a planet to sustain life as we know it," he concludes.