Elementary Collision

Most of Earth’s essential elements for life — including most of the carbon and nitrogen in you — 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 a recent study by Rice petrologists.

“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 Rajdeep Dasgupta, professor of Earth, environmental and planetary sciences. “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.”

Dasgupta’s lab specializes in studying geochemical reactions that take place deep within a planet under intense heat and pressure. Dasgupta, graduate student Damanveer Grewal and postdoctoral researcher Chenguang Sun used data from high-temperature, high-pressure experiments and a computer simulation to find the most likely scenario that produced Earth’s volatiles. Finding the answer involved varying the simulation’s 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.

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 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,” he said. “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.”