Earth: Over 4 Billion Years in the Making

Introduction

‘You have to know the past to understand the present.’

Carl Sagan

S. ANDREWS (HARVARD-SMITHSONIAN CFA); B. SAXTON (NRAO/AUI/NSF); ALMA/ESO/NAOJ/NRAO/SCIENCE PHOTO LIBRARY

The TW Hydrae protoplanetary disc photographed by the Atacama Large Millimeter Array (ALMA) telescope. The multiple rings and gaps indicate the presence of emerging planets as they sweep their orbits clear of dust and gas.

Every great story has a beginning, and the story of the Earth is no different. It’s a tale that began over 4.5 billion years ago, when this 6-billion-trillion-tonne ball of beautiful, life-sustaining rock began forming out of the dust of the newly developing Solar System, which had a young Sun at its centre and clouds of gas and dust encircling it. The planet you are sitting on right now was, as unbelievable as it seems, slowly pieced together by the collision of endless cosmic scraps over millions upon millions of years. Formed by nothing more than the force of gravity, dust became rock, rock became boulder and boulder became a molten lump so vast it would pull itself into the near-perfect sphere that we know and inhabit today.

But although this was the beginning of our planet’s story, it was far from the beginning of a world that we could recognise today. At its birth there was no life, no ever-changing sky, and there were no oceans. This was a planet born out of a series of endless cosmic collisions, and in its early years it was still reeling from the violence of that creation. To understand how it would become not just a planet but a home, we need to weave our way through the events of this young, hostile world’s early life, as it hurtled from one catastrophic event to another. At times it seemed as if there would be no life-filled future for Earth, as again and again it faced the prospect of becoming a sterile world, choked by conditions that had become too extreme – whether too hot, too cold or just too toxic – to allow anything to survive.

And yet, despite everything that was thrown at it, life would ultimately conquer and flourish. Our planet has created every living thing – including you and me. Today, we see all of this beauty, but we also take it for granted. This is a living, breathing, life-sustaining planet that is more fragile than we like to admit and more indifferent to us than we want to acknowledge.

The story of our planet is long, and we are just beginning to understand the extraordinary moments, the endless twists and turns and, to be blunt, the incomprehensible luck that has led to all of this. This is because the Earth’s transformation was not an inevitability – in fact, it wasn’t even likely.

This is the almost implausible story of our Earth; how it went from a fiery, dead planet to a beautiful, living, breathing, blue bubble floating in the darkness of space.

DETLEV VAN RAVENSWAAY/SCIENCE PHOTO LIBRARY

Illustration of our Solar System’s formation. The Sun’s gravity attracted dust and small particles that gradually became asteroids and eventually planets.

Geologic Timeline of Earth’s History

CORDELIA MOLLOY / SCIENCE PHOTO LIBRARY

Geologic Time Scale

Franco Banfi/naturepl.com

Life on Earth is a glorious abundance, richly diverse and phenomenally complex and interconnected. Our seas sparkle with fish species like these sardines, our skies ring with the cries of birds and our forests are shaded by towering trees. It’s worth taking a moment to appreciate how fantastic and how unlikely all this life really is, and to consider how we can protect and support it for all our futures.

No. 1

atmosphere

‘Understanding how Earth got its atmosphere is the first step in understanding how to protect it.’

Professor Peter Girguis, Harvard University

Science Photo Library

THE JAWS OF HELL

We begin our story with a young, unrecognisable and alien planet. Just a few million years after its birth, this is a volatile world, still burning with the residual heat of its formation and under almost constant bombardment from a chaotic, asteroid-filled solar system. In a sky that is black both by day and night, there is no atmosphere to diffuse the Sun’s rays, because space meets Earth directly at its surface. The Sun burns in this inky sky as a ball of brilliant white light, while a newly formed moon hangs heavy, still molten from its own formation. The lunar presence sits tucked in close to its parent planet, illuminating a landscape that is hellish in every direction.

This is the Hadean Eon, a period of Precambrian time named after the Greek god of the Underworld, the first and perhaps the most mysterious of Earth’s geological time frames. The Hadean began with the formation of the planet 4.5 billion years ago and ended over half a billion years later. Little direct evidence remains of this vast era of time, because the constant bombardment of the Earth’s crust by meteorites, combined with the dynamism of the planet’s ever-changing exterior, has long since destroyed or hidden any trace of its ancient surface. Only the rarest of fragments remain for us to examine and ponder what that ancient surface might have looked like.

