For my mum, and in memory of my dad, neither of whom ever told me I needed to get a proper job
ENTER, STAGE LEFT
I grew up in a village with no street lights. The night sky was a vast bowl of dark, its edges lightened by the sodium glare of the nearby towns, its interior scratched by the navigation lights of airliners drifting down towards Heathrow. Venus, the bright morning and evening star, could hang on the horizon like a white jewel for days on end, and Jupiter and Saturn danced high in the sky. They wandered through the constellations, but were otherwise indistinguishable from stars. Mars, though, was different: from Earth it was clearly visible as a wavering point of red light. Identifiably, unmistakably red.
Mars was an inspiration to me, even if it always remained tantalisingly out of reach. But I did have a close encounter with it. I had progressed in an entirely unexceptional way from a degree in geology to a master’s in geophysics, and a mid-course sidestep into a doctorate on the magnetic properties of meteorites. The secrets of how and when the planets formed are locked up in these rocks, and the information I needed was held in microscopic fragments of raw, unaltered iron–nickel alloys. Later, in my first and only postdoctoral post, I ordered a piece of a meteorite from NASA’s collection. It duly arrived and I ran the usual tests on it, only to be disappointed. The magnetic minerals had been weathered to rust. It was useless to my research, so I put it to one side and concentrated on other, more promising samples.
I had been right that there was no trace of iron metal in that sample; it had all been converted to iron oxides. But it hadn’t happened here on Earth, and I should have looked harder, questioned more. Six months later, a laboratory in Japan analysed the trapped gases in the rock and announced their results. The previously miscategorised ALH84001 had formed on Mars, and the few grams of material I’d already sent back to the USA were now part of a tiny group of known Martian rocks, far too precious for a junior researcher to work on. I’d missed my chance. The economic tide receded. I left academic research. Eventually, I became a science-fiction writer. I even wrote stories set on Mars.
That lost opportunity was thirty years ago, always remembered with a sense of regret. Then, this unlooked-for grace. I’m fully aware of how ridiculous it sounds, but writing this book feels like completion, finally closing a circle I’d left half-drawn for far too long. I had held Mars in my hands once and now I could hold it again. And if I can convey even the smallest portion of the unfathomable joy I feel about this book, these words, this glorious scrapheap of a planet, then the journey to this point will have been worth it, because Mars is yours as much as it is mine.
PART ONE
DAWN ON MARS
It’s cold. So cold that the frost glittering on the rocks has been wrung out of the air itself, frozen and turned to ice. The Sun won’t rise for another hour, but it’s already light enough to see by, although too diffuse to cast shadows. Above you is a sky of pale pink and pastel blue, shot through with white, hair-thin streamers that might be high clouds chasing away from the dawn.
The landscape is flat: not sheet flat, but plains flat, badlands flat. It’s rough and rubbly, dominated not by the few variations in height, but by its sheer breadth and width and its utter emptiness. The distance to the horizon is an illusion you’ve not yet got used to: you think you can see forever, but it’s not even the distance of a parkrun. Still, you know, because you’ve seen the maps and the satellite images, that from where you stand there’s nothing between you and the frozen north pole but an unending, monotonous flatness, punctuated by craters of every size that have been gouged out of the ground. The geography only begins to change far to the south, with a planet-girdling slope that climbs inexorably upwards for a hundred kilometres until it plateaus on the pockmarked highlands. Also beyond your closely curved horizon to the south-west, out of sight but looming heavily on your mind, is the land between the red soil and the bright sky where the giant volcanoes squat. They manage to be vastly broader than they are tall, but they still grow so high that they reach above even the clouds.
You feel light because you are light – a third of your usual weight. Walking is difficult. You’re used to a falling gait, a motion that rocks you from right to left and back again, heel to toe. Using it here is not just inefficient but unwieldy. You try a few steps, as if you’re a toddler again, and find yourself slow and unbalanced. Then you remember what they taught you and move into something halfway between a skip and a lope. You spend far longer in the air than you ever thought possible. Bringing both feet down within seconds of each other, you push off again for another metres-long stride. It’s too similar to the dreams you’ve had of running without touching the ground to be entirely comfortable.
