Scientists have discovered the age of our water, and it’s old : ScienceAlert
The origin of Earth’s water has been an enduring mystery. There are different hypotheses and theories that explain how the water got here, and a lot of evidence to support them.
But water is ubiquitous in protoplanetary disks, and the origin of the water may not be so mysterious after all.
A research article in GeoScienceWorld Items shows that other young solar systems have abundant water. In solar systems like ours, water accompanies it as the young star grows and planets form. The evidence is in Earth’s heavy water content, showing that our planet’s water is 4.5 billion years old.
The article is “We drink good water that is 4.5 billion years old,and the authors are Cecilia Ceccarelli and Fujun Du. Ceccarelli is an Italian astronomer at the Institute of Planetary Sciences and Astrophysics in Grenoble, France. Du is an astronomer at the Purple Mountain Observatory in Nanjing, China.
The formation of a solar system begins with a giant molecular cloud. The cloud is mostly hydrogen, the main component of water. Helium, oxygen, and carbon follow, in order of abundance.
The cloud also contains small grains of silicate dust and carbonaceous dust. The research paper takes us through the history of water in our Solar System, and this is where it begins.
Here, in the cold reaches of a molecular cloud, when oxygen encounters a dust grain, 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 jump over the frozen dust grains until they meet the oxygen.
When that happens, two types of water react and form water ice: normal water and heavy water containing deuterium.
Deuterium is an isotope of hydrogen called heavy hydrogen (HDO). It has a proton and a neutron in its nucleus. That separates it from “regular” hydrogen, called protium. The protium has a proton but no neutron. Both isotopes of hydrogen are stable and persist to this day, and both can combine with oxygen to form water.
When water ice forms a blanket over dust grains, the authors call it the cold phase, the first step in the process they describe in their paper.
Gravity begins to exert itself on the cloud as matter accumulates in the center. More mass falls into the center of the molecular cloud and begins to form a protostar. Part of gravity is converted to heat, and in a few astronomical units (AU) from the center of the cloud, gas and dust in the disk reach 100 Kelvin (-280 Fahrenheit).
100 K is very cold in terrestrial terms, only -173 degrees Celsius. But in chemical terms, it’s enough to trigger sublimation and the ice changes phase to water vapor. Sublimation occurs in a region of hot corin, a warm envelope that surrounds the center of the cloud.
Although they also contain complex organic molecules, water becomes the most abundant molecule in corines.
Water is plentiful at this point, though all is steam. “…a typical hot corino contains about 10,000 times the water in Earth’s oceans,” say the authors. write.
That’s step two 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 rotating disk called protoplanetary disk. Everything that will eventually become the planets of the solar system and other features is within that disk.
The young protostar is still accumulating mass, and its main-sequence fusion lifetime is still 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 farthest from the young protostar are the coldest. What happens next is crucial, according to the authors.
The water ice that formed in step one turns to gas in step two, but recondenses in the coldest places in the protoplanetary disk. The same population of dust grains is again covered by an icy blanket.
But now, the water molecules in that icy mantle contain the history of water in the Solar System. “Thus, the dust grains are the guardians of the inheritance of water,” say the authors. write.
That is step three in the process.
In step four, the Solar System begins to take shape and looks like a more complete system. All the things we are used to, like planets, asteroids, and comets, begin to form and take up their orbits. And what do they originate from? Those tiny grains of dust and their twice-frozen water molecules.
This is the situation we find ourselves in today. While astronomers can’t travel back in time, they are getting better at observing other young solar systems and searching for clues to the whole process. Earth’s water also contains a critical clue: the ratio of heavy water to regular water.
Some details are left out of the simple explanation given so far. 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 water ice than at other temperatures.
The deuterium was only formed in the seconds following the big Bang. Not much was formed: only one deuterium for every 100,000 protium atoms.
That means that if deuterium were to mix uniformly with water in the Solar System, the abundance of heavy water would be expressed as 10-5. But there is more complexity to come.
In a hot corino, the abundance changes. “However, in hot corinos, the HDO/HtwoO ratio is only slightly less than 1/100,” the authors explain. (HDO are water molecules that contain two isotopes of deuterium and HtwoOr it’s regular water that contains two isotopes of protium.)
