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TO ANNA – AMONGST THE VASTNESS OF THIS STORY HOW LUCKY AM I TO HAVE FOUND YOU.

ANDREW COHEN

COVER

TITLE PAGE

DEDICATION

SOLAR SYSTEM

AN INTRODUCTION

MERCURY + VENUS

A MOMENT IN THE SUN

EARTH + MARS

THE TWO SISTERS

JUPITER

THE GODFATHER

SATURN

THE CELESTIAL JEWEL

URANUS NEPTUNE PLUTO

INTO THE DARKNESS

INDEX

ACKNOWLEDGEMENTS

ABOUT THE AUTHOR

ABOUT THE BOOK

ABOUT THE PUBLISHER

AN INTRODUCTION

SOLAR SYSTEM

PROFESSOR BRIAN COX

WANDERING LIGHTS

In the daytime, our universe stretches only as far as the horizon. The Sun hides in plain sight because it is too bright for us to see it directly. Only rarely do we glimpse a watercolour moon. Unless we think hard, our intellects are confined to the surface of the Earth. After sunset, beyond cities, the Universe appears; a destination for the imagination, albeit separated by a seemingly unbridgeable gulf. This may be true for the stars, but it is not so for the planets. There are times when Mars, Venus, Jupiter and Saturn dominate the sky; bright lights that shift position nightly against the fixed stars, commanding our attention even if we aren’t certain what we’re looking at. The distances are still vast by terrestrial standards, but despite appearances the gulf is certainly not unbridgeable, because we have visited all of these planets and taken our first steps into the outer reaches of the Solar System beyond. And yet the wandering lights in the dark still feel detached from human affairs, and the time and effort we’ve spent in visiting them might seem to be an indulgence. This assumption, however, is profoundly wrong.

The exploration of the planets is not an indulgence. If we want to know how we came to be here we need to understand the histories of the planet that gave birth to us and the system that gave birth to it. We are children of Earth and also children of the Solar System.

The Milky Way in the night sky over Sliding Spring Observatory, New South Wales, Australia.

The Solar System is a system. The Sun and the eight major planets and countless billions of minor planets, moons, asteroids, comets and unclassified lumps of ice and rock were formed back in the mists of time and they continue to evolve as one. We rarely notice the dynamic, interconnected nature of our system, although asteroid strikes on our planet are not such a rare occurrence. The Chelyabinsk impact in February 2013 injured 1,500 people when a 12,000-tonne asteroid broke up as it entered the Earth’s atmosphere at 60 times the speed of sound, and the Tunguska airburst in Siberia in 1908 flattened 800 square miles of forest in an explosion comparable to that of the most powerful hydrogen bomb ever tested. The surface of the Moon bears testament to a record of violence and destruction from the skies that the Earth has also endured, but the relentless erasure of craters by weathering and our good fortune that no major impacts have occurred in recorded human history are the reason for our misplaced sense of isolation from the heavens.

The Tunguska airburst of 1908 is the largest impact event on Earth in recorded history. It flattened 800 square miles of forest.

Meteor showers are not uncommon, but as most meteors are smaller than sand grains, they disintegrate before hitting the Earth’s surface.

The interdependent nature of the Solar System has become more evident as we have begun to understand its history. It is tempting to imagine that the physical layout of the planets is a fossilised remnant of primordial patterns in the collapsing dust cloud around the newly ignited Sun 4.6 billion years ago, but our exploration of the planets, coupled with increasingly powerful computer simulations of the evolution of the Solar System, has revealed that this is not the case. Planetary orbits are prone to instability; particularly so in the early, more chaotic years when our Solar System was young. The details of precisely how the planetary orbits have shifted are still uncertain, but we now suspect that Mercury, the innermost planet, began life much further out and was deflected inwards to its present-day seared orbit. Jupiter and Saturn may have drifted inwards shortly after their formation, before reversing their course and retreating, but not before affecting the distribution of material out of which Mars and Earth would later form. Around the time that life began on Earth, Neptune and Uranus may have been flung outwards, disrupting the orbits of billions of smaller objects far from the Sun. The record of this time of unprecedented violence, known as the Late Heavy Bombardment, is written across the scarred surface of the Moon, itself most likely formed in a glancing interplanetary collision between Earth and a Mars-sized planet 4.5 billion years ago. The planets are like snowflakes; the detail of their structure – their composition, size, spin and climate – are a frozen record of their past.