The soil is brittle and it snaps and squeaks as you press your deeply ridged boots into it. The dust that coats the planet from top to bottom is clay-fine, almost oily. Where it becomes loose, it drifts in persistent, dirty puffs and sticks to everything. Your bright white spacesuit is no longer pristine and it’ll never be clean again. You wear an ochre cast, deepening in colour towards your feet. It’s your badge, your sign, and it’ll mark out your stay in days and weeks as it darkens.
The rover you’re going to be using this morning is both basic and robust. It’s little more than an oversized electric go-cart, designed so that as little can go wrong with it as possible. Its frame is made from open struts of lightweight – but strong – alloy, and its four wheels are independently powered by the fuel cell slung underneath the driver’s seat. Each wheel is as tall as you are, and the treads around the circumference consist of hundreds of ridged metal plates. Each plate is sprung to provide suspension, and the amount of springiness is controlled by a computer, allowing the vehicle to navigate soft sand as well as hard rock and ice. The cargo you’re driving is strapped to the back of the chassis, already covered in a thin film of dust.
You swing yourself up easily – climbing in one-third Earth’s gravity isn’t a problem – and settle in the driver’s seat. The controls are designed to be foolproof. There’s a steering wheel, a trigger to accelerate and brake, and a switch to put the whole vehicle in reverse. The screen in front of you tells you where you are and, critically, how much power you have left. You know that if you abuse that, you’re going to be in real trouble. Someone will have to come out and find you before your air runs out. And perhaps they won’t make it in time. The only thing keeping you alive at the moment – and every moment you’re outside – is the suit you’re wearing. Your life is measured in the minutes of air on your back and the watts in your batteries. If the tanks run empty, you will die quickly. If the heater circuits fail, you will die slowly.
You start up the rover and steer steadily north. Compass directions are by convention only, because there’s nothing for a magnetic compass needle to find here. All your navigation is done by reference to the small constellation of satellites in orbit overhead. The landscape itself gives very few clues and you can see so little of it anyway. The best way back home is to follow your own tyre tracks, which will persist for a few weeks before being covered by wind-blown dust.
You suppose you’re closing on your intended destination. The ground becomes more uneven, with broken blocks of stone buried beneath the subtly ridged terrain. If you go too fast, you could hit one of these sharp-edged rocks and break a tyre-plate. You’ve been here for less than a week and already the metal of the wheels is starting to look pitted – not just mechanical wear, but chemical erosion from being in contact with the corrosive soil. So you slow down and make a special effort to steer around the most obvious obstacles protruding from the surface – a surface that seems to have flowed in the past, as if it was once thick mud that simply lost momentum and froze in place.
Your position in your seat makes you realise that you’ve been driving subtly uphill for a while. The terrain is increasingly chaotic, as if something has struggled its way out of the plain and dragged the subsurface with it. There are ridges of rock, hard hills emerging from the pitted plain, fringed by skirts of frost-damaged rock and curtains of landslipped debris. The going is tough. You find yourself driving wheel-first into shallow hollows and losing sight of the horizon until you claw your way back out again.
The gradient gets steeper. There is more rock – not just broken, but shattered, loose and open-jointed, both fragile and lethal. There are cliffs that you cannot possibly ascend, but there is also a way up – not exactly smooth, but passable with care. There are banked ridges to contend with, like driving up concrete steps, but finally, at the limit of your range and the capacity of the rover’s fuel-cell battery, you reach your destination, the place to plant your weather station. It’s just one part of the expanding network of remote data collection sites designed to feed the appetite of scientists back on Earth. Tomorrow, there’ll be another.
