‘Nebular neon’ confirmed deep inside the Earth
Discovery adds to debate over how the planet formed. Ben Lewis reports.
The early formation of Earth was a relatively rapid process that trapped water and gases in the planet's mantle from the cloud of gas and dust surrounding the sun, according to researchers from the US.
The finding will add fuel to a long-standing debate about whether nebular gases were dissolved into a magma ocean during the early stages of Earth’s accretion, and they are preserved in the mantle to the present day.
The study, by Curtis Williams and Sujoy Mukhopadhyay at the University of California Davis, is published in the journal Nature.
Scientists around the world hold three competing ideas about how the Earth formed from a protoplanetary disk of dust and gas. One theory is that the planet grew relatively quickly over two to five million years, capturing gases from the nebula as it accreted.
Another suggests dust particles formed in the nebula and were irradiated by the Sun for some time, before then condensing into small planetesimal objects that were incorporated into the growing planet. The third option is that Earth formed relatively slowly, and the gases were delivered by carbonaceous chondrite meteorites rich in water, carbon and nitrogen.
To try to unpick the origins, the researchers turned to neon as a proxy for other volatile gases such as water, carbon dioxide and nitrogen, which would have been condensing into Earth at the same time from the same source.
Unlike these compounds that are essential for life, neon is an inert noble gas, not influenced by chemical or biological processes.
"We're trying to understand where and how the neon in Earth's mantle was acquired, which tells us how fast the planet formed and in what conditions," Williams says.
By examining the relative amounts of two neon isotopes, the researchers were able to distinguish between different sources of volatile chemicals in the planet’s interior. With each isotope being stable and non-radioactive, the amounts have been constant since formation and will remain so forever, say the researchers.
The three most likely sources of the two neon isotopes – nebular gas, solar-wind-irradiated planetesimals and chondrite meteorites – are each predicted to have distinct ratios.
The researchers took measurements from ocean-floor basalts formed when flows from deep within the Earth spilled out and cooled in the ocean, and compared them to measurements from solar wind particles, irradiated lunar soils, and meteorites.
The ratios of Earth-bound neon in the deep basalts closely matched the values from the solar nebula, well above those for the “irradiated particles” or “late accretion” models. And this, says Williams, supports the model of Earth’s rapid early formation.
“This is a clear indication that there is nebular neon in the deep mantle,” explains Williams.
According to Williams, to absorb these vital compounds a planet needs to reach a size equivalent to Mars, or a little larger, before the solar nebula dissipates.
Further measurements found differences in the neon isotope ratios between the deep mantle plumes and mid-ocean ridges which, according to the authors, is best explained by a component of volatile gases also being provided by chondrite meteorites during the main phase of accretion, after nebula gases had already been captured in the early stages.
With results from the ALMA telescopes in Chile showing protoplanetary discs with dark bands where the dust had been depleted – thought to be from planetary accretion – this same process could be occurring in other systems.
“We can observe planet formation in a gas disk in other solar systems, and there is a similar record of our own solar system preserved in Earth's interior,” says Mukhopadhyay.
“This might be a common way for planets to form elsewhere.”