John Cancalosi/Alamy Stock Photo

The oldest-known material of terrestrial origin: zircon 4.4 billion years old found in the Jack Hills, Western Australia.

The oldest of all of these, in fact the oldest piece of the Earth we have ever discovered, was found in the Jack Hills of the Narryer Gneiss Terrane, in an area of Western Australia known for its ancient rock formations. This is no majestic slab of ancient rock, though; it is the merest trace of zircon, a crystal that is found in tiny amounts in almost all of the Earth’s igneous (volcanic) rocks. Zircon is an incredibly hard, durable and chemically inert mineral, and for that reason it can survive for immense periods of time. It also contains trace elements of uranium and thorium, whose slow but predictable radioactive decay can provide the perfect method for accurately measuring the age of a rock sample. Through these radiometric dating techniques we’ve been able to put an age on samples of zircon from the Jack Hills that date back over 4.4 billion years, right to the middle of the Hadean. This is the oldest-known material of any kind formed on our planet, and it gives us a window into that alien world around 100 million years or so after its birth, to a time when the Earth was cooling just enough for this rock to be perhaps part of that first crust, the first solid planet, and a stage upon which the greatest of stories could begin to play out.

Today our world couldn’t be more different. It’s a place of vivid colour, where oceans give way to land, creating myriad environments and habitats filled with a seemingly endless array of life forms. And yet all that divides our living world from the uninhabitable conditions of the cosmos beyond is gossamer-thin layers of gas stretching upwards by barely over 100 kilometres. This atmosphere is our great protector, a thin blue line that nurtures all life beneath it and marks out our planet as perhaps unique in the known Universe.

In this chapter we will tell the almost implausible story of how this great protector came to be, how it was born out of a turbulent, explosive young planet and how it created the setting for a water world to take shape. We will also explain how these events allowed a set of conditions to form where life could emerge and flourish, and begin an intimate four-billion-year dance between our atmosphere and life itself, creating, shaping and calibrating that atmosphere in ways that we are only just beginning to fully understand.

NASA/METI/AIST/Japan Space Systems, and U.S./Japan ASTER Science Team

These iron-rich rocks in northwestern Australia formed before the presence of atmospheric oxygen, and life itself.

A Timeline of the Earth

JACK DYKINGA/NATURE PICTURE LIBRARY/SCIENCE PHOTO LIBRARY

A bird comes into view; its feathers are silvery, rippling in the wind as it glides along the edge of this escarpment.

With a wingspan of more than 3 metres, the Andean condor weighs up to 15 kilograms, so in terms of their weight and wingspan these are the largest flying birds on Earth.

The condor launches itself out into the air to catch a thermal, and in this way it can travel hundreds of kilometres without hardly ever beating its wings. Watching these giant Andean condors soaring here reveals how their life is completely intertwined with that thin cloak of air that is wrapped around our planet. But when you think about it, everything – every plant, every fungus, every bacterium, every tiny insect, every giant reptile, even every human being – is completely dependent on this atmosphere.

A THIN BLUE LINE

Let’s start by looking up. Wherever you are on the planet at this very moment, you, like every single one of us, are swimming in an ocean of air that is as ethereal as it is majestic. Stretching 100 kilometres above you, this is an ocean made of nitrogen (78.08 per cent), oxygen (20.95 per cent), argon (0.93 per cent), carbon dioxide (0.038 per cent) and a collection of other gases in trace amounts. Held to the surface of the planet by nothing more than gravity, this protective blanket that we collectively call air has an overwhelming effect on the characteristics of our planet and all life upon it. Without the pressure created by our atmosphere there would be no liquid water for life to exist in, ultraviolet solar radiation would bombard the surface, the temperature would vary dramatically between day and night, and the whole planet wouldn’t be able to hold onto the heat that, through a process we know as the greenhouse effect, helps maintain a stable, life-supporting climate. It is the air that we inhale, filling our cells with the combustible oxygen we need to drive every process in our body. It also provides carbon, the crucial ingredient for all photosynthesising plants and the foundation of almost every food chain on Earth. This is the basis of our weather and the reason we live under the most brilliant blue skies and experience the multitude of colours that usher the Sun in and out of our lives each day.

But our atmosphere is not one homogeneous entity or a single layer of nurturing gas, it is a complex and dynamic system that varies significantly with altitude. From the surface of the Earth upwards, the general rule is that the air pressure and density decrease consistently as you ascend. However, the temperature of the atmosphere does not follow such a straight trajectory and may remain constant or even increase with the changing altitude. We’ve plotted this temperature signature as you climb into the atmosphere and away from Earth in great detail, using weather balloons containing ever-more-advanced instrumentation. Known as the lapse rate, this rate of temperature change with altitude is used to demarcate the structure of the Earth’s atmosphere into five distinct layers.

