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3825 - EARTH'S
WATER - how did it get here? The origin of Earth’s water has been an enduring mystery. There
are different hypotheses and theories explaining how the water got here, and
lots of evidence supporting them. But,
water is ubiquitous in protoplanetary disks, and water’s origin may not be so
mysterious after all.
--------- 3825 - EARTH'S WATER - how did it get here?
- Other young solar systems have abundant
water. In solar systems like ours, water is along for the ride as the young
star grows and planets form. The evidence is in Earth’s heavy water content,
and it shows that our planet’s water is 4,500,000,000 years old.
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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,
H2O.. 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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- Stars form in molecular clouds, vast conglomerations
of mostly hydrogen. 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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- 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 vapour. 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 vapour. Image
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- 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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- 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 we’re
accustomed to, 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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- A protoplanetary disc surrounding the young
star “HL Tauri”. These new ALMA
observations reveal substructures within the disc that have never been seen
before and even show the possible positions of planets forming in the dark
patches within the system.
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- Earth’s water contains a critical hint,the
ratio of heavy water to regular water.
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. But there’s more complexity to come.
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- In a hot corino, the abundance changes. 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 ratios contain such large abundances of
deuterium because of super-deuteration. 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.
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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,
and the question becomes, “How much of that ancient water reached Earth?”
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- Hot corinos are the only place we’ve
observed HDO in any still-forming, solar-type planetary systems. In previous
research, 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 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.
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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. The issue is quite involved because
the origin and evolution of Earth’s water is inevitably connected with other
important participants on this planet, carbon, molecular oxygen, and the
magnetic field.
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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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- A good fraction of terrestrial water likely
formed at the very beginning of the Solar System’s birth when it was a cold
cloud of gas and dust, frozen and conserved during the various steps that led
to the formation of planets, asteroids, and comets and was eventually
transmitted to the Earth just coming into existence.
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January 10, 2022 EARTH'S WATER
- how did it get here? 3825
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--------------------- --- Sunday, January 15, 2023 ---------------------------
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