Planet Earth/4a. The Air You Breathe

Take a Deep Breath
Take a deep breath. The air that you inhale is composed of a unique mix of gasses that form the Earth’s atmosphere. The Earth’s atmosphere is the gas-filled shell around a sphere representing the outer most portion of the planet. Understanding the unique mix of gasses within the Earth’s atmosphere is of vital importance to living organisms that require the presence of certain gasses for respiration. Air in our atmosphere is a mix of gasses with very large distances between individual molecules. Although the atmosphere does vary slightly between various regions of the planet, the atmosphere of Earth is nearly consistent in its composition of mostly Nitrogen (N2), representing about 78.08% of the atmosphere. The second most abundant gas in Earth’s atmosphere is Oxygen (O2) representing 20.95% of the atmosphere. This leaves only 0.97%, of which 0.93% is composed of Argon (Ar). This mix of Nitrogen, Oxygen and Argon is very unique among the solar system especially compared to neighboring planets, as Mars has an atmosphere of 95.32% Carbon dioxide (CO2), 2.6% Nitrogen (N2) and 1.9% Argon (Ar). While Venus has an atmosphere of 96.5% Carbon dioxide (CO2), 3.5% Nitrogen (N2) and trace amounts of Sulfur dioxide (SO2). Earth’s atmosphere is strange in its abundance of Oxygen (O2) and very low amounts of Carbon dioxide (CO2). However, evidence exists that Earth began its early history with an atmosphere similar to Venus and Mars, an atmosphere rich in carbon dioxide.

Earth’s Earliest Atmosphere
Evidence for Earth’s early atmosphere comes from the careful study of moon rocks brought back to Earth during the Apollo missions, which show that lunar rocks are depleted in carbon, with 0.0021% to 0.0225% of the total weight of the rocks composed of various carbon compounds (Cadogan et. 1972: Survey of lunar carbon compounds). Analysis of Earth igneous rocks show that carbon is more common in the solid Earth, with percentages between 0.032% and 0.220%. If Earth began its history with a similar rock composition found on the Moon (during its molten early history), most of the carbon on Earth would have been free as gasses of carbon dioxide and methane in the atmosphere, accounting for an atmosphere that was an upwards of 1,000 times denser, and containing a majority of carbon dioxide, similar to Venus and Mars. Further evidence from ancient zircon crystals indicate low amounts of carbon in the solid Earth during its first 1 billion years of history, and supports an early atmosphere composed mostly of carbon dioxide.

Today Earth’s rocks and solid matter contain the vast majority of carbon (more than 99% of the Earth’s carbon), and only a small fraction is found in the atmosphere and ocean, whereas during its early history, the atmosphere appears to have been the major reservoir of carbon, containing most of the Earth’s total carbon, with only a small fraction lockup in rocks. Over billions of years, in the presence of water vapor, the amount of carbon dioxide in the atmosphere decreased, as carbon was removed from the atmosphere in the form of carbonic acid, and deposited as calcium carbonate (CaCO3) into crustal rocks. Such scrubbing of carbon dioxide from the atmosphere did not appear to occur on Venus and Mars which both lack large amounts of liquid water and water vapor on their planetary surfaces. This also likely resulted in a less dense atmosphere for Earth, which today has a density of 1.217 kg/m3 near sea level. Levels of carbon dioxide in the Earth’s atmosphere dramatically decreased with the advent of photosynthesizing life forms and calcium carbonate skeletons which further pulled carbon dioxide out of the atmosphere and accelerated the process around 2.5 billion years ago.

Water in the atmosphere
It should be noted that water (H2O) makes a significant component of the Earth’s atmosphere, as evaporated gas from Earth’s liquid oceans, lakes, and rivers. The amount of water vapor in the atmosphere is measured as Relative humidity. Relative humidity is the ratio (often given as a percentage) between the partial pressure of water vapor to the equilibrium pressure of liquid water at a given temperature on a smooth surface. A relative humidity of 100% would mean that the partial pressure water vapor is equal to the equilibrium pressure of liquid water, and would condense to from droplets of water, either as rain or dew on a glass window. Note that relative humidity is not an absolute measure of atmospheric water vapor content, for example a measured relative humidity of 100% does not mean that the air contains 100% water vapor, nor does 25% relative humidity mean that it contains 25% water vapor. In fact, water vapor (H2O) accounts for only between 0 to 4% of the total composition of the atmosphere, with 4% values found in equatorial tropical regions of the planet, such as rainforests. In most places, water vapor (H2O) represents only trace amounts of the atmosphere and is found mostly close to the surface of the Earth. The amount of water molecules air can hold is related to both its temperature and pressure. The higher the temperature and lower the pressure the more water molecules are found in the air. Water molecules are at an equilibrium with Earth’s air, however, if temperatures on Earth’s surface were to rise above 100° Celsius, the boiling point for water, the majority of water on the planet would be converted to gasses, and make up a significant portion of Earth’s atmosphere as water vapor. Scientists debate when temperatures on Earth’s surface dropped below this high value and when liquid oceans first appeared on the surface of the planet, but by 3.8 billion years ago, Earth appeared to have oceans present.

Before this, was the period of time of a molten Earth called the Hadean (named after Hades, the underworld of Greek mythology). Lasting between 500-700 million years, the Hadean was an Earth that resembled the hot surface of Venus, but consistently bombarded with meteorite impacts and massive volcanic eruptions and flowing lava. Few, if any rocks are known from this period of time, since so much of the Earth was molten and liquid at this point in its history. Temperatures must have dropped, leading to the appearance of liquid water on Earth’s surface and resulting in less dense atmosphere. This started the lengthy process of cleansing the atmosphere of carbon dioxide.

For the next 1.3 billion years, Earth’s atmosphere was a mix of water vapor, carbon dioxide, nitrogen, and argon, with traces of foul-smelling sulfur dioxide (SO2), nitrogen oxides (NO2). It is debated whether there may have been pockets of hydrogen sulfide (H2S), methane (CH4) and ammonia (NH4), or whether these gas compounds were mostly oxidized in the early atmosphere. However, free oxygen (O2) was rare or absent in Earth’s early atmosphere, as free oxygen as a gas would only appear later, and when it did, it would completely alter planet Earth.