by Ben Lewis
is a science communicator
with the Royal
Institution of Australia.
An artist's impression of a
proto-planetary disc surrounding a young star.
European Southern Observatory
over how the
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 (Capture
of Nebular Gases during Earth's Accretion is Preserved in
Deep-Mantle Neon), by Curtis Williams and Sujoy Mukhopadhyay
at the University of California Davis, is published in the journal
Scientists around the world hold three competing ideas about how the
Earth formed from a proto-planetary 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
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
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
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
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,
...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,"
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
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
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."