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Episode 1. Life as we know it!

  • Writer: Abigail James
    Abigail James
  • Jan 29, 2020
  • 17 min read

Updated: Feb 2, 2020

This blog post is a transcript of Series 1, Episode 1 of my science podcast, Don't touch my Radium! All sources, references and recommended material can be found here.


The first exoplanet was discovered in 1992, and as of today there are approximately 4,100 confirmed exoplanets, 2600 of these discovered by the Kepler Space Telescope. And that doesn’t include over 3000 more that Kepler has reported on but which have yet to be confirmed. Given the limited region of space that Kepler focused on, the estimations are that there could be around 40 billion exoplanets, in the milky way alone! And there are billions of galaxies out there. When you think about these numbers, it seems that life must exist in other solar systems throughout the universe however, what form of life is the question.

So let’s start with life as we know it, here on Earth and look at how this beautiful, rocky biosphere came into existence and developed all the right traits that allows us to be here, and not be burned up by radiation from the big shiny Sun.


Formation of the Solar System


So, how did our solar system come into existence, or rather where did our Sun come from and then how did we come along? As well as taking a theoretical approach, modern observations of stellar systems along with computer simulations combine to create an overall theory of formation. There are a lot of theories out there but today we’ll look at an extension of the nebular hypothesis. The original concept was proposed and developed over a period of 60 years in the 18th century by Emanuel Swedenborg, Immanuel Kant and Pierre-Simon Laplace. However it was not widely believed until 1969, when a Russian scientist, Victor Safronov, released a book titled 'Evolution of the proto-planetary cloud and formation of the Earth and the planets'. Where he proposed solutions for many of the major problems of the nebular hypothesis and developed what is now known as the Solar Nebula Disk Model.

About 4.5 billion years ago, a dense interstellar cloud filled with dust particles would have inhabited our place in the milky way. As the particles move with individual random motion collisions form small molecules of gas, while the motion creates a net rotation on the cloud. Early cold temperatures would have led these gas particles to clump together forming denser materials. These denser areas in the cloud want to collapse under their own gravity but the rising internal pressures want to act outwards, this causes a balance in the forces and keeps the cloud from collapsing altogether. So we now have a balanced, rotating molecular cloud hanging out in space, waiting for something to change. The prevalent theory is that a nearby supernovae exploded which sent a shock wave through space and disrupted the balance in our molecular cloud. This shock would have provided enough energy to excite the molecules and cause a gravitational collapse at the centre of the cloud. As the cloud contracts, the gravitational potential energy of the cloud is converted to kinetic energy of individual gas particles leading to collisions that generate heat. In physics there are a number of quantities that need to be conserved, angular momentum is one of them. This conservation of angular momentum meant that the increase in the density of the materials at the centre would have instigated an increase in the rotation of the cloud and this coupled with the continuing increase in pressure at the core is what eventually lead to the formation of a proto-Sun at the center. As the proto-Sun began to form and the rotation of the cloud increased, the material spread out into a flat disk we refer to as the proto-planetary disk.

Eventually the core is heated to a point where the pressure and density of hydrogen leads to thermonuclear fusion igniting our Sun! The burning plasma core of our Sun has a temperature of about 15 million Kelvin. Within this core hydrogen atoms can fuse to create helium. There are a few paths this process can take but our Sun mostly uses the proton-proton cycle whereby repeated nuclear fusion reactions occur until a helium nucleus is created. This process is an exoergic reaction which means that there is a net release of energy, basically the resulting mass of helium is lighter than hydrogen so some of the mass of hydrogen gets converted to energy. This energy is released and travels to the surface where it is radiated outwards as a solar wind, this formation of helium is the main source of the energy that is emitted by our Sun.

As the temperature, reaction rates and density of the central core continued to increase the outward pressures also increased until after about 50 million years, hydrostatic equilibrium stabilized our star by balancing the gravitational force with the internal pressures, ending the stellar formation period.


Nice youtube video visualising the process of solar nebula disk formation.