Two views of the Chelyabinsk meteor fireball caught on camera in 2013.

An understanding of the planets beyond Earth is therefore a prerequisite for understanding our home world, and that in turn is a prerequisite for understanding ourselves. Earth is unique in the Solar System because it is a planet with a complex ecosystem. The genesis and subsequent 4-billion-year evolution of life on Earth required planetary characteristics which are necessarily linked to the evolution of the system as a whole. There had to be liquid water on the surface, and much of this water was delivered after the Earth’s formation by icy, water-rich asteroids and comets, possibly deflected inwards from the outer Solar System by Jupiter. These rivers, seas and lakes of extra-terrestrial origin had to persist for the best part of 4 billion years, which required a stable atmosphere to maintain surface temperatures and pressures within a limited range. Four billion years is a long time – around a third of the age of the Universe. The Sun has brightened by 25 per cent since the Earth formed, which makes the stability of our environment all the more difficult to understand. In a chaotic system of planets around an evolving star, a planet with life-supporting properties and the remarkable stability enjoyed by Earth over billions of years may be extremely unusual. The study of our sister worlds, Mars and Venus, has proved instructive in understanding just how fortunate we may have been and how delicate our position today might be.

Satellite image of the Earth, centred on the Pacific Ocean. Water dominates this hemisphere of the Blue Planet.

The far side of the Moon bears the scars of the Late Heavy Bombardment. Photographed from Apollo 16 in 1972.

This colour-enhanced satellite photo of the Mississippi River Delta shows a lush, watery landscape.

Four billion years ago, as life began on Earth, Mars was also Earth-like.

Four billion years ago, as life began on Earth, Mars was also Earth-like. It had oceans and rivers and active geology and complex surface chemistry; the ingredients of life. One of the primary goals of the fleet of spacecraft currently in orbit around and exploring the surface of Mars is to search for evidence of past or even present life, and to understand why the red planet was transformed from a potential Eden at the dawn of the Solar System to the frigid desert world we observe today. The story is complex, but one of the most important differences between the two worlds is size. Mars is just one-tenth the mass of Earth; too small to hang on to its internal heat, its protective magnetic field and its atmosphere for much more than a billion years after its formation. Yet Mars formed in a similar region of the Solar System to Earth and Venus, so why is it so small? The answer may lie in the fast-changing orbits of Jupiter and Saturn early in the Solar System’s history. These surprising findings will be explored later in this book.

NASA’s Mars Reconnaissance Orbiter captured this photograph of Aram Chaos, an ancient impact crater that once held a lake.

The Apollo 11 mission in July 1969 changed human history, landing the first people on the surface of the Moon.

‘Life, forever dying to be born afresh, forever young and eager, will presently stand upon this Earth as upon a footstool, and stretch out its realm amidst the stars.’

H.G. Wells

The history of Venus is perhaps even more puzzling, in part because of the immense difficulty of exploring the planet. Venus is often described as a vision of hell; surface temperatures are high enough to melt lead, and the atmospheric pressure is 90 times that on Earth. Sulphuric acid raindrops fall from its clouds. And yet, long ago, Venus too may have been Earth-like. Perhaps there were once Venusians, before a runaway greenhouse effect took hold and began destroying Venus’s temperate climate around 2.5 billion years ago – although this date is highly uncertain.

Taken together, the stories of the three large terrestrial planets are salutary. If an alien astronomer observed our Solar System from afar, they would classify Mars, Earth and Venus as potentially living worlds, orbiting as they do inside the so-called habitable zone around the Sun – the region within which, if atmospheric conditions are right, liquid water can exist on the surface of the planets. All three worlds may have once been habitable, and all three worlds may have once harboured life, but now only Earth supports a complex ecosystem, let alone a civilisation.

Understanding why Mars and Venus diverged so significantly from Earth over the last 4 billion years will provide great insight into the fragility of worlds and perhaps suggest whether our own good fortune is near-impossible to comprehend or merely outrageous. Planets change. Ours could change at any moment. A stray comet from the frozen Kuiper Belt beyond the orbit of Neptune could put an end to our story. We could also put an end to ourselves. The study of Venus might help us avoid one of the ways by which we could destroy our civilisation, because it shows us what greenhouse gases can do to a world. I think one of the reasons why anthropogenic climate change is so difficult for a certain type of person to accept is that atmospheres seem ethereal and tenuous and incapable of trapping enough heat to modify the temperatures on a planet significantly. For such people I suggest a trip to Venus, where they will be squashed and boiled and dissolved on the surface of Earth’s twin.

