Earth formed from dry, rocky building blocks

Date:
July 5, 2023
Source:
California Institute of Technology
Summary:
A new study shows that the so-called planetesimals that accreted
together to form Earth must have been composed of dry, rocky
materials.


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FULL STORY ========================================================================== Billions of years ago, in the giant disk of dust, gas, and rocky
material that orbited our young sun, larger and larger bodies coalesced
to eventually give rise to the planets, moons, and asteroids we see
today. Scientists are still trying to understand the processes by which planets, including our home planet, were formed. One way researchers
can study how Earth formed is to examine the magmas that flow up from
deep within the planet's interior. The chemical signatures from these
samples contain a record of the timing and the nature of the materials
that came together to form Earth -- analogous to how fossils give us
clues about Earth's biological past.

Now, a study from Caltech shows that the early Earth accreted from hot
and dry materials, indicating that our planet's water -- the crucial
component for the evolution of life -- must have arrived late in the
history of Earth's formation.

The study, involving an international team of researchers, was
conducted in the laboratories of Francois Tissot, assistant professor of geochemistry and Heritage Medical Research Institute Investigator; and
Yigang Zhang of the University of Chinese Academy of Sciences. A paper describing the research appears in the journal Science Advances.Caltech graduate student Weiyi Liu is the paper's first author.

Though humans do not have a way to journey into the interior of our
planet, the rocks deep within the earth can naturally make their way to
the surface in the form of lavas. The parental magmas of these lavas can originate from different depths within Earth, such as the upper mantle,
which begins around 15 kilometers under the surface and extends for
about 680 kilometers; or the lower mantle, which spans from a depth of
680 kilometers all the way to the core- mantle boundary at about 2,900 kilometers below our feet. Like sampling different layers of a cake --
the frosting, the filling, the sponge - - scientists can study magmas originating from different depths to understand the different "flavors"
of Earth's layers: the chemicals found within and their ratios with
respect to one another.

Because the formation of Earth was not instantaneous and instead involved materials accreting over time, samples from the lower mantle and upper
mantle give different clues to what was happening over time during
Earth's accretion.

In the new study, the team found that the early Earth was primarily
composed of dry, rocky materials: chemical signatures from deep within
the planet showed a lack of so-called volatiles, which are easily
evaporated materials like water and iodine. In contrast, samples of the
upper mantle revealed a higher proportion of volatiles, three times of
those found in the lower mantle. Based on these chemical ratios, Liu
created a model that showed Earth formed from hot, dry, rocky materials,
and that a major addition of life-essential volatiles, including water,
only occurred during the last 15 percent (or less) of Earth's formation.

The study is a crucial contribution to theories of planet formation, a
field which has undergone several paradigm shifts in recent decades and
is still characterized by vigorous scientific debate. In this context,
the new study makes important predictions for the nature of the building
blocks of other terrestrial planets -- Mercury and Venus -- which would
be expected to have formed from similarly dry materials.

"Space exploration to the outer planets is really important because a
water world is probably the best place to look for extraterrestrial life," Tissot says. "But the inner solar system shouldn't be forgotten. There
hasn't been a mission that's touched Venus's surface for nearly 40 years,
and there has never been a mission to the surface of Mercury. We need
to be able to study those worlds to better understand how terrestrial
planets such as Earth formed." The paper is titled "I/Pu reveals Earth
mainly accreted from volatile-poor differentiated planetesimals." In
addition to Liu and Tissot, co-authors are Zhang of the University of
Chinese Academy of Sciences; Guillaume Avice of the Universite' Paris
Cite', Institut de physique du globe de Paris; Zhilin Ye of the Chinese
Academy of Sciences; and Qing-Zhu Yin of the University of California,
Davis. Funding was provided by the Chinese Academy of Sciences, the
National Science Foundation, a Packard Fellowship for Science and
Engineering, the Heritage Medical Research Institute, and Caltech.

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========================================================================== Journal Reference:
1. Weiyi Liu, Yigang Zhang, Franc,ois. L. H. Tissot, Guillaume Avice,
Zhilin
Ye, Qing-Zhu Yin. I/Pu reveals Earth mainly accreted from
volatile-poor differentiated planetesimals. Science Advances,
2023; 9 (27) DOI: 10.1126/sciadv.adg9213 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230705142944.htm

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