Formation of the Earth and Moon


Once fusion had occurred, the Sun consumed 99.8% of the mass of the solar nebula while a stellar wind would have blown the remaining debris along the disk and past the point where it would collapse into the star leaving a spread of microscopic material orbiting the newly formed Sun. As you move away from the Sun the temperature decreases so that there is now a range of temperatures along the disk. Condensation is the method by which the heaviest molecules combined to form tiny solid or liquid droplets. This occurs at different temperatures for different classes of materials. The temperature at which the different classes of materials condense is what determines how these tiny planetesimals were dispersed along the proto-planetary disk laying the seeds from which our planets were formed.

The exact method of planet formation is still unknown but a few models do exist that have the most credible theories. The most popular one is the core accretion method, whereby the condensed planetesimals are pulled towards each other by their gravitational force with the denser objects gaining more and more mass over millions of years until they eventually have enough mass to then accrete gaseous elements. This theory poses a problem for the formation of gas giants as it is believed that they would need to form much faster than core accretion allows due to the high temperatures of the early solar system. In recent years both the disk instability model and the pebble accretion model have been proposed as possible explanations for the expected rapid formation of gas giants but continued observations of our solar system as well as other systems in our galaxy are needed before one of these will surpass core accretion as the accepted model.

Once the Sun had formed and solar winds spread the material out along the proto-planetary disk creating the planetesimals, metals and silicates came together to form larger solid materials, the beginnings of our planet. This initial increase in energy and pressure due to radioactive decay of elements as well as the effects of some larger collisions led to the temperatures increasing until the materials melted and the Earth became molten. In this liquid form, the denser metals could sink to the centre holding the lighter silicates around it by its gravitational pull. Over time, as the planet cooled, these outer layers formed a crust around the molten layers of its core and mantle. This formation of a metal core, rocky mantle and a solid surface coupled with our position from the Sun and our spherical shape, classify us as a ‘terrestrial’ planet. The final stage in our planets formation led to the formation of our own natural satellite, the Moon.

It’s believed that an impact with another body led to the creation of our Moon. The reason that we think this, has to do with isotopes. It’s common for elements to have slight variations in the number of nucleons contained in the nucleus such as Oxygen-16 and Oxygen-18. We refer to these variations as isotopes. Studies have shown that the isotopic signatures of the bodies in our solar system are different from Earths, indicating that they were formed separately. However, the Earth and Moon have the same oxygen isotopes, which is an extremely unlikely natural occurrence. Secondly, the Moon is very low on iron, this would be unusual if it had gone through the same formation process as the other bodies in the solar system. It’s believed that in these early days, the Earth had a twin. Another planet about the size of Mars known as Theia. The Giant Impact Hypothesis suggests that Theia collided with the proto-Earth with a force large enough to melt both bodies and send pieces of the Earths mantle into space as debris orbiting our planet. This explains the similarity in the oxygen isotopes and also the lack of iron on the Moon. There was one other consequence of this collision, it is thought that this is the point when Earth was knocked off its axis so that it rotates at an angle of 23.5 degrees.

In terms of satellite to planet ratios, our moon is the largest in the solar system. It's size relative to us means that it also exerts a significant gravitational pull on the Earth. If an orbiting body rotates about its own axis in the same amount of time as its orbital period around a partner, then it is tidally locked. The Moon takes about 27 days to rotate on its own axis and the same time to rotate around the Earth, so that the same face is always showing and the Moon is tidally locked. This is a natural consequence of the gravitational distortions that are induced by one body on another due to the difference in strength of their respective gravitational fields. Both bodies exert a tidal force on each other, a pulling and stretching of the body that is strongest at the points where they face each other. This tidal force causes distortions on both bodies which, for us, is seen in the motion of the Earth's oceans that we refer to as the tides. If the Moon is being pulled and stretched by the tidal forces of the Earth, why doesn’t it get torn up? The Roche radius is the distance an object would need to be from Earth for the tidal forces to be great enough to tear it apart. For a solid satellite this is about 9500 km from earth, our Moon is 385000km from Earth so is outside of the Roche radius where its own gravitational pull is strong enough to keep it together.

