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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    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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   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   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230705142944.htm   
      
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