But water is ubiquitous in protoplanetary disks, and its origin may not be all that mysterious.
The Elements research article in GeoScienceWorld shows that other young solar systems have a lot of water.
In solar systems such as ours, water moves as young stars grow and planets form. The proof is the content of heavy water on Earth, and it shows that the water on our planet is 4.5 billion years old.
The article is titled “We Drink Good Water 4.5 Billion Years Old” and its authors are Cecilia Ceccarelli and Fujun Du. Ceccarelli is an Italian astronomer at the Institute for Planetary Sciences and Astrophysics in Grenoble, France. Du is an astronomer at the Purple Mountain Observatory in Nanjing, China.
The formation of the solar system begins with a giant molecular cloud. The cloud is mostly made up of hydrogen, the main component of water. This is followed by helium, oxygen, and carbon, in order of their abundance.
The cloud also contains tiny grains of silicate and carbonaceous dust. The research article introduces us to the history of water in our solar system, and here it begins.
Here, in the cold confines of the molecular cloud, when oxygen collides with a grain of dust, it freezes and sticks to the surface.
But water isn’t water until hydrogen and oxygen combine, and the lighter hydrogen molecules in the cloud bounce over frozen dust particles until they meet oxygen.
When this happens, they react and form water ice – two types of water: ordinary 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. This distinguishes it from the “ordinary” hydrogen called protium.
Protium has a proton, but no neutron. Both of 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 authors call this the cold phase, the first step in the process they outline in their paper.
Gravity begins to act in a cloud when matter accumulates in the center. More mass falls into the center of the molecular cloud and begins to form a protostar.
Some of the gravity is converted to heat, and within a few astronomical units (AU) of the center of the cloud, the temperature of the gas and dust in the disk reaches 100 Kelvin (-280 Fahrenheit).
100 K is bittersweet. cold by earthly standards, only -173 degrees Celsius. But from a chemical point of view, this is enough to cause sublimation, and the ice changes phase to water vapor.
Sublimation occurs in the hot corino region, the warm shell surrounding the center of the cloud.
Although they also contain complex organic molecules, water becomes the most abundant molecule in corino.
Water is plentiful at this point, although it is all steam.”… a typical hot corino contains about 10,000 times more water than Earth’s oceans,” the authors write.
This is the second step in the process described by the authors, and they call it the protostar phase.
The star then begins to rotate, and the surrounding gas and dust form a flattened spinning disk called a protoplanetary disk.
Everything that will eventually become the planets of the solar system and other objects is inside this disk.
The young protostar is still gaining mass, and its main-sequence fusion life is still far ahead.
A young star gives off some heat from jolts on its surface, but not much. Thus, the disk is cold, and the regions farthest from the young protostar are the coldest.
According to the authors, what happens next is critical.
The water ice formed in the first stage turns to gas in the second stage, but condenses again in the coldest parts of the protoplanetary disk. The same population of dust particles is again covered by an icy mantle.
But now the water molecules in this icy mantle contain the history of water in the solar system. “Thus, the dust particles are the guardians of the heredity of water,” the authors write.
This is the third step in this process.
In the fourth stage, the solar system begins to take shape and resemble a more complete system. Everything that we are used to, such as planets, asteroids and comets, begin to form and take their orbits.
And where do they come from? Those tiny dust particles and their doubly frozen water molecules.
This is the situation we are in today. Although astronomers cannot travel through time, they are getting better at observing other young solar systems and finding clues to the whole process.
Terrestrial water also contains an important clue: the ratio of heavy water to ordinary water.
Some details are left out of the simple explanation given so far. When water ice forms in the first stage, the temperature is very low.
This causes an unusual phenomenon called superdeuteration. Superdeuterium introduces more deuterium into water ice than at other temperatures.
Deuterium was formed only a few seconds after the Big Bang. Not much was formed: only one deuterium for every 100,000 protium atoms.
This means that if deuterium is evenly mixed with the water of the solar system, the heavy water content will be expressed as 10-5. But there are still many challenges ahead.
In hot Corino, abundance changes. “However, in hot corinos, the HDO/H 2 O ratio is only slightly less than 1/100,” the authors explain.