Earth’s Atmosphere

AMERICAN PHILOSOPHICAL SOCIETY/SCIENCE PHOTO LIBRARY

The Explorer II was a manned balloon launched on 11 November 1935 to study the stratosphere. It reached a record altitude of 22,066 metres and carried a two-man crew inside a sealed spherical gondola.

‘Our atmospheric composition may actually be the thing that is screaming out to the cosmos, a signal to the Universe that we’re here, that life exists on our world.’

Dr Michael Wong, Carnegie Institution for Science

The first of these is the troposphere, which extends from the surface of the Earth up to an average height of 12 kilometres – being lowest at the poles and highest at the equator. This layer contains the vast majority of the mass of the atmosphere (up to 80 per cent), even though it is only a thin slither sitting beneath another 10,000 kilometres of air. In fact, 50 per cent of the total mass of the Earth’s atmosphere sits in just the first 5.5 kilometres. The troposphere contains nearly every living thing on the planet, supporting every plant with the carbon dioxide needed for photosynthesis and every animal with the oxygen needed for life. It also contains 99 per cent of all the water vapour on the planet, which means this is where the weather happens, because almost every cloud in the sky sits in the troposphere, except for the tips of cumulonimbus thunderclouds whose summits can peek up into the level above.

Every day, the heat of the Sun that has been absorbed radiates out from the surface of the Earth and up into the troposphere, which means that the lowest parts of the troposphere are the warmest and the highest parts the coolest, creating the powerful convection currents that help drive our weather.

It’s this gradual reduction in temperature with altitude that demarcates the troposphere from the atmospheric layer that sits directly above it – the stratosphere. At the boundary of these two distinct layers, around 12 kilometres above the surface of the Earth, is the tropopause, the place in the planet’s atmosphere where the temperature stops dropping with altitude and instead stays stable or inverts with a layer of warm air sitting above a cooler one.

JIM REED/SCIENCE PHOTO LIBRARY

Meteorologists from the Severe Thunderstorm Electrification and Precipitation Study (STEPS) prepare to launch a weather balloon into a storm to gather data on temperature, pressure, wind speed and electrical fields within the thunderstorm itself.

Eric Baccega/naturepl.com

In strong sunlight, the hippopotamus sweats a red substance that blocks out UV rays and thus acts as a natural sunscreen.

Associated Press/Alamy Stock Photo

A university student smells a magnolia through a gas mask on the first Earth Day in 1970, protesting the pollution that has damaged the ozone layer.

‘If we don’t understand the history of the atmosphere, how can we possibly be the stewards of it moving forward?’

Dr Lynn Rothschild, NASA Ames Research Center

It’s this change in the lapse rate that signals we are now entering the stratosphere, the second-lowest atmospheric layer, which extends from around 12 kilometres above the surface of the Earth to a maximum altitude of approximately 55 kilometres. The stratosphere is defined by a temperature signature totally unlike that of the troposphere – here the temperature rises with increased altitude, in complete contrast to the layer below. Although at first this may seem counterintuitive, the reason for this sudden change is in fact down to a single property of the stratosphere – the ozone layer.

The ozone layer plays an important role in protecting all life on the surface of the planet, including us, so when, in 1974, scientists raised concern about a hole that had appeared in it due to our use of CFCs (Chlorofluorocarbons – a chemical used in aerosols), it was essential to act quickly to repair the damage to this crucial part of our atmosphere. (An interesting example of humans actually acting on the evidence when faced with a growing atmospheric threat!)

To understand the importance of the ozone layer you have to understand the chemistry of its key singular ingredient. Ozone is what is known as an allotrope of oxygen, a structurally different form of the gas with the chemical formula O3, as opposed to O2. Just as allotropes of carbon, such as diamond and graphite, have strikingly different physical properties, ozone differs fundamentally from the O2 that fills the troposphere, most significantly in its relationship with ultraviolet light raining down from the Sun. Found mainly in the lower levels of the stratosphere, at a height of between 20 and 40 kilometres, the ozone layer absorbs huge amounts of the Sun’s ultraviolet radiation – over 97 per cent of its medium-frequency ultraviolet light. This is the characteristic that makes this layer so important in protecting the life forms beneath it from the potentially damaging effects of UV exposure, particularly UV-C exposure, which is harmful to almost all living things because of its ability to cause genetic damage.

