In a new paper in the journal Nature Geoscience, researchers offer a new answer to a long-debated question: how did carbon-based life develop on Earth, given that most of the planet’s carbon should have either boiled away in the planet’s earliest days or become locked in its core?
The ratio of volatile elements in Earth’s mantle suggests that virtually all of the planet’s life-giving carbon came from a collision with an embryonic planet about 100 million years after Earth formed. Image credit: A. Passwaters / Rice University / NASA / JPL-Caltech.
Earth’s core, which is mostly iron, makes up about one-third of the planet’s mass. The planet’s silicate mantle accounts for the other two-thirds and extends more than 1,500 miles below the surface.
The mantle, atmosphere and crust constantly exchange elements, including the volatile elements needed for life.
If Earth’s initial allotment of carbon boiled away into space or got stuck in the core, where did the carbon in the mantle and biosphere come from?
“One popular idea has been that volatile elements like carbon, sulfur, nitrogen and hydrogen were added after Earth’s core finished forming,” said lead author Dr. Yuan Li, of Rice University and Guangzhou Institute of Geochemistry.
“Any of those elements that fell to Earth in meteorites and comets more than about 100 million years after the Solar System formed could have avoided the intense heat of the magma ocean that covered Earth up to that point.”
“The problem with that idea is that while it can account for the abundance of many of these elements, there are no known meteorites that would produce the ratio of volatile elements in the silicate portion of our planet.”
The team decided to conduct experiments to gauge how sulfur or silicon might alter the affinity of iron for carbon.
“We thought we definitely needed to break away from the conventional core composition of just iron and nickel and carbon,” said co-author Prof. Rajdeep Dasgupta, also of Rice University.
“So we began exploring very sulfur-rich and silicon-rich alloys, in part because the core of Mars is thought to be sulfur-rich and the core of Mercury is thought to be relatively silicon-rich.”
“It was a compositional spectrum that seemed relevant, if not for our own planet, then definitely in the scheme of all the terrestrial planetary bodies that we have in the Solar System.”
A schematic depiction of early Earth’s merger with a Mercury-like planetary embryo, a scenario supported by new high-pressure, high-temperature experiments at Rice University. Magma ocean processes could lead planetary embryos to develop silicon- or sulfur-rich metallic cores and carbon-rich outer layers. If Earth merged with such a planet early in its history, it could explain how Earth acquired its carbon and sulfur. Image credit: Rajdeep Dasgupta.
The experiments revealed that carbon could be excluded from the core – and relegated to the silicate mantle – if the iron alloys in the core were rich in either silicon or sulfur.
“The key data revealed how the partitioning of carbon between the metallic and silicate portions of terrestrial planets varies as a function of the variables like temperature, pressure and sulfur or silicon content,” Dr. Li said.
The scientists mapped out the relative concentrations of carbon that would arise under various levels of sulfur and silicon enrichment, and compared those concentrations to the known volatiles in Earth’s silicate mantle.
“One scenario that explains the carbon-to-sulfur ratio and carbon abundance is that an embryonic planet like Mercury, which had already formed a silicon-rich core, collided with and was absorbed by Earth,” Prof. Dasgupta said.
“Because it’s a massive body, the dynamics could work in a way that the core of that planet would go directly to the core of our planet, and the carbon-rich mantle would mix with Earth’s mantle.”
“In this paper, we focused on carbon and sulfur. Much more work will need to be done to reconcile all of the volatile elements, but at least in terms of the carbon-sulfur abundances and the carbon-sulfur ratio, we find this scenario could explain Earth’s present carbon and sulfur budgets.”
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Yuan Li et al. Carbon and sulfur budget of the silicate Earth explained by accretion of differentiated planetary embryos. Nature Geoscience, published online September 5, 2016; doi: 10.1038/ngeo2801
