Wednesday, January 4, 2023

3807 - WATER - where did Earth's water come from?

 

     3807  - WATER  -  where did Earth's water come from?   But water is ubiquitous in protoplanetary disks, and water’s origin may not be so mysterious after all.  In other young solar systems there is an abundance water. In solar systems like ours.

           


            ---------  3807  -  WATER  -  where did Earth's water come from?

            -    I finally found something older than me and the guys in our coffee club.  It is the water in our coffee.  Earth’s water is 4,500,000,000 years old.  The origin of Earth’s water has been an enduring mystery. There are different hypotheses and theories explaining how the water got here.

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            -  (  also check out Review 3810 about water world on other planets )

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            -   The formation of a solar system starts with a giant molecular cloud. The cloud is mostly hydrogen, water’s main component. Next are helium, oxygen, and carbon, in order of abundance. The cloud also contains tiny grains of silicate dust and carbonaceous dust.

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            -    Out here in the cold reaches of a molecular cloud, when oxygen encounters a dust grain, it freezes and adheres to the surface. But water isn’t water until hydrogen and oxygen combine, and the lighter hydrogen molecules in the cloud hop around on the frozen dust grains until they encounter oxygen. When that happens, they react and form water ice, two types of water: regular water and heavy water containing deuterium.

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            -    Deuterium is an isotope of hydrogen called heavy hydrogen (HDO.) It has a proton and one neutron in its nucleus. That separates it from “regular” hydrogen, called “protium”. Protium has a proton but no neutron. Both these hydrogen isotopes are stable and persist to this day, and both can combine with oxygen to form water.

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            -    When water ice forms a mantle on dust grains,  the cold phase, step one in the process.  Gravity begins to exert itself in the cloud as matter clumps in the center. More mass falls into the center of the molecular cloud and starts forming a protostar. Some of the gravity is converted into heat, and within a few astronomical units (AU) of the cloud’s center, the gas and dust in the disk reach 100 Kelvin.

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            -   100 K is bitterly cold in Earthly terms, only -173 degrees Celsius. But in chemical terms, it’s enough to trigger sublimation, and the ice changes phase into water vapor. The sublimation occurs in a hot corino region, a warm envelope surrounding the cloud’s center. Though they also contain complex organic molecules, water becomes the most abundant molecule in corinos.

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            -    Water is abundant at this point, though it’s all vapor, a typical hot corino contains about 10,000 times the water in the Earth’s oceans.  In step two, the protostar hasn't begun fusion yet. But it still generates enough heat to sublimate the water ice on dust grains into vapor.

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            -  Step two in the processnis called the protostar phase.  Next, the star begins to rotate, and the surrounding gas and dust form a flattened, rotating disk called a protoplanetary disk. Everything that will eventually become the solar system’s planets and other features is inside that disk.

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            -        The young protostar is still gathering mass, and its life of fusion on the main sequence is still well in its future. The young star generates some heat from shocks on its surface, but not much. So the disk is cold, and the regions furthest away from the young protostar are the coldest.

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            -        The water ice that formed in step one is released into gas in step two but recondenses again in the coldest reaches of the protoplanetary disk. The same population of dust grains is again covered in an icy mantle. But now, the water molecules in that icy mantle contain the history of the water in the Solar System.

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            -  That’s step three in the process.   As the protostar continues to gather mass, it begins to rotate. The gas and dust form a rotating disk centred on the star. The water vapor from step two recondenses, and the dust grains are again covered in icy mantles.

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            -        As the protostar continues to gather mass, it begins to rotate. The gas and dust form a rotating disk centred on the star. The water vapor from step two recondenses, and the dust grains are again covered in icy mantles. But this time, the water ice retains a record of what it’s been through.

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            -        In step four, the Solar System begins to take shape and resemble a more fully-formed system. All the things like planets, asteroids, and comets, start forming and taking up their orbits. And what do they originate from? Those tiny dust grains and their twice-frozen water molecules.

