Juno: Our boat is now transitioning from the Brahmaputra to the Jamuna, making this the perfect time to begin discussing the Chemical Era. The connection between Krishna of Mathura and the Taj Mahal of Agra offers a deep metaphorical link to Yamuna’s lifeblood. But to start this era, we must first revisit Earth’s 4.5-billion-year history because the first 4 billion years are essentially Earth’s Chemical Era.

Socrates: Does this mean we’ll only focus on Earth during the Chemical Era?

Juno: Since the complex chemistry of only one planet is fully known to us, it makes sense to concentrate on Earth during this era. However, by the end of the discussion, we’ll also talk about ways to search for complex molecules and life on other planets within or beyond the Solar System. In fact, our approach here is quite similar to the Planetary Era. During the Planetary Era, Hermes mainly focused on the Solar System but concluded by discussing the discovery of planets around other stars.

Socrates: A good plan. Then begin.

Juno: About 4.5 billion years ago, Earth was born. For the first 500 million years, its surface was extremely hot, dominated by volcanoes, and it rotated rapidly on its axis, completing a single rotation in just 12 hours. On top of that, leftover fragments of rocks and comets from the formation of the inner planets bombarded Earth during the Late Heavy Bombardment. This era is known as the Hadean Era. Some intact zircon (ZrSiO₄) crystals from that time indicate that oceans already existed.

Socrates: How did the oceans form?

Juno: Water vapor escaped from Earth’s interior through volcanic activity and cracks in the crust, a process called outgassing. Once the Earth cooled, this vapor condensed to form clouds, which eventually brought rain. This rain contributed to the formation of the oceans. A significant portion of the ocean water likely also came from comets and asteroids during the bombardment. At that time, the Earth was likely covered entirely by ocean with no large continents. Scattered volcanic islands, essentially the peaks of underwater volcanoes, dotted the ocean. While water and vapor contained oxygen, and oxygen was present in zircon crystals, there was no free molecular oxygen (O₂) in the atmosphere.

Socrates: Does the figure here show the increase in atmospheric oxygen?

Juno: It shows not only the rise of oxygen in the atmosphere but also significant chemical changes in the oceans. The Archean Era began after the Hadean, approximately 4 billion years ago. However, Earth’s crust began to stabilize around 3.8 billion years ago, when the precursors of modern continents, called microcontinents, started to form. You can see in the figure that many critical events occurred around 3.5 billion years ago. During this time, microbial mats—layered colonies of cyanobacteria—existed on the ocean’s surface. The top layer of cyanobacteria had already started producing oxygen through photosynthesis by combining sunlight, carbon dioxide, and water. Over time, these mats thickened and eventually fossilized into rocks called stromatolites. By analyzing stromatolites, we learned when free oxygen started to appear.

Socrates: On the x-axis, the figure shows time, but the y-axis doesn’t directly indicate oxygen levels. Instead, it shows oxygen’s contribution to atmospheric pressure, where 1 represents 100%, 0.1 represents 10%, and 0.01 represents 1%. Does this indirectly represent oxygen levels?

Juno: Yes, you can think of it that way. Today, oxygen levels are about 21%, and it started rising from near-zero around 3.2 billion years ago, as marked by the dashed line. Early photosynthesis wasn’t oxygenic, meaning it didn’t produce oxygen. During this period, bacteria combined oxygen with iron and water, forming iron compounds at the ocean floor. Oxygenic photosynthesis began in earnest about 3 billion years ago. Around this same time, microcontinents merged to form larger landmasses. The newly produced oxygen reacted with iron in the oceans, filling them with iron compounds. This is what the figure refers to as an “Iron Ocean.”

Socrates: The figure suggests that the amount of oxygen in the atmosphere didn’t increase significantly until much later, despite its early production. Oxygen production began around 3.1 billion years ago, but the Great Oxidation Event (GOE) happened 2.1 billion years ago. Does this mean oxygen couldn’t accumulate in the air for nearly a billion years due to iron in the ocean?

Juno: Yes, iron was one reason. Another was the presence of many microbes in the ocean that used oxygen for metabolism. Only after the iron available for oxidation in the ocean decreased did cyanobacteria-produced oxygen start to mix into the air. In a relatively short time, atmospheric oxygen rose to nearly 1%. This oxygen then oxidized sulfur, dissolving it into the oceans, leading to what we call the “Sulfur Ocean.” How oxygen levels rose from 1% to 20% is a topic for the Biological Era, not now.

Socrates: Then let’s return to the roots of the Chemical Era. You mentioned elements like zirconium, silicon, oxygen, iron, sulfur, and carbon. But we know the universe was primarily made of hydrogen and helium after the Big Bang. Where did all the other chemical elements come from?