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 ====== 5. Chemical Age ======
-**Juno:** Our boat is now moving from the Brahmaputra into the Jamuna, this is just the right time to start talking about the Chemical Age, because through Krishna of Mathura and the Taj Mahal of Agra the Jamuna has a profound symbolic connection with life. But to begin this age we need to brush up a little on Earth’s 4.5-billion-year history, because the first 4.0 billion years of it will be our terrestrial Chemical Age.
-**Socrates:** So in the Chemical Age we will focus only on Earth?
-**Juno:** Since we know the history of complex chemistry on only one planet, it is logical in this age to think about Earth. But at the end of the discussion we will also talk about ways of searching for complex molecules and life on other planets, inside or outside the Solar System. In fact, our plan here is quite similar to that of the Planetary Age. In the Planetary Age Hermes mainly focused on the Solar System, but in the end he also spoke about the discovery of planets around other stars.
-**Socrates:** Good plan. Then begin.
 ===== - Seas and Atmosphere =====
-**Juno:** About 4.5 billion years ago Earth was born. For the first 500 million years the surface was very hot and full of volcanoes, and it spun very fast on its own axis, taking only 12 hours for one rotation. On top of that, many leftover pieces of rock and comets from the construction of the inner four planets kept crashing onto Earth from space during the Late Heavy Bombardment. This period is called the Hadean Eon. Some intact zircon (ZrSiO$_4$) crystals from that time have been found, from which we understand that oceans already existed then.
-**Socrates:** How did the oceans form?
-**Juno:** Through volcanoes and all the cracks in the crust, water vapor rose from inside Earth into the atmosphere, this process is called outgassing. After Earth cooled, this vapor formed clouds, and clouds produced rain. One reason for the oceans is this rain. But a large part of ocean water probably also came through asteroids and comets during the bombardment. At that time the whole Earth was probably surrounded by ocean, with no large continents. Scattered across the ocean were volcanic islands, the peaks of various volcanoes. There was oxygen in vapor and water, and oxygen in zircon, but there was no free molecular oxygen (O$_2$) in the atmosphere at all.
-**Socrates:** Is the rise of oxygen in the atmosphere what this figure shows?
 {{:bn:courses:ast100:oxygen.webp?nolink&800|}}
-**Juno:** I am showing not only the increase of oxygen in the air, but also important changes in the chemical composition of the oceans. After the Hadean, the Archean Eon began 4.0 billion years ago. But Earth’s crust began to stabilize around 3.8 billion years ago, when the precursors of today’s continents, various microcontinents, started forming. In the figure you can see that many important events took place 3.5 billion years ago. At that time there were microbial mats, that is, layered colonies of cyanobacteria on the surface of the ocean water. The top layer of cyanobacteria had already begun producing oxygen by combining sunlight, carbon dioxide, and water, a process called photosynthesis. Over time these mats thickened and eventually turned into rock, known as stromatolites. By analyzing stromatolites we have learned how long ago free oxygen first began to be produced.
-**Socrates:** On the X axis the figure shows time, but on the Y axis it does not show oxygen content directly, it shows the contribution of oxygen to atmospheric pressure, where 1 means 100%, 0.1 means 10%, 0.01 means 1%. Does this basically indicate oxygen concentration?
+===== - Periodic Table =====
-**Juno:** Yes, you can take it that way. At present oxygen makes up about 21% of the atmosphere, and it began to rise from zero around 3.2 billion years ago, where the dashed line starts. The earliest photosynthesis was not oxygenic, meaning it did not produce oxygen. At that time bacteria mixed oxygen with iron and water to form iron compounds on the ocean floor. Oxygenic photosynthesis began properly around 3.0 billion years ago. At the same time microcontinents were joining to form various continents. The newly produced oxygen reacted with iron on the seafloor, filling the oceans with iron. In the figure this is what is meant by “Iron Ocean.”
-**Socrates:** From the figure it seems that for a long time after free oxygen began to be produced the amount of oxygen in the atmosphere did not increase. Oxygen production began about 3.1 billion years ago, but the Great Oxidation Event happened 2.1 billion years ago. For almost a billion years did oxygen in the air fail to rise because of the iron in the oceans?
