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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Pass the salt: This space rock holds clues as to how Earth got its water   
      
      
     Date:   
         June 13, 2023   
     Source:   
         University of Arizona   
     Summary:   
         The discovery of tiny salt grains in a sample from an asteroid   
         provides strong evidence that liquid water may be more common in   
         the solar system than previously thought.   
      
      
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   FULL STORY   
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   Sodium chloride, better known as table salt, isn't exactly the type   
   of mineral that captures the imagination of scientists. However,   
   a smattering of tiny salt crystals discovered in a sample from an   
   asteroid has researchers at the University of Arizona Lunar and Planetary   
   Laboratory excited, because these crystals can only have formed in the   
   presence of liquid water.   
      
   Even more intriguing, according to the research team, is the fact that   
   the sample comes from an S-type asteroid, a category known to mostly lack   
   hydrated, or water-bearing, minerals. The discovery strongly suggests   
   that a large population of asteroids hurtling through the solar system   
   may not be as dry as previously thought. The finding, published inNature   
   Astronomy, gives renewed push to the hypothesis that most, if not all,   
   water on Earth may have arrived by way of asteroids during the planet's   
   tumultuous infancy.   
      
   Tom Zega, the study's senior author and a professor of planetary sciences   
   at the UArizona Lunar and Planetary Laboratory, and Shaofan Che, lead   
   study author and a postdoctoral fellow at the Lunar and Planetary   
   Laboratory, performed a detailed analysis of samples collected from   
   asteroid Itokawa in 2005 by the Japanese Hayabusa mission and brought   
   to Earth in 2010.   
      
   The study is the first to demonstrate that the salt crystals originated   
   on the asteroid's parent body, ruling out any possibility they might have   
   formed as a consequence of contamination after the sample reached Earth,   
   a question that had plagued previous studies that found sodium chloride   
   in meteorites of a similar origin.   
      
   "The grains look exactly like what you would see if you took table salt   
   at home and placed it under an electron microscope," Zega said. "They're   
   these nice, square crystals. It was funny, too, because we had many   
   spirited group meeting conversations about them, because it was just so   
   unreal."  Zega said the samples represent a type of extraterrestrial rock   
   known as an ordinary chondrite. Derived from so-called S-type asteroids   
   such as Itokawa, this type makes up about 87% of meteorites collected on   
   Earth. Very few of them have been found to contain water-bearing minerals.   
      
   "It has long been thought that ordinary chondrites are an unlikely   
   source of water on Earth," said Zega who is the director of the Lunar   
   and Planetary Laboratory's Kuiper Materials Imaging & Characterization   
   Facility. "Our discovery of sodium chloride tells us this asteroid   
   population could harbor much more water than we thought."  Today,   
   scientists largely agree that Earth, along with other rocky planets such   
   as Venus and Mars, formed in the inner region of the roiling, swirling   
   cloud of gas and dust around the young sun, known as the solar nebula,   
   where temperatures were very high -- too high for water vapor to condense   
   from the gas, according to Che.   
      
   "In other words, the water here on Earth had to be delivered from the   
   outer reaches of the solar nebula, where temperatures were much colder and   
   allowed water to exist, most likely in the form of ice," Che said. "The   
   most likely scenario is that comets or another type of asteroid known as   
   C-type asteroids, which resided farther out in the solar nebula, migrated   
   inward and delivered their watery cargo by impacting the young Earth."   
   The discovery that water could have been present in ordinary chondrites,   
   and therefore been sourced from much closer to the sun than their "wetter"   
   kin, has implications for any scenario attempting to explain the delivery   
   of water to the early Earth.   
      
   The sample used in the study is a tiny dust particle spanning about 150   
   micrometers, or roughly twice the diameter of a human hair, from which   
   the team cut a small section about 5 microns wide -- just large enough   
   to cover a single yeast cell -- for the analysis.   
      
   Using a variety of techniques, Che was able to rule out that the sodium   
   chloride was the result of contamination from sources such as human sweat,   
   the sample preparation process or exposure to laboratory moisture.   
      
   Because the sample had been stored for five years, the team took   
   before and after photos and compared them. The photos showed that the   
   distribution of sodium chloride grains inside the sample had not changed,   
   ruling out the possibility that any of the grains were deposited into the   
   sample during that time. In addition, Che performed a control experiment   
   by treating a set of terrestrial rock samples the same as the Itokawa   
   sample and examining them with an electron microscope.   
      
   "The terrestrial samples did not contain any sodium chloride, so that   
   convinced us the salt in our sample is native to the asteroid Itokawa,"   
   he said. "We ruled out every possible source of contamination."  Zega said   
   tons of extraterrestrial matter is raining down on Earth every day, but   
   most of it burns up in the atmosphere and never makes it to the surface.   
      
   "You need a large enough rock to survive entry and deliver that water,"   
   he said.   
      
   Previous work led by the late Michael Drake, a former director of the   
   Lunar and Planetary Lab, in the 1990s proposed a mechanism by which water   
   molecules in the early solar system could become trapped in asteroid   
   minerals and even survive an impact on Earth.   
      
   "Those studies suggest several oceans worth of water could be delivered   
   just by this mechanism," Zega said. "If it now turns out that the   
   most common asteroids may be much 'wetter' than we thought, that will   
   make the water delivery hypothesis by asteroids even more plausible."   
   Itokawa is a peanut-shaped near-Earth asteroid about 2,000 feet long   
   and 750 feet in diameter and is believed to have broken off from a much   
   larger parent body. According to Che and Zega, it is conceivable that   
   frozen water and frozen hydrogen chloride could have accumulated there,   
   and that naturally occurring decay of radioactive elements and frequent   
   bombardment by meteorites during the solar system's early days could have   
   provided enough heat to sustain hydrothermal processes involving liquid   
   water. Ultimately, the parent body would have succumbed to the pummeling   
   and broken up into smaller fragments, leading to the formation of Itokawa.   
      
   "Once these ingredients come together to form asteroids, there is   
   a potential for liquid water to form," Zega said. "And once you have   
   liquids form, you can think of them as occupying cavities in the asteroid,   
   and potentially do water chemistry."  The evidence pointing at the salt   
   crystals in the Itokawa sample as being there since the beginning of the   
   solar system does not end here, however. The researchers found a vein of   
   plagioclase, a sodium-rich silicate mineral, running through the sample,   
   enriched with sodium chloride.   
      
   "When we see such alteration veins in terrestrial samples, we know   
   they formed by aqueous alteration, which means it must involve water,"   
   Che said. "The fact that we see that texture associated with sodium and   
   chlorine is another strong piece of evidence that this happened on the   
   asteroid as water was coursing through this sodium-bearing silicate."   
       * RELATED_TOPICS   
             o Space_&_Time   
                   # Asteroids,_Comets_and_Meteors # Solar_System #   
                   Space_Missions # Space_Exploration # Nebulae # NASA #   
                   Sun # Extrasolar_Planets   
       * RELATED_TERMS   
             o Definition_of_planet o Asteroid_belt o Asteroid   
             o Ceres_(dwarf_planet) o Gas_giant o Solar_system o   
             Extrasolar_planet o Comet   
      
   ==========================================================================   
   Story Source: Materials provided by University_of_Arizona. Original   
   written by Daniel Stolte.   
      
   Note: Content may be edited for style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. Shaofan Che, Thomas J. Zega. Hydrothermal fluid activity on asteroid   
         Itokawa. Nature Astronomy, 2023; DOI: 10.1038/s41550-023-02012-x   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/06/230613190805.htm   
      
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