Angular momentum is present in both the orbital and rotational motion of an object. Most of the angular momentum of the Earth-Moon system is held in the orbital motion, which is not common in the other moons in our solar system. The Moon revolves in the same direction as Earth's rotation but as the Earth rotates at a faster period the pull on the tides is always ahead of the Moon pulling and stretching it, so the tides dissipate energy and the angular momentum is transferred from the rotation of the Earth to the Moon allowing the Moon to gain energy and gradually spiral outwards, receding its orbit by around 38mm/year. As the total angular momentum must be conserved the Earth compensates for the transfer by slowing its spin at a rate of about 23 microseconds per year. When our Moon first formed, the Earth would have rotated at a speed where one day would have been 5 hours. The pull of the Moon on the Earth slows us down and over billions of years got us to our 24 hour day.

We’ve been talking about how our Moon has an effect on the tides of our oceans. But just how did our oceans come to be.

Water to Earth


We are the only planet in our solar system to have liquid water on the surface. But how, throughout the stages of our planets’ formation, did this occur? The exact timing and method of how water came to Earth is up for discussion however it is generally thought to have occurred during an ice storm from space.

Earlier we were talking about how isotopic signatures in the Earth and Moon led us to the theory of the Moon formation. Similar analysis can help us to understand where the water in our oceans came from. Deuterium is an isotope of hydrogen; by looking at the ratio of deuterium to normal hydrogen in our oceans we can compare this to other potential water sources in the solar system to get a better idea of its source. Some of the oldest meteorites found on Earth are nearly as old as the planet, dating from around 4.5 billion years ago so they must have arrived pretty soon after the Earth formed. These ‘carbonaceous chondrites’ contain liquid water with the same deuterium/hydrogen ratio as our oceans. It is generally thought that a bombardment of these ice asteroids lasting millions of years is what brought water to our oceans. However, this may not be the complete story.

It was previously thought that water would not have been able to condense to form the oceans during the planets early formation as the temperatures would have been so high, however recent models indicate that this may not be the case. Samples have been taken from parts of the ocean that are closer to the core and mantle boundary that have a lower ratio of deuterium, this region also holds isotopic signatures of helium and neon that would have originated in the solar nebula. The models presented suggest that the ‘nebular’ hydrogen would have been pulled towards the core due to a chemical attraction to iron and thereby leaving behind the heavier isotope of deuterium to cool in the mantle. This could explain why samples from the Moon suggests that water may have been present before it’s formation. The research estimates that 1 in 100 water molecules could originate from the solar nebula. The majority of our water likely came from ice storms, however a fraction of it may have come from the solar nebula. Why is this important? Because it’s an indication of something that goes beyond our solar system. If water was able to condense on our rocky planet during Earth's formation, then this indicates the potential for similar occurrences on terrestrial exoplanets meaning that there could be more worlds out there with liquid water than were previously thought.

A liquid medium is vital to the existence of life as we know it. For us, it’s water. The first signs of life were created in our oceans, billions of years ago. The common hypothesis favours an evolutionary process, whereby the complexity of life was gradually increased over time, rather than a single event resulting in complex life forms with the ability to self-replicate and self-assemble. The earliest, confirmed, fossils are dated to 3.5 billion years ago, there may be earlier signs but they have not been conclusively proven as of yet. The first forms of life were single-cell organisms that extract energy from their surroundings for their survival but they would have lived in environments that current life would not be able to exist in.


Formation of our Atmosphere


We know that in order for life to exist and thrive, as far as life on Earth is concerned, water is required. But what of the building blocks of water?

Oxygen is a chemical substance that makes up nine tenths the mass of water, it accounts for two thirds the mass of our body weight, contributes to around half the mass of the Earth's crust and makes up roughly 21% of our atmosphere. Oxygen is a chemical substance that is required by all complex life forms as we currently know them. It is the most abundant element on our planet's surface and third most abundant behind hydrogen and helium, in the known universe. However, we normally see oxygen as a component of molecules, free oxygen is much less common so the creation of free oxygen to form our atmosphere, is a unique event. In fact the existence of an atmosphere is not a commonly occurring event in our solar system. Earth is one of only a few planets - that we know of - to have an atmosphere.

