Traditional theories held that the Earth’s core and mantle formed over tens of millions of years due to gradual, random collisions between large bodies. New research supports a rapid-accretion model for Earth, which means that this pivotal formation stage lasted only about five million years instead.
A match made in the heavens
The iron isotope composition of various meteorites was measured at the finest-ever precision by researchers. They found one type with an isotope composition that is essentially indistinguishable from the bulk of the Earth’s mantle.
That match came from a group of meteorites called CI chondrites. These carbonaceous chondrites are believed by scientists to best represent the bulk solar system composition when the Sun and planets were forming – the “cosmic dust” of our solar system.
One type of meteor matches the Earth’s bulk. The same type also matches the cosmic dust, present only during a specific phase of our solar system’s creation that spanned five million years. That match suggests the planetary formation process was much faster than previously realized.
Terrestrial planet formation was previously thought to take tens of millions of years
Previous models held that the rocky, inner planets of our solar system took form in many stages over tens of millions of years:
- Bodies of a few hundred kilometers condense and form rapidly.
- The swirling cosmic dust kickstarts gas-assisted accretion of millimeter-sized particles onto those bodies.
- The gases dissipate, leaving many Mars-sized planetary embryos.
- Embryos collide with each other, combining and becoming larger, but also ejecting bits of material back into space – some come back later as meteorites.
- Some of the embryos become large enough to form iron cores.
- Water and other oxidizing elements are delivered to the cores, possibly from the outer solar system as the gaseous planets form and shift.
- The cores oxidize, but they continue to collide with each other, getting larger but still ejecting pieces that would become meteorites.
- These proto-planets use up most of the material around the Sun and become large enough to dominate their orbits.
- With fewer collisions and activity, their mantles cool enough for planetary crusts to form.
The cosmic dust settles
Instead of a gradual process of embryos colliding over tens of millions of years, the researchers found that the Earth’s core formed and oxidized within a 5-million-year span, as the cosmic dust settled.
A wide mix of compositions and meteor types would be expected if collisions occurred over longer time spans. The planet’s formation would have to occur rapidly to explain how the iron isotope composition is only comparable to one type of meteorite.
Original Study Citation: http://dx.doi.org/10.1126/sciadv.aay7604
Schiller, M., Bizzarro, M., & Siebert, J. Iron Isotope Evidence for Very Rapid Accretion and Differentiation of the Proto-Earth. Sci. Adv. 6, eaay7604 (2020).