The exploration of the planets, then, is not an indulgence. If we want to know how we came to be here we need to understand the histories of the planet that gave birth to us and the system that gave birth to it. We are children of Earth and also children of the Solar System. Understanding our history is important because it places our existence in context. The more we learn about the events that led to the emergence of humans on this planet only a few hundred thousand years ago, the more we are forced to marvel at the sheer unlikeliness of it all. We needed Jupiter and the comets and asteroids and countless collisions and mergers and near-catastrophes stretching back 4.6 billion years. There are valid objections to this way of thinking; it is an objective fact that we are here, and our future should be our primary concern. That may be so, but I argue that a deeper understanding of the evolution of the planets is essential for our continued prosperity and existence on this one. The threat of catastrophic climate change is an obvious example, but there are many other reasons why knowledge is important. There is feedback in human affairs; our collective state of mind affects the decisions we make. To confine our imaginations to the surface of the Earth is to ignore both our immense good fortune and the fragility of our position. A wider knowledge of both will, I believe, help secure a safer and more prosperous future.

No 1

MERCURY
+
VENUS

A MOMENT IN THE SUN

ANDREW COHEN

DAYS YET TO COME

E arth sits a mere 150 million kilometres from the Sun – not too hot, not too cold, with surface temperatures ranging from minus 88 to plus 58 degrees Celsius. This ‘Goldilocks’ location has created a stability of climate that, despite the best efforts of ice ages and impacts, has allowed life to maintain an unbroken chain for nearly 4 billion years, and yet we know for certain that it cannot last.

Our Sun, like every star in the universe, is far from static. Stars have life cycles of their own and, eventually, the hydrogen fuel that powers the nuclear reactions within a star will begin to run out and the star will enter the final phases of its lifetime. It will expand, cool and change colour to become a red giant. Small stars, like our sun, will undergo a relatively peaceful and beautiful death, which will see it pass through a planetary nebula phase to become a white dwarf, which will cool down over time to leave a brown dwarf. Life on Earth has prospered through our sun’s middle years, but these optimum conditions are waning. At first the changes will be invisible, but a billion years from now they will be obvious to any life forms left on the planet – an immense sun, filling the sky, will warm and transform itself and the Earth that it shines upon. The Sun is both the giver and the taker of life on our planet.

‘What has been is what will be, and what has been done is what will be done, and there is nothing new under the sun.’

Ecclesiastes 1:9

It is one of the great paradoxes of the Universe that as the life of a star like ours begins to wane, its size and luminosity will increase. A rise in luminosity of just 10 per cent will see the average surface temperature on Earth rise to 47 degrees Celsius instead of the 15 degrees Celsius that it is today. The effect of this rise in temperature manifests in a lifting of vast amounts of water vapour from the oceans into the atmosphere, creating a greenhouse effect that could quickly and rapidly run out of control, evaporating the oceans and sending the surface temperature skyrocketing. Astrobiologist David Grinspoon explains,

The greenhouse effect is the name we give to the physical process by which planets heat up through the interaction of their atmospheres and solar radiation. Solar radiation comes in what we call the visible wave lengths, primarily wave lengths that we can see, and most atmospheric gases are very transparent to visible radiation. So light from the Sun comes through pretty much unimpeded by an atmosphere and reaches the surface of a planet. Then the surface of the planet reradiates that radiation in infrared, because planets are much cooler than the Sun. And that means they radiate at much longer wave lengths – what we call infrared. That infrared radiation doesn’t make it through an atmosphere so easily. Some of the atmospheric gases, the ones we call greenhouse gases, block infrared radiation and so therefore the more of those greenhouse gases that are in a planet’s atmosphere the harder it is for that surface radiation to make it back out into space and the more that planet will heat up.

Estimates of the timescale that will see our oceans disappear vary massively, and are heavily influenced by a multitude of factors, but few are in doubt that by the time our planet reaches its 8-billionth birthday (in 3.5 billion years’ time) the end will be in sight. With temperatures heading above a thousand degrees, life will have long disappeared from a surface that is beginning to melt under the burning Sun.

