The Mystery of the Missing Matter

Beneath Earth’s solid crust are the mantle, the outer core, and the inner core.
Credit: World Book illustration by Raymond Perlman and Steven Brayfield, Artisan-Chicago

Observations about the early formation of Earth may answer an age-old question about why the planet’s mantle is missing some of the matter that should be present, according to UBC geophysicist John Hernlund.

Earth is made from chondrite, very primitive rocks of meteorites that date from the earliest time of the solar system before the Earth was formed. However, scientists have been puzzled why the composition of Earth’s mantle and core differed from that of chondrite. Solving this mystery could help scientists understand the formation and evolution of our planet. This information would, in turn, help astronomers search for similar Earth-like planets beyond our Solar System.

Hernlund’s findings suggest that an ancient magma ocean swirled beneath the Earth’s surface and would account for the discrepancy.

"As the thick melted rock cooled and crystallized, the solids that resulted had a different composition than the melt," explains Hernlund, a post-doctoral fellow at UBC Earth and Ocean Sciences.

"The melt held onto some of the elements. This would be where the missing elements of chondrite are stored."

Planets are thought to form from dusty disks that surround stars. As particles of dust collide, they eventually gather together to form planets. Objects like meteorites are thought to be left-over chunks of rock from this early planet-forming phase of our Solar System.
Credit: NASA/Spitzer

He says this layer of molten rock would have been around 1,000 km thick and 2,900 km beneath the surface."

Published in the journal Nature, Hernlund’s study explores the melting and crystallization processes that have controlled the composition of the Earth’s interior over geological time. Co-authors are Stéphane Labrosse, Ecole Normale Superieure de Lyon and Nicolas Coltice, Université de Lyon.

The centre of Earth is a fiery core of melted heavy metals, mostly iron. This represents 30 per cent while the remaining 70 per cent is the outer mantle of solid rock.

Traditional views hold that a shallow ocean of melted rock (magma) existed 1,000 km below the Earth’s surface, but it was short lived and gone by 10 million years after the formation of Earth.

This image shows a polished face of the Mezö-Madaras meteorite from Romania – one example of a common chondrite. The material that composes meteorites like Mezö-Madaras is thought to be the same material that formed the planets of our Solar System.
Credit: Arizona State University

In contrast, Hernlund’s evolutionary model predicts that during Earth’s hotter past shortly after its formation 4.5 billion years ago, at least one-third of the mantle closest to the core was also melted.

The partially molten patches now observed at the base of the Earth’s mantle could be the remnants of such a deep magma ocean, says Hernlund.

Related Web Sites

Earth’s Life-Giving Rocks
Earth’s Super Rotating Core
Different at the Core
Earth’s Early Quakes
Reading History in Rocks
Decoding Dusty Disks