In truth, the primordial Sun was dim, and carbon dioxide that may have frozen the planet was present—the astonishing, exquisite balance of Earth's atmosphere.
//Summary - Level-B2//
The early Earth faced many challenges after the giant impact that formed the Moon. Its first hydrogen–helium atmosphere escaped, and volcanic activity created a new one rich in toxic gases, especially carbon dioxide. At the same time, the Sun was about 30% dimmer, which should have frozen the planet. Instead, the greenhouse effect of carbon dioxide kept Earth warm enough for liquid water to exist, allowing oceans to form. Over time, carbon dioxide levels decreased, preventing overheating as the Sun grew brighter. This delicate balance between atmosphere and sunlight made Earth habitable and shaped its unique history.
1)
The “sea” that nurtured life unique in the universe, the “mountains” that seem so commonplace, and the mysterious “rivers”… The countless undulations and depressions etched into Earth's face – “terrain”. Tracing how such landscapes formed reveals Earth's history.
Focusing on the acclaimed “Earth's Resilience Trilogy” – “How Did Rivers Form?”, “How Did the Sea Form?” and “How Did Mountains Form?” – we present selected topics on terrain.
2)
Having experienced the Giant Impact theory introduced in the previous article, what was the next challenge Earth faced...? This time, we explain the complex trials Earth endured before its atmosphere formed.
The term “greenhouse effect”, now familiar in discussions of global warming, actually points to a fascinating theory: carbon dioxide was likely crucial in saving the primordial Earth from freezing temperatures.
In fact, it wouldn't have been at all strange if ‘Earth had been born only to perish immediately’... The startling details of the boldest theory on the Moon's origin: the ‘Giant Impact’ hypothesis
https://gendai.media/articles/-/156324
3)
What is the ‘Earth Calendar’ mentioned in this article?:
To grasp Earth's history – spanning roughly 4.6 billion years – intuitively, we use a ‘Earth Calendar’ as a scale. This converts 4.6 billion years into a single “year”, representing each era as a ‘date’.
Converting 4.6 billion years into a single ‘year’ yields the following:
1 month…………Approx. 380 million years
1 week…………approximately 88 million years
1 day……………approximately 12.6 million years
1 hour…………approximately 530,000 years
1 minute……………approximately 8,800 years
1 second……………approximately 146 years
Please note that due to the precision of dating methods and other factors, there is not always agreement among researchers, and new findings may also cause variations.
4)
The Birth of the Atmosphere:
We noted that various theories exist regarding how oceans filled with water emerged from the primordial magma ocean, and determining which is correct remains challenging.
However, it is certain that the formation of the oceans, alongside the establishment of the atmosphere and landmasses, and the completion of Earth's internal structure, occurred almost simultaneously and stemmed from a common cause.
This was the gravitational rearrangement of matter that happened after the melting and volatilisation of primordial substances due to the intense heat brought about by the heavy bombardment of meteorites and the giant impact.
5)
Let us first focus on the atmosphere and trace this change.
The composition of the atmosphere when the Earth first formed was vastly different from that of the present atmosphere.
This initial atmosphere is termed the “primary atmosphere”, consisting solely of hydrogen and helium – the very gases composing the Sun itself (the Sun and gases like helium will be explained shortly).
It is thought these gases escaped into space due to some cause. Many researchers believe this cause was the giant impact described previously.
At this point, the entire primary atmosphere had escaped from Earth.
6)
How Earth became shrouded in carbon dioxide:
The “secondary atmosphere” is thought to have formed when volcanic activity originating from the magma ocean caused a phenomenon called “degassing”, where magma released gases. These gases gradually accumulated in the upper atmosphere to form the atmosphere. They were gases trapped within the meteorites that formed the Earth.
7)
Its composition is thought to have closely resembled the gases emitted by modern volcanic activity. For instance, gases released from Hawaii's Kilauea volcano include hydrogen, water vapour, carbon dioxide, carbon monoxide, nitrogen, argon, chlorine gas, hydrochloric acid, sulphur, and sulphurous acid gas.
Most are highly toxic to humans. The oxygen essential to us was not yet present in the atmosphere at this time (see diagram: “Historical Changes in Atmospheric Composition”).
8)
However, on the high-temperature Earth, light substances like argon and volatile compounds could not be retained by Earth's gravity; much escaped into space. The light substances that remained on Earth initially formed an atmosphere, condensing in the colder upper atmosphere to become rain, snow, and ice.
9)
Yet, upon reaching the surface, these precipitation forms evaporated due to the high surface temperatures, escaping back into space. This implies that while the Earth's surface, specifically the landmasses, remained hot, oceans filled with liquid water could not form.
Thus, the composition of this secondary atmosphere also changed, eventually becoming dominated by carbon dioxide, supplied in great quantities by volcanoes.
10)
The Sun was dim during the Hadean Eon:
Now that carbon dioxide has become the “king” of the atmosphere, let me share a curious tale.
The immense heat and energy emitted by the Sun stem from nuclear fusion reactions converting hydrogen into helium. This process delivers heat and light even to Earth, situated 150 million kilometres away.
11)
However, during the Hadean Eon when the Sun was newly formed, nuclear fusion had only just begun. Consequently, it is thought that the Sun was not particularly bright. Estimates suggest it was approximately 30% dimmer than today.
If this were true, calculations indicate Earth's surface temperature would have fallen below 0°C. This would not merely preclude a magma ocean; it would have resulted in global glaciation.
12)
Of course, we know this cannot be the case, so we must consider whether the premise that the Sun was dim at that time is incorrect.
13)
Too bright also creates inconsistencies:
However, if the Sun had been as bright then as it is now, considering the subsequent increase in fusion reaction energy due to the Sun's growth, calculations show Earth would have long since become a scorching inferno uninhabitable for life.
In other words, whether the Sun was dim or bright at that time creates a contradiction, known as the “faint young Sun paradox”.
14)
This poses a significant problem for the theme of “How did the oceans form?”. If the Sun were dim, all water would have frozen into ice (solid). If it were bright, the water would eventually evaporate into water vapour (gas). Either way, oceans filled with liquid water could not have formed.
15)
The greenhouse effect of carbon dioxide holds the key:
The solution lay in carbon dioxide.
You are likely familiar with the term ‘greenhouse effect’ (see diagram ‘The Greenhouse Effect of Carbon Dioxide’). As mentioned earlier, when the magma ocean formed, the ‘primary atmosphere’ had entirely dissipated, giving way to a ‘secondary atmosphere’. At its core was carbon dioxide.
16)
As it remains a prime target in current global warming debates, carbon dioxide is the most potent greenhouse gas. It is thought that its overwhelming dominance in the atmospheric composition allowed the Earth to remain sufficiently warm, even with a dimmer Sun, enabling the formation of oceans filled with liquid water.
17)
However, had the greenhouse effect of carbon dioxide remained as potent as it was then, the subsequent increase in solar heat might have rendered the Earth super-hot, causing the water-filled oceans to evaporate.
This did not occur because the proportion of carbon dioxide in the atmosphere gradually decreased over time, thereby reducing the greenhouse effect. The reason for this will be explained in detail later, but for now, let us state that “by an exquisitely delicate balance, the oceans were able to remain oceans”.
In truth, the primordial Sun was dim, and carbon dioxide that may have frozen the planet was present—the astonishing, exquisite balance of Earth's atmosphere.
https://gendai.media/articles/-/156385?imp=0
