Origins

Formation Of The Atmosphere and Oceans

Yay!  The Earth now has layers!

Now what?

Formation of the First Atmosphere

Not long after the formation of our moon, the first atmosphere formed via outgassing.  What's outgassing?  Volcanoes erupt not just lava, but gasses as well.  Gravity traps these gasses close to the planet, creating an atmosphere. 

However, solar radiation stripped away the initial atmospheres of the planets and also blew away the remains of nebular gases in the solar system.  The planets in the inner solar system - Mercury, Mars, Earth, and Venus - were affected more than the planets in the outer solar system (everyone else).  Why?  Proximity to the Sun!  Solar radiation is the strongest closest to the sun, so the radiation hits the inner planets with more force than the outer ones. 

Think of a fan.  You feel the air coming from the fan more strongly than if you're standing further away.  Now, imagine you are holding a birthday cake with candles on it - the amount of candles is up to you.  The closer the cake is to the fan, the easier it is for the candles to be blown out, right?  The same is true for the atmospheres of planets.  The planets closer to the sun will have their atmospheres blown away more easily than those planets that are further away.

Eventually, though, things calmed down and gravity kept Earth's atmosphere close to the planet.

Water, Water . . . When did we get water?

Where Earth’s water originated from is hotly debated.  Initially, it was assumed that it came from the condensation of gases into clouds, then formed rain.  But recent studies suggest that the origins of Earth’s water are more complex.

The first sources are called ‘internal’ sources – that is, sources that come from the Earth itself.  The first is volcanic outgassing - yes, the same outgassing that created the atmosphere.  Water vapor and other gases are emitted from a volcano when it erupts.  There was a lot of volcanic activity on our planet back then, so this certainly accounts for some of the water.  The next is the melting of water-rich minerals.  Many minerals – like olivine – have a lot of water in their structure.  When melted, that water is released.  Many of these minerals exist deep inside the Earth.  However, when these minerals are brought to the surface, the lowered pressure can cause the minerals to melt.

Not all of Earth's water was derived from internal sources.  Recent research indicates that additional water was brought to primordial Earth from extraplanetary sources, like comets, meteoroids, and protoplanets.  We'll take a look at these sources later.

Back to the atmosphere.  At this time, around 3.8 billion years ago, Earth was very hot.  Eventually, the atmosphere cooled enough to form clouds, and then those clouds formed the first rains.  These rains did three things:

1. Cooled the surface of Earth
2. Began to erode the surface rocks
3. Formed first oceans

It's estimated that the rains may have lasted as long as 25 million years, and water may have covered the Earth’s surface for 200 m.y.  How do we know this?  The evidence comes in the form of rocks.  At first, the only rocks were basalt, a type of extrusive igneous rock formed by shield volcanoes like Kilauea in Hawai'i.  Then the first sedimentary rocks enter into the rock record. 

How can we tell?  Sedimentary rocks form from sediments - broken bits of rock.  Thus, the only way you can get sedimentary rocks is from sediments, and sediments come from broken bits of rock, and the only rocks available were the original basalts.

The erosion of basaltic crustal rocks at the surface had two important effects:

• “locked” CO₂ into rocks
• Formed first salts in the oceans

How can scientists tell that CO₂ was "locked" into rocks?  By the formation of a new type of rock called limestone.  Most elements are friendly and want to bond with one another.  Calcium is an element common in rocks like basalt, and CO₂ was abundant in the early atmosphere due to volcanic outgassing.  The Calcium dissolved in the water got together with the CO₂ dissolved in the water and a new compound, Calcium Carbonate (CaCO₃ aka calcite) was born.  Calcite will form a mud on the seafloor, then eventually will lithify into a rock we call limestone.

Image source:  Sonjia Leyva, own work, © 2020.

The broken bits of rocks did something else - formed the raw materials for salts in the ocean.  Data suggests that ocean salinity has been nearly constant for the past 4 billion years

Image source:  Sonjia Leyva, own work, © 2020.

Formation of our current atmosphere

Take a look at the chart below.  The numbers themselves aren't as important as the trend of the numbers.  Note how similar the compositions of Venus, Early Earth, and Mars are, then compare those to the atmospheric composition of Earth today.

Around 3.5 billion years ago, our atmosphere began to change towards the present due to:

1. Breakdown of crust into sediments to form ROCKS (see above).
2. Release of oxygen by the ancestors of green plants (2 billion years ago).

The original crustal rocks were likely basalts.  Calcium, released from minerals by the physical weathering of basalt, began to combine with CO₂ to create a new mineral - CaCO₂, aka calcite, which then created limestones.  This combination, over time, helped to deplete our atmosphere of CO₂.

Plants are too sophisticated a species at this time in Earth's history.  Cyanobacteria are one of the most successful species on our planet, forming at least 3.5 billion years ago, and they’re still around today.  Plants take in CO₂ and release O₂, slowly changing the composition of our atmosphere.

Thus, between the formation of limestones (and other minerals & rocks), and photosynthesis by plants, our atmosphere changed from CO₂ rich and O₂ poor to O₂ rich and CO₂ poor.

Wait . . . 🌳 Plants?  When did lifeforms enter into the picture?