The absorption of all this UV light by ozone molecules in turn triggers a chemical reaction, whereby the ozone is photolysed (broken down) by the UV into O and O2, which rapidly re-forms into ozone and results in the release of heat. It’s this process of photolysis (the separation of molecules by the action of light) and rapid reformation of ozone that creates the characteristic temperature inversion of the stratosphere. The heat generated by this process causes temperatures to rise from around -51 degrees Celsius at the base of the stratosphere to as much as 0 degrees Celsius at its highest levels. With very little mixing between the different temperature levels due to the lack of convection currents, this layer is stratified into layers of warmer air sitting above cooler layers below, and it is for this vertical stratification that it is named. This process also gives the layer a degree of stability; in the absence of convection currents and the associated weather systems that are constantly driven through the troposphere, there is hardly any turbulence in the stratosphere. This makes the lower levels the ideal altitude for commercial airliners to cruise in, because the lower density of the air combined with the lower temperature means this flight path is the most fuel-efficient. Beyond these reaches of the stratosphere the air becomes too thin to support commercial flight, so we leave behind the last layer of the Earth’s atmosphere regularly populated by humans and make our way upwards towards the next layer, known as the mesosphere.

The third layer of our atmosphere, the mesosphere, rises up from an altitude of around 50 kilometres and extends up to 85 kilometres above sea level. Here, once again, the temperature dynamic follows a simple relationship between increasing altitude and decreasing temperature. This means that by the time we reach the top of the mesosphere, at a region known as the mesopause, we find ourselves at the coldest place around Earth, with an average temperature of -85 degrees Celsius. The air is so cold at this altitude that any water vapour forms into what are known as noctilucent clouds, or, more poetically, night shining clouds, around the polar regions of the planet. These are the highest clouds in our atmosphere and if you are lucky enough to be at the right latitude at the right time, when the Sun is rising or setting, this rare phenomenon can be visible to the naked eye.

JOYCE PHOTOGRAPHICS/SCIENCE PHOTO LIBRARY

Cirrostratus clouds are similar to cirrus, but form more of a widespread, veil-like layer.

PHIL DEGGINGER/SCIENCE PHOTO LIBRARY

5km

Cumulonimbus clouds are menacing-looking multi-level clouds, extending high into the sky in towers or plumes. More commonly known as thunderclouds, cumulonimbus is the only cloud type that can produce hail, thunder and lightning.

2.5km

JOHN MEAD/SCIENCE PHOTO LIBRARY

Cumulus are fluffy, fair-weather clouds that sometimes look like pieces of floating cotton wool. The base of each cloud is often flat and may be only 1,000m above the ground. These clouds grow upward, and they can develop into cumulonimbus.

DAVID R. FRAZIER/SCIENCE PHOTO LIBRARY

Stratus clouds are uniform and flat, producing a grey layer of cloud cover which may be precipitation-free or may cause periods of light precipitation or drizzle.

PEKKA PARVIAINEN / SCIENCE PHOTO LIBRARY

Cirrus comes from the Latin word meaning ‘curl of hair’. These are high clouds, forming between 6.2 and 13.7km. They are made up of ice crystals and tend to appear before a low-pressure area like a storm system in the middle latitudes or a tropical system like a hurricane. Their delicate, feathery shape comes from wind currents that twist and spread the ice crystals into strands.

PEKKA PARVIAINEN/SCIENCE PHOTO LIBRARY

Cirrocumulus clouds are thin cloud patches found high in the troposphere and are made up of individual ‘cloudlets’. They appear between 5 and 12km and contain a small amount of liquid water droplets as well as ice crystals.

PEKKA PARVIAINEN/SCIENCE PHOTO LIBRARY

Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapour to condense.

PEKKA PARVIAINEN/SCIENCE PHOTO LIBRARY

Altocumulus clouds are typically found in groups clumped together. They’re found in the middle layer of the troposphere, lower than cirrocumulus and higher than their cumulus and stratocumulus counterparts. The term mackerel sky is also common to altocumulus clouds that display a pattern resembling fish scales.

GEORGE POST/SCIENCE PHOTO LIBRARY

Stratocumulus clouds are a mix of stratus and cumulus, forming a low, fluffy layer. They may indicate storms to come.

CORDELIA MOLLOY/SCIENCE PHOTO LIBRARY

Nimbostratus clouds are multi-level, amorphous, nearly uniform and often dark grey, and they usually produce continuous rain, snow or sleet but no lightning or thunder.