(HDO are water molecules containing two isotopes of deuterium, and H 2 O is ordinary water containing two isotopes of protium.)
There are even more extremes. “To make things even more extreme,” the authors explain, “doubly deuterated water D2O is 1/1000 of H 2 O, which is about 107 times what would be estimated from the D/H element abundance ratio.”
The ratios contain such a high deuterium content due to overdeuteration. At the moment when ice forms on the surface of the dust grains, the number of D atoms increases in comparison with the H atoms that fall on the surface of the dust grains.
An in-depth chemical explanation is beyond the scope of this article. article, but the conclusion is clear.
“There are no other ways to obtain such a large amount of heavy water in hot corinos,” the authors write.
“Therefore, the abundance of heavy water is a hallmark of water synthesis in a cold molecular cloud cluster during the STEP 1 era.”
At the moment, what is important is that there are two episodes of water synthesis. The first happens when the solar system has not yet formed and is just a cold cloud. Secondly, when the planets are formed.
Both occur under different conditions, and these conditions leave their isotopic imprint on the water.
The water of the first fusion is 4.5 billion years old, and the question arises: “How much of this ancient water reached the Earth?”
To find out, the authors observed the only two things they could: the total amount of water and the amount of deuterated water.
As the authors put it,”…namely, the ratio of heavy water to ordinary water, HDO/H 2 O”.
More than enough water has been created to make up the earth’s water supply. Remember that the amount of water in the hot corino was 10,000 times greater than the water on Earth, and its HDO/H 2 O ratio is different from the water formed in the original cloud.
How much Corino water has reached Earth? A hint can be found by comparing the HDO/H 2 O values in terrestrial water with those of hot corinos.
Hot Corinos is the only place where we have observed HDO in any still waters. -forming solar-type planetary systems. In previous studies, scientists have compared these relationships with those of objects in our solar system comets, meteorites and Saturn’s icy moon Enceladus.
So, they know that the abundance of heavy water on Earth, HDO/H 2 O, is about ten times greater than in the universe and at the beginning of the solar system.
“Heavy water on Earth is about ten times greater than the elemental D/H ratio in the universe and therefore at the birth of the solar system, in the so-called solar nebula,” the authors explain.
The results of all this work show that between 1 and 50 percent of the water on Earth came from the initial phase of the birth of the solar system. It’s a wide range, but it’s still an important piece of knowledge.
The authors summarize in their conclusion.
“Water in comets and asteroids (of which the vast majority originate meteorites) has also been inherited from the beginning in large quantities.
The Earth likely inherited its original water primarily from planetesimals, which are assumed to be the precursors of the asteroids and planets that formed the Earth, rather than from comets that rained down on it. .”
Delivery by comets is another hypothesis for terrestrial water. According to this hypothesis, frozen water from 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 this may not be true.
However, it still leaves questions unanswered. This does not explain how all the water reached the Earth. But the study shows that the amount of heavy water on Earth is at least the beginning of understanding.
“In conclusion, the amount of heavy water on Earth is our Ariadne thread that can help us get out of the maze of all the possible paths the solar system could take,” they explain.
The waters of the Earth are 4.5 billion years old, as the title says. At least some of them.
According to the authors, planetozimals probably brought it to Earth, but exactly how this happens is unclear. Scientists have more complexities to deal with before they can figure it out.
“The question is rather complex, because the origin and evolution of terrestrial water are inevitably related to other important players on this planet, such as carbon, molecular oxygen and the magnetic field,” the authors write.
All these things are closely related to how life began and how worlds formed. Water likely played a role in the formation of the planetesimals that brought it to Earth.
Water likely played a role in the release of other chemicals, including the building blocks of life, on the rocky bodies that brought them to Earth.
Water is at the center of it all, and by showing that some of its dates are before the very beginning of the solar system, the authors provided a starting point for figuring out the rest of it.
“Here we have presented a simplified early history of Earth’s water in accordance with the most recent observations and theories,” they write.
“A significant portion of Earth’s water probably formed at the very beginning of the birth of the solar system, 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 transferred to the nascent Earth.
“How the last pass happened is another fascinating chapter,” they conclude.
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