The formation of our planet occurred over billions of years and went through a few different stages, so it stands to reason that our atmosphere also developed and changed over time. It is believed that we have had three different atmospheres in the course of Earths' existence. The first atmosphere would have consisted of helium and hydrogen elements that came from the creation of the solar nebula. Over time, solar winds and the heat of the first earth's body would have driven these elements away into space stripping the planet of its first atmosphere. The next atmosphere occurred after the Moon was created when the Earth was heated to molten temperatures; volatile gases would have been released with eruptions from volcanoes contributing further gases to the surface generating a second atmosphere with very high quantities of greenhouse gases and very little oxygen. The atmosphere as we currently know it, arose through what is termed the 'Great Oxidation Event'.

Up to about 2.5billion years ago the existence of life on earth was termed as anaerobic: simple life forms that didn’t need oxygen to live. There likely would have been a lot of different variations, but the one we’re most interested in is cyanobacteria. This is the guy that evolved to harness the energy from sunlight to make glucose using water and co2. This is the process of photosynthesis which is how all green plants get their food.

Through this reaction, oxygen is created alongside the glucose but is not required by the plants and so gets released as a waste product into the environment around the cyanobacteria. The early oceans would have been full of dissolved iron which when bonded to oxygen produced sedimentary rocks. Over time it is thought that the cyanobacteria became more effective at harnessing sunlight and would have reproduced more efficiently eventually leading to more oxygen than iron and so it had nothing to bond to. This free oxygen was available to be released into the atmosphere.

The addition of oxygen to our atmosphere led to the structure that we see today. Our atmosphere consists of a number of layers that each contribute to promoting and protecting our existence. The first layer is the troposphere which holds nearly all the water vapour and dust and is where all the weather develops, showing daily changes, seasonal patterns and climate systems. The seasonal patterns contribute to the biodiversity of plant life all over the planet while the rain collects dissolved co2, returning it to the surface where the carbon elements gather at the base of the ocean cooling down the planet. The stratosphere is the layer that contains oxygen in the form of the o3 molecule, which is more commonly referred to as ozone. This is where the ozone layer is. Oxygen in this form can absorb harmful UV rays from the Sun, preventing them from reaching the surface, if they did Earth's surface would be scorched from radiation and be uninhabitable.Unfortunately it is uneven and thinner near the poles making it fragile. The Mesosphere is the next layer which is very hard to learn about as it is too high for aircraft/weather balloons and too low for spacecraft. We use sounding rockets to get as much information as we can. What is important is that this is the layer where meteors burn up protecting the surface from collisions from outer space. The Ionosphere is made up of a layer of free electrons and ions and is where radio waves are reflected (am radio). We then have the thermosphere, this region is the thickest layer and is mostly made up of oxygen, helium and hydrogen, tiny gas molecules that absorb x-rays and uv radiation. We commonly refer to this region as ‘low-earth orbit’ and it is where the ISS and the Hubble space telescope orbit the planet. Finally we have the exosphere, a very thin layer which is dominated by widely dispersed hydrogen and helium particles. This area is governed by the sun and solar storms and is where weather satellites orbit.

So this brings us to the magnetosphere which is technically not a part of our atmosphere, rather it is the region that we know as the magnetic field generated by the planet.


Magnetic Field


It resides above the ionosphere protecting us from solar winds and cosmic rays that would strip away the ozone layer. The mechanism by which the Earth generates a magnetic field is referred to as a 'Dynamo'. Dynamo theory explains how the rotation, convection and electrically conducting properties of a fluid material can generate and maintain a magnetic field over the time scales that astronomical objects can survive in our universe. It requires an electrically conductive fluid, planetary rotation to provide kinetic energy and an internal energy source to push convective motion in the fluid. For Earth, the iron core is made up of a solid centre with a molten outer core whose motion gives rise to these convection currents allowing heat to escape. The heat is generated through the release of potential energy as the heavier materials sink towards the core and radioactive elements in the interior decay. This motion of an electrically conducting fluid, acting as an electrical current, generates the magnetic field through what is termed as a 'feedback loop'.

Why is the presence of a magnetic field so important to us? There is evidence that Mars may have had a magnetic field billions of years ago along with a thick atmosphere. However, for reasons that are still being investigated, this field weakened over time until it shut down. It is believed that the lack of a magnetic field on Mars is what contributed to its atmosphere escaping and ultimately allowing solar radiation to destroy the planet's surface, rendering it uninhabitable.