These computer artworks show how the Earth might appear in 5–7 billion years’ time. As the Sun swells and becomes a red giant, temperatures on our planet’s surface will soar, making life untenable.

Our Sun is far from static, and NASA’s Solar Dynamics Observatory regularly and consistently tracks its rise to solar maximum. This composite image shows 25 shots taken between 16 April 2012 and 15 April 2013, which reveal an increase in solar activity.

This series of photos, captured by the Hubble Space Telescope in 2002, demonstrates the reverberation of light through space. A burst of light from an unusual star in the constellation spreads through space and reflects off surrounding dust. During this activity, the red star at the centre brightens to more than 600,000 times the Sun’s luminosity. It will continue to expand before eventually disappearing.

Astrobiologist David Grinspoon on the greenhouse effect

‘The greenhouse effect gets a bad rap these days and that’s understandable because we are tweaking it in ways we don’t fully understand, in changing the climate balance that we depend upon on this planet.

‘And yet it’s important to understand that the greenhouse effect is an essential part of what makes the Earth a habitable planet. Without some measure of greenhouse effect, Earth would be completely frozen over and life would not be possible on this planet.

‘Thirty degrees or so of a greenhouse effect on a planet like the Earth is absolutely wonderful and crucial and what keeps us alive, what makes Earth such a great place for life because it keeps us in that liquid water zone.

‘But, of course you can have too much of a good thing, look at Venus for a possible image of Earth’s future, if we’re not careful.’

Moving even further into the future the outlook becomes bleaker. As the Sun enters old age it will grow into a red giant, engulfing the Earth within its expanding atmosphere. Moonless, lifeless and perhaps reduced to its inner core, our planet and the civilisations it once harboured will be nothing but a distant memory, etched in the atoms that made us all as they are dispersed amongst the cosmos.

For planet Earth, the clock is ticking and time is slowly running out, but ours is far from the only world to enjoy its moment in the sun. Across the history of our Solar System, stretching deep into its ancient past and reaching far into its future, we see stories of worlds in a constant battle with our ever-changing star. Close in, ancient worlds such as Mercury, which long ago lost their fight with the Sun, are taunted by views of Earth, and what might have been. Further out and even hotter, Venus circles, shrouded in a choking cloak of cloud, and even further beyond the Earth, Mars sits cold and barren. Beyond these planets, frozen worlds await, huddled in perpetual hibernation; anticipating the moment when the warmth of the Sun reaches out far enough, with sufficient heat, to trigger a first spring. On that day, mountains of ice will melt, rivers of water will flow, and where there was once only bleakness, in the distant future on planets once frozen and lifeless we might find a place that looks very much like home.

The story of our Solar System is not as we once thought it – eternal and unchanging. Instead it is a place of endless transformation. It is a narrative that repeats itself with a predictable rhythm, and as one world passes, another comes into the light. Only one planet has maintained stability for almost the entire life of the Solar System: the Earth has remained habitable for at least 4 billion years while change has played out all around it. What makes the Earth so lucky compared to all of its terrestrial siblings? To answer that question we need to look not just at our planet but at the whole of the Solar System, going right back to the very beginning.

The last colourful hurrah of a star. Ultraviolet light from the dying star causes a glow around the white dwarf at the centre, where the star has burned out. This planetary nebula tells the story of the demise of our own Sun.

IN THE BEGINNING

This artist’s image represents a dead star known as a pulsar. The disc of rubble that surrounds it resembles the protoplanetary discs of gas and dust that are found around young stars, which collide and coalesce under the gravitational force of attraction to create young planets.

For the first few million years after its birth, there were no terrestrial worlds to see the Sun rise, and there were no days, no nights, no circular tracks around it. Instead, surrounding our infant star was a vast cloud of dust and gas. A tiny fraction of the material left over from the Sun’s formation, this swirling cloud would one day coalesce to form the various planets of the Solar System, and many other smaller bodies, but at this time, 4.7 billion years ago, there was nothing but tiny specks of dust reflecting back the light of our slowly growing star.

Only time – vast amounts of empty time – would allow enough of this gas and dust to catch and cluster, randomly forming the smallest of seeds. Most of these seeds would hardly get the chance to grow at all, smashed apart and returned to the immense swirl of dust from whence they arose. Just a few would grow big enough and survive long enough to capture and condense more of the cloud, slowly increasing their mass and density.