+[[https://pubchem.ncbi.nlm.nih.gov/periodic-table/|{{:bn:courses:ast100:periodic-table.webp?nolink|}}]]
-**Juno:** Yes, iron was one reason. Another reason may have been microbes in the ocean that lived by metabolizing oxygen. Only after the amount of oxidizable iron in the ocean decreased did the oxygen produced by cyanobacteria start mixing into the air, and in a very short time oxygen in the atmosphere rose to nearly 1%. Because of this oxygen, sulfur was oxidized and began to mix into the ocean, giving us the “Sulfide Ocean.” How oxygen rose from 1% to 20% is a subject for the Biological Age, not now.
-**Socrates:** Then now go back to the beginning of the Chemical Age. You have mentioned zirconium, silicon, oxygen, iron, sulfur, carbon and many elements. But we know that after the Big Bang the universe mainly produced hydrogen and helium. Where did all the other chemical elements come from?
-===== - Periodic Table =====
+{{:bn:courses:ast100:starfurnace.webp?nolink|}}
-**Juno:** Before understanding where they came from, we need to look once at the periodic table. The pier you see on the left is actually Shakhahati Char, in the middle of the Jamuna. If we dock the boat at that ghat we will see a marvelous seven-story building that has been built in the form of the periodic table.
-**Socrates:** Then let’s go.
-**Juno:** Now you can all see it, the seven-story building faces the Jamuna, and each floor has 18 rooms. The top floor is number 1, and the very bottom one is number 7. From room number 3 on the bottom two floors, a two-story pier extends out toward the river. This pier is where the boat can be docked.
+===== - Life on Earth =====
-[The boat docks. Socrates and the other seven admire the building while still sitting in the boat.]
-**Ishtar:** Then by showing us the building, explain the beauty of the periodic table.
-[[https://pubchem.ncbi.nlm.nih.gov/periodic-table/|{{:bn:courses:ast100:periodic-table.webp?nolink|}}]]
+{{:bn:courses:ast100:life.webp?nolink|}}
-**Juno:** Each floor is a period (row) of the periodic table, and each room is a group (column). Since there are 7 periods, there are 18 groups. The columns can again be divided into four blocks: s, p, d, f. The first two columns make the s-block. In this block hydrogen is a nonmetal, besides that the first column are alkali metals (red), and the second column are alkaline earth metals (purple). Helium in the last column is also placed in the s-block. Except helium, everyone in columns 13 to 18 are in the p-block. Among them are metalloids, post-transition metals (green), halogens, nonmetals (yellow) and noble gases (brown). A few of these nonmetals are very important for life, especially carbon (C), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S), abbreviated as CHNOPS. Columns 3 to 12 form the d-block, everyone here is a transition metal (blue). And the pier where we have docked, all the elements here belong to the f-block (cyan), on the bottom floor the actinides (beginning with Ac), and on the floor above them the lanthanides (beginning with La); many of them are radioactive.
-**Socrates:** Why exactly 118 rooms?
-**Juno:** Because until now a total of 118 basic atoms have been discovered. Hydrogen’s atomic number is 1, oganesson’s is 118. In the nucleus at the center of an atom there are protons and neutrons, and around them are electrons. The number of protons in the nucleus is the atomic number. If there are as many electrons as protons, the atom is neutral; if there are more or fewer electrons than protons, ions are obtained: more means negative ion (since electrons are negative), fewer means positive ion. But the property of an atom is determined by the number of protons.
+{{:bn:courses:ast100:dna.webp?nolink&800|}}
-**Socrates:** What do you mean by property of an atom?
-**Juno:** You will understand with an example. Gold (Au, from the Latin Aurum) has 79 protons, and gold is such a solid that even at 1000 degrees Celsius it remains hard, to melt it requires 1064° Celsius. But by adding just one proton to it we get mercury (Hg), which melts and becomes liquid already at −40° Celsius, which is why at room temperature mercury is liquid inside a thermometer. That means just one proton can change the property of an atom so drastically.
+===== - Habitable Zone =====
-**Socrates:** I see. So the element Moscovium at number 115, does that mean it is found only in Moscow?
-**Juno:** You should cut down your annoying comedy, Socrates.
+{{:bn:courses:ast100:habitable-zone.webp?nolink|}}
-**Socrates:** Leave aside judgment and give the answer.