There are even more tips. “To make things even more extreme,” the authors explain“double deuterated water D2O is 1/1000 with respect to HtwoOr, that is, about 107 times larger than what would be estimated from the elemental abundance ratio D/H.”
The ratios contain such large amounts of deuterium due to super deuteration. At the time when ice forms on the surfaces of the dust grains, there are a greater number of D atoms compared to H atoms that fall on the grain surfaces.
The in-depth chemical explanation is beyond the scope of this article, but the bottom line is clear.
“There are no other ways to obtain this large amount of heavy water in hot corinos or in general,” the authors state. write. “Therefore, the abundance of heavy water is a hallmark of water synthesis in the cold molecular cloud group during the STEP 1 era.”
The important thing so far is that there are two episodes of water synthesis. The first occurs when the solar system has not yet formed and is just a cold cloud. The second is when planets are formed.
The two occur under different conditions, and those conditions leave their isotopic footprint in the water. The water from the first synthesis is 4.5 billion years old, and the question is: “How much of that ancient water made it to Earth?”
To find out, the authors looked at the only two things they could: the total amount of water and the amount of deuterated water.
like the authors put it“…that is, the ratio of heavy water to normal water, HDO/HtwoEITHER.”
More than enough water was created to account for Earth’s water. Remember that the amount of water in the hot cory was 10,000 times greater than the water on Earth, and its HDO/HtwoThe proportion of O is different from the water formed in the initial cloud.
How much water from the corino reached the Earth? A clue can be found by comparing HDO/HtwoO values in terrestrial waters with those of warm corinos.
The hot corynes are the only place where we have observed HDO in any forming solar-like planetary system. In previous research, the scientists compared those ratios to ratios in objects in our Solar System: comets, meteorites and Saturn’s icy moon Enceladus.
So they know that the abundance of heavy water on Earth, the HDO/HtwoThe proportion of O is about ten times greater than in the Universe and at the beginning of the Solar System.
“‘Heavy above normal’ water on Earth is about ten times larger than the elemental ratio D/H in the Universe and consequently at the birth of the Solar System, in what is called the solar nebula,” say the authors. explain.
The results of all this work show that between 1 and 50 percent of Earth’s water came from the early phase of the birth of the Solar System. That’s a wide range, but it’s still an important piece of knowledge.
The authors wrap things up at its conclusion.
“Water from comets and asteroids (where the vast majority of meteorites originate from) was also inherited early on in large quantities. Earth probably inherited its original water predominantly from planetesimals, which are assumed to be the precursors of the asteroids and planets that formed the Earth, rather than the comets that rained down 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 to the inner Solar System. The idea makes sense.
But this study shows that may not be true.
However, it still leaves unanswered questions. He doesn’t explain how all the water got to Earth. But the study shows that the amount of heavy water on Earth is at least the start of figuring this out.
“In conclusion, the amount of heavy water on Earth is our Ariadne’s threadthat can help us get out of the labyrinth of all the possible routes that the Solar System could have taken”, they explain.
Earth’s water is 4.5 billion years old, just as the title of the article says. At least some of it is. According to the authors, planetesimals probably carried it to Earth, but exactly how that happens isn’t clear. There is much more complexity that scientists must solve before they can solve it.
“The issue is quite complicated because the origin and evolution of Earth’s water are inevitably connected to other important players on this planet, for example, carbon, molecular oxygen, and the magnetic field,” the authors say. write.
All these things are involved in how life originated and how worlds were formed. Water likely played a role in the formation of the planetesimals that brought it to Earth. The water likely played a role in sequestering other chemicals, including the building blocks of life, in the rocky bodies that carried them to Earth.
Water is at the center of it all, and by showing that part of it dates back to the early Solar System, the authors have provided a starting point for discovering the rest.
“Here, we present a simplified early history of Earth’s water in accordance with the most recent observations and theories,” they said. write.
“A good fraction of the terrestrial water probably formed at the beginning of the birth of the Solar System when it was a cold cloud of gas and dust, frozen and preserved during the various steps that led to the formation of planets, asteroids and comets and was eventually transmitted to the rising Earth.
“How the final passage happened is another fascinating chapter,” they said. conclude.
This article was originally posted by universe today. Read the Original article.