You climb down from your seat and look out over the crater that stretches before you. You can’t see the other side, and the effect is such that you seem to be standing at the edge of a kilometre-high curving cliff with no end. The ramparts are overlapping arcs of wall where collapse and time have carved out bites and thrown the digested remains at their feet. You could, you think, plot a course that would get you down there, but not today and not from where you are now. You make sure to stand away from the edge and not make any sudden moves. Even if you survived the fall, you wouldn’t live long enough for the climb back up.
The crater floor has more of the same pitted terrain as the landscape you’ve just driven through, together with sinuous lines and grooves that superficially look like dried-up riverbeds. There are sheltered lees where dust has accumulated into low, rolling dunescapes. In the far distance is a hint of a mountain: triangular, jagged, with more than a hint of archetype about it. This central peak is nearly a kilometre tall, base to summit, standing alone.
The Sun, a cool pale disc, sends streams of light over the edge of the crater wall, progressively illuminating the exposed layers of rock as it rises; the deep shadow and bright light are enough of a contrast to make you squint and wait for your eyes to adjust. Where the sunlight hits, the temperature soars from its night-time low of minus seventy. The frost turns to fog, twisting and swirling, quickly vanishing, and the dust-covered surface glows its traditional rust-red. Behind you, low on the horizon, the bright spark of a moon traverses the sky, its motion peculiar and unsettling.
This is Mars, as it is today, at this hour, this moment. This is Mars, dominated now by ice and time – not inactive, but its processes moving at a glacial speed.
Later in the afternoon, insubstantial winds will pick up and blow the fine dust around half the globe, a distant echo of when those same winds could tear and howl. Where there is now ice, there was once an ocean. Where there is now pink haze, there were once black clouds of volcanic ash. Where this crumbling, moth-eaten crater now sits, the roots of mountains once shook with the violence of impacts so great they threatened to break the planet apart. Mars was built from stories such as these, chapter after savage chapter, creating an entire, glorious world out of grains of dust and starlight.
Mars is unique and everything about it is extraordinary. Even – especially – its future.
MARS AS AN UNRELIABLE NARRATOR
We know what Mars looks like. We can measure it and take increasingly detailed photographs of it from space. We can send probes to the planet and scrape our way into its surface rocks. Our robots can sniff its air and taste its dust. We can find out how hot or cold it is, discover whether it is wet or dry, hard or soft, made of frozen lava or washed sand. We can admire its mountains and its plains and its canyons, vast and broad and deep.
But the moment we ask ‘How did it get this way?’, we stand on more uncertain ground. There are different routes Mars could have taken to reach the same point – roads that are equally possible – but we don’t know which one it actually travelled, or even whether it travelled just one. What we see might be the result of many paths coming together to produce a seamless whole. Each theory, each route to the present configuration, gives us the same Mars.
This presents a problem to anyone who wants to tell a story like this, and to anyone who wants to listen to it. Anything I offer you will look like the truth if I don’t tell you about the other possible options that could also get us to where we are now. We can point to an object on the planet’s surface – a feature, an anomaly – and while we can know a very great deal about it, we sometimes can’t say for certain how it got there. This is not because of scientific inaccuracies or mismeasurements, but because there are times when several proposed scenarios fit the data well enough to be plausible and none of them can be ruled out.
The history of Mars is drawn not just on its surface but also down into its broken bedrock and up into its frigid air. Most of all, it stretches back into deep time, where the trackways of the past have been obliterated by later events: there’s no discernible trace of where they started from or how they travelled, only where they ended up. The history of Mars is simultaneously obvious and hidden. We have to unravel it, see through each layer, to determine the order in which it was made. And sometimes we’ll get it wrong.
We’re going to find plenty of examples of this. Add to the mix that whatever I tell you could conceivably be true at the moment, to the best of our knowledge, but that by the time you read these words they could have been completely debunked. Science progresses and it doesn’t respect reputations. Sometimes I think it gleefully trashes them, leaving a trail of broken hearts and shattered careers in its wake.
