A planet called 'Home'
- Shaun Smillie and Schalk Mouton
Our home planet Earth is unique, not only in its position in space but in the way it manages energy to create a comfortable spot for us to inhabit.
Our planet Earth is our perfect home, the only one we have. But it was not always hospitable. In the beginning, just after what is known as the ‘Big Bang’ about 4,6 billion years ago, our planet was a raging mass of hot chemicals. There were no atmosphere, no oceans, and no life. This blob of a protoplanet comprised various bits of accumulated leftovers from the birth of our sun.
Professor Gillian Drennan, Head of the School of Geosciences at Wits, explains what happened next: “These leftover particles were pulled together and collided with one another under the influence of gravity. That was 4,6 billion years ago and if someone had caught a glimpse of this new protoplanet back then, it would have appeared as a big three-dimensional pizza. Imagine a ball of pizza with pepperoni, onion and tomato equally distributed throughout. That is what the early Earth was like – a complete mishmash of ingredients.”
As collisions continued and the protoplanet (a large body of matter in orbit around the sun) grew, it heated up. It became so hot that it began partially to melt, explains Drennan.
“As it melted it began to differentiate or separate into different layers of increasing density towards the centre of the Earth. Melting allowed various volatiles in the accreted [accumulated] particles to escape, giving rise to the development of an atmosphere and even the oceans.” Volatiles are the group of chemical elements and chemical compounds that can be readily vaporised.
“It is also thought that the oceans might even have resulted from the accretion of some comets,” adds Drennan.
The Goldilocks Zone
For Earth to sustain life, several things had to emerge from this hot globule to make it the only planet in our Universe where life exists (as far as we know at the moment).
Firstly, our planet’s position in our solar system had to be just right. Our planet would not be what it is today if it wasn’t for our sun, believed to be a third or fourth generation star.
The Earth is perfectly positioned from the sun to support life, in what is often referred to as the ‘Goldilocks zone’, a habitable band where water remains liquid.
“If we were any closer to the sun, we would all fry. And if we were any further away, we would all freeze,” says Professor Mary Scholes in the School of Animal, Plant and Environmental Sciences.
Secondly, our planet is still in the process of cooling down from its original cataclysmic formation and the heat produced by radioactive elements in its interior. The most efficient way for this to happen is convection, says Professor Roger Gibson in the School of Geosciences.
This convection occurs in Earth’s mantle where the rocks are so hot that they are molten and able to flow and even melt. As the upwelling mantle nears Earth’s surface, it starts to flow laterally, splitting apart the thin, cold crust above it. Hot magma rises along these giant cracks and cools down, forming new crust. And if hot magma is rising to the surface somewhere, in other places, cold crust is sinking back down into the mantle.
This conveyor-belt of crust formation and destruction shifts the continental fragments across the planet on which we live, causing them not only to break apart or collide, but physically to move into different latitudes over timespans of tens of millions of years, thus driving significant climatic shifts.
Magnetic protective umbrella
The next ingredient in the process to create a liveable environment was for the planet to form a protective umbrella to shield us from objects from space. This is the magnetic field that is believed to have developed around 3,5 billion years ago.
“Our magnetic field protects us from cosmic rays and from high energy particles associated with coronal mass ejections, those high energy particles that come out of the sun and get deflected around the Earth,” says Professor Susan Webb in the School of Geosciences, who studies Earth’s earliest magnetic field.
“Earth happens to have a large liquid core and the rotation rate is such that, between the rotation of the planet and the chemical and thermal buoyancy, we get a dynamo action, which is basically turning mechanical energy into electromagnetic energy that generates a magnetic field,” explains Webb.
Other planets in our solar system lack a magnetic field. Venus, for instance, is rotating too slowly, while Mars is so small that scientists think its core has mostly frozen and doesn’t have enough liquid iron to generate a magnetic field.
Protection from our magnetic field is thought to have been important for evolution, as plants, animals and humans on Earth are protected from genetic damage from these high energy particles.
Magnetic minerals preserved in rocks provide evidence of the strength of the magnetic field in the past and how it, along with the positions of the continents, has changed over time. With rocks as old as 3,5 billion years and as the home to one of Earth’s earliest recognisable continents, South Africa is a rich laboratory in which to study these secular changes, says Gibson.
“Fluctuations in the magnetic field could in the future help explain how life emerged on Earth,” says Webb.
The Great Oxidation Event
Another important role of this magnetic field is that it prevents the Earth’s atmosphere from being stripped away.
Our atmosphere is believed to have formed after the planet cooled down and grew large enough to trap gasses around it, through gravitational force. Other gasses, such as hydrogen sulphide, methane, and carbon dioxide, were spewed into the atmosphere through volcanoes. It took about half a billion years for Earth’s surface to cool down and solidify enough for water to collect on it.
The early atmosphere was highly reducing and anaerobic microbes such as archaebacteria (such as methanogens, or sulphur-reducing bacteria) persisted until the advent of cyanobacteria, which flourished around 2,5 billion years ago. This resulted in oxygenic photosynthesis and oxygenation of our atmosphere for the first time.
“This global phenomenon is known as the Great Oxidation Event (GOE),” says Professor Pierre M. Durand in the Wits Evolutionary Studies Institute. “The build-up of free oxygen in the atmosphere created suitable environments for eukaryotes [cells with a nucleus] or even complex multicellular life to evolve later in Earth's history. However, this phenomenon may have led to the extinction of various anaerobic [without oxygen] microorganisms at that time.”
The planet’s atmosphere gives us the environment in which we can live and breathe. It is also important as a temperature regulator and one gas plays a lifesaving role: Carbon dioxide (CO2) has a bad rap because of the role it plays in climate change, but it is needed to trap heat.
“The heat that gets trapped does not get reflected back into the atmosphere when the sun goes to sleep, and therefore keeps our planet at a habitable temperature for 24 hours a day,” says Scholes.
Motion of the ocean
A global average mean temperature of between 16 and 18 degrees Celsius must be maintained, and weather worldwide plays its part in maintaining habitability on Earth. Helping drive these systems are oceanic currents.
“Ocean currents play a very important role in maintaining patterns of rainfall distribution as well as overall temperature on the land,” says Scholes.
Different forms of life have also emerged to play a role in weather. Vegetation type is linked to rainfall distribution. Keeping ‘the blue planet’, Earth, alive relies on a dance of different parts, where each must work in harmony, even when climate change threatens. But it’s not always a smooth ride and the best way to describe this, explains Scholes, is to compare these working parts to an orchestra:
“You’ve got some things that are constantly going on in the background, like you may always have a singular violin being played. And then every so often you might get a perturbation [disturbance] to the global planet. These are the cymbals coming in; this is something like a big riff, like a tsunami.”
- Shaun Smillie is a freelance writer.
- Schalk Mouton is Senior Communications Officer for Wits University.
- This article first appeared in Curiosity, a research magazine produced by Wits Communications and the Office of the Deputy Vice-Chancellor: Research and Innovation.
- Read more in the 15th issue, themed: #Energy. We explore energy research into finding solutions for SA's energy crisis, illuminate energy needs of people with disabilities, address the energy and digital divide in an unequal society, and investigate the origins of fire control.