The excited atoms in the magnetic field absorb the harmful radiation emitted by the sun, protecting us from atmospheric escape while allowing a comfortable surface temperature to remain promoting an environment where life can survive. You can see this in action when solar winds hit the magnetic field causing the aurora, northern and southern lights. Electrons and protons from the solar winds cause ionisation and excitations of oxygen and nitrogen in the Earth’s upper atmosphere. As these atoms return to their ground (or non-excited) state, they release the excess energy as photons and it is these photons that we see as those beautiful lights.


Final Stages


So now we have a protective atmosphere that holds a weather system helping to water and cool the planet, promoting the growth of the plant life and filling the air with oxygen. But one more event had to happen for us to develop from simple bacteria. Bacteria have been known to survive without oxygen by living off heavy materials, however oxygen is a reactive gas and so was a potential source of energy for organisms that could find a way to harness it. The growth, size and complexity of our physical life is connected to the existence of an oxygen rich atmosphere.

The exact time of when multicellularity occurred has not been conclusively determined. Some studies suggest that it occurred before the oxidation event but either way we can be sure that the oxidation event is what led to the complexity of life as we know it today.

It is understood that the transition from simple microbial life was the result of one, potentially spontaneous event that is likely to be an 'Earth only' occurrence. This was the development of mitochondria. It seems that mitochondria arose only once in all of evolution. They originated via endosymbiosis whereby a host cell develops a symbiotic relationship with another bacteria cell, one that used oxygen as a production mechanism. Mitochondria are often referred to as the ‘powerhouse’ of the cell, they convert energy from food into energy that can run the biological processes we need to survive, by using oxygen and water to turn glucose into energy via cellular aerobic respiration. We quite simply could not survive without mitochondria. The mechanism which powers all life as we know it, coupled with an increase in oxygen levels, allowed for life to evolve in both size and complexity and then through natural selection to large, complex species.

What can we take away from all this? We exist at a distance from the sun that allows us to have a rocky planet with oceans of liquid water. We have an oxygen rich environment that can be used as an energy source allowing us to have large and complicated species. We have a protective atmosphere along with a magnetic field that absorbs and reflects the harmful UV radiation from the sun protecting the surface of the planet from radiation. A rare and spontaneous event created mitochondria allowing intelligent life to evolve. So how likely is it that we are not alone? Very unlikely. But how likely is it that we are the only intelligent life forms? We are potentially a rare, Earth only event, our existence may be the only intelligent life, or life as we know it. So what do we need to do to find out? As our biological ancestors moved from the depths of the oceans onto dry land, our planet eventually evolved into a protective biosphere full of diverse species all powered by the same fundamental elements, leaving signatures all over the surface of our planet, that we are here. Maybe, just maybe, there will be other life forms out there that have left their own mark, we just need a way to look for them!


References and Sources


Articles


Lecture Notes / Education Sites


Papers

  • Canup, R., Asphaug, E. Origin of the Moon in a giant impact near the end of the Earth's formation. Nature 412, 708–712 (2001). DOI: doi.org/10.1038/35089010

  • Wu, J., Desch, S. J., Schaefer, L., Elkins‐Tanton, L. T., Pahlevan, K., & Buseck, P. R. ( 2018). Origin of Earth's water: Chondritic inheritance plus nebular ingassing and storage of hydrogen in the core. Journal of Geophysical Research: Planets, 123, 2691– 2712. https://doi.org/10.1029/2018JE005698

  • Adam R. Sarafian, Sune G. Nielsen, Horst R. Marschall, Francis M. McCubbin, Brian D. Monteleone (2014). Early accretion of water in the inner solar system from a carbonaceous chondrite-like source. Science, 346, 623-626. DOI: 10.1126/science.1256717


Documentaries/Youtube Videos


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During its flight, NASA’s Galileo spacecraft returned images of the Earth and Moon. Separate images of the Earth and Moon were combined to generate this view. Image and Caption was sourced from NASA's public image library.

 
 
 

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