So to be plain with you, this book is not going to tell the whole story. That’s not to say I won’t try, but when I come across alternative explanations of the same phenomenon, I may pick my favourite. Everything here is accurate, but I’m not going to pretend I’m impartial. Mars will always remain over-explained, rather than lacking any explanation at all, and I am not here to stab you with facts and leave you bleeding by the end of chapter three. I’m here to tell you about Mars, with all its ambiguities, and about the times when, honestly, I think it’s lying to us.
So rather than me being your unreliable narrator, I want Mars to take that role. Rather than putting the Martian story on rails, I want to open the map out, creases and all. I want to show you Mars, on its own terms, whichever of its stories we decide in the end to believe.
WHY IS MARS SO DIFFERENT?
Physics and chemistry are universal. This is good, because we have a common language to make sense of the results we get from our experiments, whether we do them here on Earth or on Mars.
In a straightforward way, we can talk about Mars’s gravity as being just over a third of Earth’s – on Mars, you’d weigh a third of your current weight. A Martian day is properly called a sol. In a strange and completely unconnected coincidence, a sol lasts just thirty-seven minutes longer than a day on Earth, but a Mars year is 668.6 sols long, as opposed to our 365.3 days.
But there are times when comparisons with Earth are going to be of limited use because conditions on Mars are so different. Where the differences really begin to bite is when we try to describe what Mars’s surface is like, both now and in the past. There is no getting around this, so I’m going to use water as both our example and our guide.
We already know that the temperature at which water freezes or boils depends on air pressure. At sea level, at one standard Earth atmosphere, which is roughly one bar (or more accurately, 1,013 millibars), water freezes at 0°C and boils at 100°C. But we’ve heard stories from mountaineers that at the top of Everest, where the pressure is only 300 millibars, water boils at 70°C and getting a properly brewed cup of tea is impossible. We might also have, or know someone with, a pressure cooker. Food cooks faster in one of these because the internal pressure can reach 2,000 millibars – nearly two standard Earth atmospheres – and water will only start to boil when it reaches 121°C. The freezing and boiling points of water are far from fixed.
What’s going on is this. The water we interact with every day generally comes in three states: solid, liquid and gas, which we call ice, water and steam. What determines water’s state is the strength of the bonds between one water molecule and the next. The bonds between the molecules are strongest in ice, weak in water and non-existent in steam. An increase in temperature means that the bonds vibrate more and eventually tear apart. An increase in pressure keeps the molecules closer together and makes the bonds more resistant to breaking. That’s why, at lower pressures, water boils at lower temperatures, and vice versa.
Off Earth, on other planets, moons and asteroids, the conditions for liquid water are rare. At the very highest pressures, water will almost always be in the form of ice, no matter the temperature, which sounds preposterous, but it’s true. At the very lowest pressures, including in the vacuum of space, the only possible states are ice and water vapour, with no liquid phase between. Rather than condensing into a liquid and then freezing, the vapour turns directly into a solid, a process known as deposition, and rather than melting and then evaporating, the solid turns directly into a gas, which is called sublimation. The Celsius scale is peculiarly parochial to Earth and it obscures the fact that for most of its occurrence across the universe, water is either a solid or a gas, not a liquid.
The knowledge to hold on to here is that all matter, all physical stuff, responds differently to changes in temperature and pressure. The major component of the Martian atmosphere is carbon dioxide, and that too can freeze solid to form ice if it gets cold enough – much colder than Earth’s air – but at most pressures it never becomes a liquid: it only moves between being a solid and being a gas.
Likewise, cold rock is brittle and it shatters when struck or stressed. Very hot rock melts and forms a liquid, which we know as lava when it’s above the ground and magma when it’s below. But rock under extreme pressure – tens or hundreds of kilometres underground – is neither solid nor liquid. Instead, it becomes plastic: that is, when pushed or pulled, it flows very slowly, like soft wax. It moves only centimetres a year, but if Mars has an abundance of anything, it’s time.