-**Juno:** Uranium’s nucleus has 92 protons, plutonium heavier than that has 94. Heavier than this are not found naturally in nature, scientists synthesized them artificially in the lab. In making Moscovium, Moscow’s scientists had the greatest contribution, that is why such a name.
-**Socrates:** Why are there no elements heavier than plutonium in nature?
+{{https://upload.wikimedia.org/wikipedia/commons/thumb/f/fb/Diagram_of_habitable_zone_rocky_exoplanets%2C_from_NASA_Exoplanet_Archive_and_Gaia_DR3_data.png/1280px-Diagram_of_habitable_zone_rocky_exoplanets%2C_from_NASA_Exoplanet_Archive_and_Gaia_DR3_data.png?nolink}}
-**Juno:** To understand that we need to go back to the story of the Particle Age. We heard from Ravi that if you bring two particles with the same charge very close, then stronger than their electromagnetic repulsion becomes the attraction of the strong force. That is why so many protons with positive charge can stay together in a nucleus. But if there are more than 94 protons, the nucleus cannot remain stable. That is why they are not in nature, they have to be made in the lab.
-**Socrates:** Suppose we made them in the lab, but how did so many elements form naturally? In the Particle Age we saw that after the Big Bang the universe mainly produced hydrogen (76 percent) and helium (24 percent). Where did all the other elements come from?
-{{:bn:courses:ast100:starfurnace.webp?nolink|}}
-**Juno:** That is what is shown in this diagram, through a star 500 times bigger than the Sun. A few elements heavier than helium were produced in very small amounts right after the Big Bang, but almost all the elements of the periodic table were born inside stars. The first generation of stars were much more massive than the Sun, which is why they were able to produce many heavy elements. How that happens we heard in the Stellar Age.
-**Socrates:** Actually we did not hear. Mars said that in the cores of massive stars elements up to iron are created, but he did not explain well how.
+===== - Searching for Life =====
-**Juno:** Then look again at the diagram above. You will see that in the final stage of life the core of a massive star looks like an onion, meaning it has many layers. At the very center is iron, outside it are several shells: first shell silicon, then successively magnesium, neon, oxygen, carbon, helium, and hydrogen shells. The star has built these shells of elements throughout its life. Let us hear again how. After all the hydrogen in the core has been converted into helium, nuclear fusion stops, and without outward pressure gravity compresses and heats the star. Then at one point the helium at the core’s center produces carbon, while outside remains a shell of helium. After all the helium in the core turns into carbon, fusion again stops, and as before the star compresses and heats up. As a result carbon in the core produces oxygen, outside remains a shell of carbon, and outside that still the previous helium shell. In this way, through the interplay of fusion and gravity, one layer after another is born. The heaviest element is at the center, the further out we go the lighter the elements we find.
-**Socrates:** On the right side of the diagram is it showing how long each reaction takes?
+{{:bn:courses:ast100:spectra.webp?nolink|}}
-**Juno:** It is showing how long each reaction continues in the womb of the star, and along with that the temperature required for each fusion reaction is shown in megakelvin and gigakelvin units. Hydrogen fusion that happens at 5 megakelvin lasts for 7 million years. But silicon fusion that happens at 2.5 gigakelvin takes place within just 1 day. That means the iron core of a massive star is made in just one day, this day can be called the last day of its life.
-**Socrates:** But iron has only 26 protons. Then where did all the elements from cobalt with 27 protons up to plutonium with 94 protons come from? The star, you said, has already reached the last day of its life.
-**Juno:** In the Stellar Age we heard that at death such massive stars explode as supernovae. At death through one enormous explosion the star gives us all the other elements as a gift. This is the star’s last donation to the universe. Let me explain how. Elements heavier than iron cannot be formed through normal fusion, because to make them requires investing more energy than is returned after they are made. Nature does not allow such losing reactions to happen. But during a supernova explosion suddenly such a vast amount of energy is released inside the star that it can be invested in reactions to make heavier elements. At the moment of explosion within only a few seconds many elements heavier than iron are born. But besides supernovae there are also some slower processes through which heavy elements can be made. We will not go into that detail.
+{{:bn:courses:ast100:transmission-spectra.webp?nolink|}}