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.
-
- ( also
check out Review 3810 about water world on other planets )
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.)
-
- 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.
-
- 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.
-
- 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?
-
- 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.
-
- 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.
-
- ‘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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
January 1, 2022 WATER -
where did Earth's water come from?
3807
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“Jim Detrick” -----------
--------------------- --- Wednesday, January 4, 2023 ---------------------------
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