The chief differences between our planets, though, are purely physical. Mars is slightly more than half the diameter of Earth and it weighs ten times less. Mars’s moons are so small as to be inconsequential to the behaviour of the planet they orbit, while Earth’s ridiculously oversized Moon has consequences far beyond that of twice-a-day tides. Mars orbits the Sun at an average distance of 228 million kilometres, while Earth lies closer in, at a distance of 150 million kilometres. These three things – the size of Mars, its moons and its distance from its primary star – have influenced every moment of Mars’s history, and will also determine its future.
MAPPING MARS
In order to make sense of Mars, we need an actual map: one that pins features to its surface and gives us fixed points of reference. Unlike the maps found in the front of epic fantasy books, we won’t be visiting every last place that has a name, but like an epic fantasy, some of the places we will end up going to have difficult-to-pronounce names. I can only apologise for that.
Making maps of Mars has a long and somewhat ignoble history. Accurate maps – ones that might help us navigate its surface – have only been around for the fifty years since the Mariner 9 space probe took the first detailed pictures from orbit in 1972. Before that, much of what was drawn was not just inaccurate but fanciful and frankly wrong. Credit where it’s due for the attempts made by Johann Mädler in 1840, Richard Proctor in 1867 and Giovanni Schiaparelli in 1893: they used serious scientific observations to create their maps, but their technology – ground-based optical telescopes – was simply too limited to pick out anything but Mars’s brightest and darkest features.
After Mariner 9 had taken photographs covering most of Mars, the process of making the first proper map could begin. The map-makers divided Mars up into thirty quadrants: eight rectangles above and eight below the equator, six circling the north pole, six around the south pole and the two polar regions themselves. Any attempt to represent a spherical object on a flat sheet of paper is going to distort it, so the polar quadrants aren’t rectangles but are instead circular, and each quadrant varies in the size of the area it maps, from nearly 7 million square kilometres at the north and south poles to 4.5 million for the equatorial quadrants.
Each of these quadrants needed a name other than the prosaic nomenclature of MC-1 (short for Mars Chart) to MC-30. These thirty names became wedded to the areas they describe, and they roll off the tongue like a shipping forecast for another planet. In order, from north to south and west to east: Mare Boreum, Diacria, Arcadia, Mare Acidalium, Ismenius Lacus, Casius, Cebrenia, Amazonis, Tharsis, Lunae Palus, Oxia Palus, Arabia, Syrtis Major, Amenthes, Elysium, Memnonia, Phoenicis Lacus, Coprates, Margaritifer Sinus, Sinus Sabaeus, Iapygia, Mare Tyrrhenum, Aeolis, Phaethontis, Thaumasia, Argyre, Noachis, Hellas, Eridania, Mare Australe…
The list of names was compiled from the older maps of Mars, and each quadrant was given the name of a principal albedo feature within that quadrant (albedo is the visible light-or-dark contrast). These names subsequently transferred to actual geographical objects on the new maps: craters, volcanoes, plains. As the maps became more and more detailed with each successive mission, they became increasingly populated with new names, many of them informal, and the International Astronomical Union became the final arbiter of nomenclature.
The quadrants have more or less served their purpose: Mars maps are now as detailed as any atlas, but the language of Martian features is idiosyncratic, a reminder of when the learned of every nation spoke Latin to each other. Astronomers in the past used it and now we’re stuck with it.
A small flat area is called a palus. A larger one is a lacus. A more extensive low-lying plain is a planitia. A whole region is a regio, and for the want of a superlative, the only use of vastitas is where it describes Mars’s extensive northern lowlands. Rugged highland plateaus are plana, or if they are extensive enough, terrae. An individual mountain is a mons, unless it’s smaller and more dome-shaped, in which case it becomes a tholus. Neither mons nor tholus refers specifically to volcanoes, but on Mars most of them are. A mountain range is a montes, while hills are colles. A mensa is like a montes, but it has a flat top and is bounded by steep cliffs. Sometimes these cliffs are rupes and sometimes they’re scopuli.
