Lecture 3 - Atmosphere, Oceans and Life On Earth Flashcards
What are conditions required for a Planet to have an atmosphere?
It needs to be a sufficient size and mass to generate a significant gravitational pull.
Cool enough so that the particles have a kinetic energy less than what is required to escape the gravitational pull.
How did the Earth’s atmosphere form?
Degassing of elements on Earth. These gases would have been released from volcanoes.
What gases do volcanoes mainly release?
Primarily water vapour but also large quantities of hydrogen, carbon dioxide and sulphur dioxide.
What evidence is there for degassing?
In the helium burning process ³⁶Ar is produced. However, ⁴⁰Ar is in the atmosphere and this is derived from the radioactive decay of ⁴⁰K.
Additionally, there is insufficient potassium at the surface to generate all of the ⁴⁰Ar in the atmosphere which shows that the decay happened deeper in the Earth’s surface and was released into the atmosphere.
What are the two factors that determine whether an element is lost from the Earth’s gravitational pull?
The mass of the element.
The gravitational force exerted on it.
Which element has a higher residence time in the atmosphere - He or Ne?
Neon as it has a greater mass.
Helium has a residence time of 1 million years whereas Neon has a residence time of several billion years.
How is hydrogen lost from the atmosphere?
Photolysis of water in the upper atmosphere generates free hydrogen atoms.
How did the oceans form?
Once the Earth cooled below 100 degrees Celcius, water vapour in the atmosphere would have condensed and rained to form the Earth’s oceans.
When did the oceans form?
3.8 billion years ago. This is known as rocks that must have formed underwater have been dated to that time.
What equations occurred when the oceans formed to impact ocean chemistry?
CO₂ + H₂O —> H₂CO₃ —> H⁺ + HCO₃
HCl + H₂O —> H⁺ + Cl⁻
SO₂ + H₂O —> H₂SO₃
Why is nitrogen the main component of the atmosphere?
It didn’t dissolve into the oceans as it is insoluble.
What is Henry’s Law?
Solubility = k x P
k = Solubility Constant
P = Overlying Pressure in the Atmosphere
What would happen to any acids formed?
They would immediately be neutralised by reaction with surface minerals.
What was the Earth’s early atmosphere like?
It was reducing and probably composed of gases including nitrogen, water, carbon dioxide and methane. Although a small amount of oxygen would form by photolysis but this would have rapidly reacted.
What feature shows that the Earth’s atmosphere took a long time to become oxidising?
Banded Iron Formations.
What is the frequency of banded iron formations?
They are common in the rock record before 1.9 billion years ago, rare between 1.9 billion years ago and 0.7 billion years ago and they are not present in the past 0.7 billion years.
How are BIFs formed?
The evaporation of seawater with high concentrations of iron would occur. For iron to be dissolved it must be in the 2+ from as Fe³⁺ is insoluble. To form the bands of iron rich rocks and iron poor rocks, the atmosphere must have been fluctuating between oxidising and reducing.
In times where the atmosphere was reducing, bands of iron would be formed. In times where the atmosphere was oxidising, the band would be deficient in iron.
How have oxygen concentrations changed over time?
Originally, any oxygen that was generated would have reacted with rocks and minerals at the surface. Once all of the rocks and minerals have been oxidised, free oxygen starts to accumulate in the atmosphere and concentrations reached current levels 400 million years ago.
What are the two mechanisms responsible for the increase in oxygen?
Photolysis of water in the upper atmosphere.
Photosynthesis.
