The planetary collision that created the moon may have also violently deposited most of Earth’s essential elements, including the carbon and nitrogen that makes up our bodies, new research suggests.
The study by Rice University petrologists, published in the journal Science Advances, suggests that the bulk of life-essential volatile elements were seeded on Earth 4.4 billion years ago in the impact that created the moon.
One of the study’s co-authors Rajdeep Dasgupta, the principal investigator on a NASA-funded effort called CLEVER Planets says: “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 have 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.”
CLEVER planets explores how life-essential elements might come together on distant rocky planets to gain a better understanding of the origin of Earth’s life-essential elements and the implications beyond our solar system.
Dasgupta continues: “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.”
Dasgupta said it does not appear that Earth’s bulk silicate, on its own, could have attained the life-essential volatile budgets that produced our biosphere, atmosphere and hydrosphere.
“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.”
Under pressure: Collecting evidence and building models
The team gathered their evidence by performing high-pressure, high-temperature experiments in a geophysical lab which specialises in studying geothermal reactions deep under the Earth’s surface.
The experiments allowed lead-author study lead author Damanveer Grewal to gather evidence to test a long-standing theory that Earth’s volatiles arrived from a collision with an embryonic planet that had a sulfur-rich core. The sulfur content of the donor planet’s core matters because of the puzzling array of experimental evidence about the carbon, nitrogen and sulfur that exist in all parts of the Earth other than the core.
Grewal, a graduate student, says: “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.”
Heating up: Competing with existing models
One long-standing idea about how Earth received its volatiles was the “late veneer” theory 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.
The team tested the idea that a sulfur-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. The experimenters created models which examined how much carbon and nitrogen made it into the core in three scenarios: no sulfur, 10% sulfur and 25% sulfur.
Explaining results, Grewal says: “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 sulfur concentration.”
Carbon, by contrast, was considerably less soluble in alloys with intermediate sulfur concentrations, and sulfur-rich alloys took up about 10 times less carbon by weight than sulfur-free alloys.
The team then used these results and known ratios of elements on both Earth and non-terrestrial bodies to design a computer simulation which could discover 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.
Grewal continues: “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.”
Original research: 10.1126/sciadv.aau3669