Planetary Collision That Formed the Moon Delivered Basic Elements of Life
"When people look at giant impacts, they always look at it as a destructive event."
Most of the carbon and nitrogen in our bodies probably came from a planet the size of Mars crashing into Earth 4.4 billion years ago, scientists say. Researchers have long thought that these elements, crucial for life, arrived on our planet aboard primitive bodies like asteroids, but a new analysis suggests that carbon and nitrogen most likely rode to Earth in a planet that had already differentiated into layers — a sign of a more mature astronomical body, possibly a planetary embryo with a mantle and a core. This same collision, they say, formed the moon.
In a paper published Wednesday in Science Advances, a team at Rice University in Texas outlined a series of experiments and simulations that support the hypothesis that a single major collision deposited the chemical foundation of life on Earth.
Damanveer Grewal, a Ph.D. student at Rice University and the study’s lead author, tells Inverse that this research changes the story of how the elemental building blocks of life came to our planet.
“The idea that has been prevalent in the scientific community has been that these elements were delivered by undifferentiated bodies after all of the Earth has almost accreted,” says Grewal. “What we are trying to say is that these elements were actually delivered by a giant impact of a large, differentiated body, rather than by smaller bodies.”
By comparing the chemical compositions of Earth’s crust with glasses on the moon, Grewal’s team concluded that they shared a common origin — the cataclysmic event that formed the moon. And then, by running simulations on how different elements settle into different parts of a planet as it differentiates, the researchers recognized that a differentiated planet that collided with Earth would have a much less carbon-rich ratio of material on its surface than an undifferentiated body would. This is because, they found, the element would settle toward the iron core, leaving less of a chemical trace in the planet’s crust. The same process, researchers say, happened in the formation of Earth’s core.
Therefore, when this embryonic planet collided with Earth, about 100 million years after our planet formed, it would have transferred material to Earth bearing the chemical signature of a planet whose carbon had settled to the core — as opposed to an undifferentiated body whose composition was relatively uniform.
And their models bore out this hypothesis, lending further support to the idea that the same planetary collision that formed the moon also deposited the very basic materials for life on our planet.
With this new paper, the team continues to add more evidence to the idea that elements essential to life were delivered by a giant impact. Grewal says the idea could change the way people look at the destructive force of planetary collisions.
“When people look at giant impacts, they always look at it as a destructive event,” he says. “But now you can actually think of it as a life-giving event as well.”
Abstract: Earth’s status as the only life-sustaining planet is a result of the timing and delivery mechanism of carbon (C), nitrogen (N), sulfur (S), and hydrogen (H). On the basis of their isotopic signatures, terrestrial volatiles are thought to have derived from carbonaceous chondrites, while the isotopic compositions of nonvolatile major and trace elements suggest that enstatite chondrite–like materials are the primary building blocks of Earth. However, the C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which rules out volatile delivery by a chondritic late veneer. In addition, if delivered during the main phase of Earth’s accretion, then, owing to the greater siderophile (metal loving) nature of C relative to N, core formation should have left behind a subchondritic C/N ratio in the BSE. Here, we present high pressure-temperature experiments to constrain the fate of mixed C-N-S volatiles during core-mantle segregation in the planetary embryo magma oceans and show that C becomes much less siderophile in N-bearing and S-rich alloys, while the siderophile character of N remains largely unaffected in the presence of S. Using the new data and inverse Monte Carlo simulations, we show that the impact of a Mars-sized planet, having minimal contributions from carbonaceous chondrite-like material and coinciding with the Moon-forming event, can be the source of major volatiles in the BSE.