We still do not fully understand the process by which grains of dust no thicker than a human hair can amass to become rocky objects the size of a car, and as yet no model exists to explain this part of the evolution of a planet. But what we do know is that once that disc of gas and dust becomes populated with clumps of rock that make it past the ‘metre-size barrier’, a powerful force comes into play to propel the process forward. These newly formed planetesimals are big enough to allow the great sculpting force of gravity to draw the clumps together, growing to sizes of over a kilometre in length. Swirling around the Sun, thousands upon thousands of these objects live and die, colliding and coalescing under the increasing gravitational forces of attraction until eventually just a few emerge as planetary embryos, moon-sized bodies known as protoplanets. In the last violent steps of the process of planetary birth these protoplanets swirl around in crowded orbits, and many are destroyed, returned to the dust of their origins, but occasionally when a collision brings two or more of these giant objects together, the size of this mass of rock becomes big enough for gravity to pull it in from all sides, creating a sphere of newly formed rock, a new world. In that moment a planet is born.

Each of the terrestrial planets in our Solar System was born this way. They are the survivors of a process that destroys far more worlds than it ever creates, and which left just four rocky planets remaining – starting closest to the Sun with Mercury, then Venus, Earth and finally, farthest out, the cold and dead world of Mars. Today these four worlds all look vastly different, and yet all were created the same way, made up of the same ingredients and orbiting the same star. So why have they ended up so distinct from each other, and with such starkly different environments? And what makes this place, the Earth, so unique, the only one of the rocks that has blossomed with life? To understand, we have to look deep into the past of our Solar System, to explore the unique history of each of the planets by means of amazing feats of human engineering across billions of miles, and into environments of unknown and unimaginable extremes.

EXPLORING MERCURY

Getting to the smallest planet in the Solar System is anything but easy. Skirting past the Sun at a distance of just 46 million kilometres at the closest point in its orbit, Mercury is a planet that is not only held deep in the gravitational grip of our massive star but is also moving at an average orbital speed of 48 kilometres per second (km/s), by far the fastest-orbiting planet in the Solar System and far outpacing the Earth’s more leisurely 30 km/s. It needs to whip around this quickly, otherwise it would have fallen into the Sun’s embrace long ago, but the combination of its speed and position make it a planet that’s immensely difficult to get close to, and even harder to get into orbit around. In order to do so, you have to travel fast enough to catch up with Mercury but not so fast that you cannot somehow slow down to prevent a headlong descent into the Sun, and that challenge has meant that until relatively recently it was the least explored of all the terrestrial planets.

For many decades our first and only close-up glimpse of the innermost rock orbiting the Sun came from the Mariner 10 spacecraft, when on three separate occasions in 1974 and 1975 it briefly flew past Mercury. This was the first spacecraft to use another planet to slingshot itself into a different flightpath, using a flyby of Venus to bend its trajectory to allow it to enter an orbit that would bring it near enough to Mercury to photograph it close up. Clad in protection to ensure it could survive the intense solar radiation and immense extremes of temperature, Mariner 10 was able to send back the first detailed images of Mercury as it flew past at just over 200 miles above its surface. It passed by the same sunlit side of Mercury each time, so it was only able to map 40 to 45 per cent of Mercury’s surface.

The first glimpse of Mercury from Messsenger, nearly 40 years after Mariner 10’s historic mission.

The spacecraft took over 2,800 photos, which gave us never-before-seen views of the planet’s cratered, Moon-like surface, a surface that we had never previously been able to fully resolve through Earth-based observation. Despite the beauty of the pictures taken, it wasn’t the images from Mariner 10 that really surprised us, it was the data the probe collected relating to Mercury’s geology, which pointed to a much more surprising history of the planet than had previously been imagined. Mercury, it seemed, was far from being just a scorched husk.

Mariner was able to sense the remains of an atmosphere consisting primarily of helium, as well as a magnetic field and a large iron-rich core, opening a mystery that would remain unexplored for another 30 years. As it flew past Mercury for the last time on 16 March 1975, the transmitters were switched off and its contact with Earth silenced. Mission completed, Mariner 10 began a lonely orbit of the Sun that, as far as we know, continues to this day.

‘We have Mercury in our sights.’

MDIS Instrument team, 10.30 am EST, 9 January 2008.

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