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            -        Astronomers are getting better at observing other young solar systems and finding clues to the entire process. Earth’s water contains a critical hint: the ratio of heavy water to regular water.

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            -        When water ice forms in step one, the temperature is extremely low. That triggers an unusual phenomenon called super-deuteration. Super-deuteration introduces more deuterium into the water ice than at other temperatures.

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            -        Deuterium was only formed in the seconds following the Big Bang. Not much of it formed: only one deuterium for every 100,000 protium atoms. That means that if the deuterium was evenly mixed with the Solar System’s water, the abundance of heavy water would be expressed as 10-5.

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            -        In a hot corino, the abundance changes.  In hot corinos, the HDO/H2O ratio is only a bit less than 1/100,” . (HDO is water molecules containing two deuterium isotopes, and H2O is regular water containing two protium isotopes.)

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            -        The doubly deuterated water D2O is 1/1000 with respect to H2O,  about 107 times larger than what would be estimated from the D/H elemental abundance ratio.  The ratios contain such large abundances of deuterium because of super-deuteration.

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            -    At the moment that ice forms on the surfaces of the dust grains, there’s an enhanced number of D atoms compared to H atoms landing on the grain surfaces.  There are no other ways to obtain this large amount of heavy water in hot corinos nor in general.  Therefore, abundant heavy water is a hallmark of water synthesis in the cold molecular cloud clump during the STEP 1 era.

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            -        The important thing so far is that there are two episodes of water synthesis. The first happens when the solar system hasn’t formed yet and is only a cold cloud. The second is when planets form. The two happen in different conditions, and those conditions leave their isotopic imprint on the water. Water from the first synthesis is 4.5 billion years old.  “How much of that ancient water reached Earth?

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            -        More than enough water was created to account for Earth’s water. Remember that the amount of water in the hot corino was 10,000 times more than Earth’s water, and its HDO/H2O ratio is different from the water formed in the initial cloud. How much of the corino water reached Earth? A hint can be found by comparing HDO/H2O values in terrestrial water with those of hot corinos.

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            -        Hot corinos are the only place we’ve observed HDO in any still-forming, solar-type planetary systems.   Scientists compared those ratios with ratios in objects in our Solar System, comets, meteorites, and Saturn’s icy moon Enceladus. So they know that Earth’s heavy water abundance, the HDO/H2O ratio, is about ten times greater than in the Universe and at the beginning of the Solar System.

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            -    ‘Heavy over normal’ water on Earth is about ten times larger than the elemental D/H ratio in the Universe and consequently at the birth of the Solar System, in what is called the solar nebula.

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            -        The results of all this work show that between 1% and 50% of Earth’s water came from the initial phase of the Solar System’s birth.

             

            The water in comets and asteroids (from which the vast  majority of meteorites originate) was also inherited since the beginning in large quantities. Earth likely inherited its original water predominantly from planetesimals, which are supposed to be the precursors of the asteroids and planets that formed the Earth, rather than from the comets that rained on it.

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            -        Delivery by comets is another hypothesis for Earth’s water. In that hypothesis, frozen water from beyond the frost line reaches Earth when comets are disturbed and sent from the frozen Oort Cloud into the inner Solar System. The idea makes sense.  But this study shows that may not be true.

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            -        It doesn’t explain how all the water reached Earth. But the study shows that the amount of heavy water on Earth is at least the beginning of figuring this out.

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            -    Earth’s water is 4.5 billion years old.  Planetesimals probably delivered it to Earth, but exactly how that happens isn’t clear. There’s a lot more complexity that scientists need to sort through before they can figure that out.

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            -        These things are all wrapped up together in how life originated and how worlds formed. Water likely played a role in forming the planetesimals that delivered it to Earth. Water likely played a role in sequestering other chemicals, including the building blocks of life, onto rocky bodies that delivered them to Earth.

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            January 1, 2022      WATER  -  where did Earth's water come from?     3807                                                                                                                              

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