A Caltech study provides insights into Earth's origins, suggesting it was formed from dry, rocky materials, and life-essential water arrived late.
Several billions of years back, amidst a colossal disk of dust, gas, and rocky matter orbiting our youthful sun, progressively larger bodies merged, eventually forming the planets, moons, and asteroids that populate our current universe. Scientists continue to explore how our home planet, Earth, came into existence. A technique used in this quest is to analyze magmas originating from Earth’s depths. These magma samples bear chemical traces which serve as a timeline and a catalogue of the constituents that coalesced to shape Earth, much like how fossils inform us about Earth’s biological history.
A recent study from Caltech reveals that the early Earth was formed from searing, dehydrated elements, suggesting that water — a critical ingredient for the evolution of life — must have reached our planet late in its developmental timeline.
From what and how did Earth form?
An international consortium of researchers undertook the study under the supervision of Francois Tissot, assistant professor of geochemistry and Heritage Medical Research Institute Investigator; and Yigang Zhang of the University of Chinese Academy of Sciences. Caltech graduate student Weiyi Liu is the lead author of the paper, published in the journal Science Advances.
While humanity lacks the means to physically explore Earth’s interior, rocks from these depths can spontaneously emerge on the surface as lavas. Parental magmas of these lavas might have their origins at varying depths within Earth, such as the upper mantle, starting approximately 15 kilometers beneath the surface and extending for around 680 kilometers, or the lower mantle, which begins at 680 kilometers deep and stretches down to the core-mantle boundary, some 2,900 kilometers beneath our feet.
Analogous to testing different strata of a cake — the icing, the filling, the base — scientists can investigate magmas from diverse depths to comprehend the unique “flavors” of Earth’s layers: the specific chemicals present and their relative proportions.
From what and how did Earth form?
Given that Earth’s formation wasn’t instantaneous but rather a prolonged amalgamation of materials, samples from the lower and upper mantle offer different insights into what transpired during Earth’s accretion. The team discovered in their new research that the initial Earth primarily consisted of arid, rocky elements. Chemical traces from deep within the planet displayed an absence of so-called volatiles, materials like water and iodine which are prone to evaporation. In contrast, samples from the upper mantle exhibited a higher percentage of volatiles, three times the amount found in the lower mantle.
Liu developed a model based on these chemical ratios, demonstrating that Earth was created from fiery, dry, rocky constituents. The addition of crucial life-supporting volatiles, including water, only took place during the final 15 percent (or less) of Earth’s formation.
From what and how did Earth form?
This research significantly contributes to theories of planetary formation, an area that has seen numerous paradigm shifts in the past few decades and remains a hotspot for scientific discussions. In this context, the new study predicts the nature of the building blocks for other terrestrial planets — Mercury and Venus — which would have been formed from comparably dry elements.
Francois Tissot explained that, “space exploration to the outer planets is crucial because a water world probably offers the best prospect for finding extraterrestrial life. But the inner solar system shouldn’t be neglected. It’s been nearly 40 years since a mission touched Venus’s surface, and a mission to Mercury’s surface has never taken place. To better comprehend how terrestrial planets like Earth were formed, we must study these worlds.”
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