Impact craters are still called craters, unless they’re paterae, which are craters with scalloped edges as if they’ve been torn carefully from a sheet of paper. Many of the structures that are paterae aren’t impact craters, but collapsed volcanic craters. Fortunately, a volcanic patera is usually found on a mons or a tholus of the same name, so it’s normally clear which is which. A chain of craters, however it’s formed, is a catena.
A valley is a vallis, but it doesn’t have to have been formed by water. A sinus looks like a bay, but it isn’t by a lake or a seashore. Insulae are islands, but they aren’t surrounded by the sea. A fluctus, however, was almost certainly formed by water – land after a flood. We can also add chaos here, which is heavily broken, unsurprisingly chaotic terrain that may be related not just to a flood but to a dam burst.
Cavi are steep-sided hollows formed by collapsing underground structures. A chasma is longer and deeper. A fossa is a crack in the crust caused by tension, but they rarely appear alone, so together they are fossae. Where valles, chasmata or fossae cross each other in a complex maze of faults and fractures, they become a labyrinthus. Land that has been squeezed so that it forms linear ridges is called dorsa. Martian wind gathers up dust and sand to form dune fields called undae, and Martian ice reveals itself in the tile-like landscape of tesserae.
Another problem with mapping Mars: where to put the prime meridian, the line of zero longitude? Every map needs a fixed reference point to mark where everything starts and finishes and to act as a ruler for measuring the planet. Competing national interests eventually settled that question on our home planet – the prime meridian passes exactly through Greenwich – but the makers of Mars maps could choose where to place it, and they did so before it was even decided for Earth.
In the 1830s, the German astronomers Beer and Mädler found a small mark just south of the Martian equator, which they denoted ‘A’. Schiaparelli continued the tradition with his 1877 map, and the mark was subsequently called Sinus Meridiani or ‘Middle Bay’. When Mariner 9 beamed back its photographic treasures in the 1970s, the quadrant-makers needed something more accurate to measure from, so in that same area they chose a small crater within a larger 40-kilometre one. The larger crater was called Airy, after the astronomer who first measured the Greenwich Meridian, and the smaller one, 500 metres across, became Airy-0. After that, with both the ability to land a probe on the surface and the increasing detail from orbital photographs, defining the very centre of Airy-0 became possible. The first lander on Mars, Viking 1, whose location could be pinpointed to the metre, was used to fix precisely where the centre of Airy-0 was, and this became Mars’s prime meridian. If Viking 1 was at 47.95137 degrees west, then Airy-0 was exactly at zero degrees.
A further problem map-makers have encountered is how to measure the relief of the land. On Earth the oceans are connected, and this gives a roughly consistent line (which we call sea level) from which to measure the height of the land or the depth of the sea. This line is then refined by accurate measurements of gravity and pressure. But on Mars there are no oceans, so there’s no consistent level to refer to, and we cannot measure absolute heights or depths without some kind of reference.
Arbitrarily assigning a zero level – the Mars datum – was necessary, but it also needed to be both useful and reproducible. Initially, it was chosen to be the level at which Mars’s air pressure matched the triple point of water: the unique lowest pressure where water can exist as either gas, liquid or solid. This is just over 6 millibars and it was entirely reasonable for Mars, but later on, more and better data led to the Martian gravity map being used to define the datum. The variation in the path of a satellite orbiting Mars can be used to calculate the planet’s centre of mass, and from there we can measure the exact average radius at the equator. Everything that is above this imaginary radius is above datum; everything beneath it is below datum. The highs and lows of Mars can now be measured. Its highest point is the summit of Olympus Mons, 21 kilometres up, and its lowest is the bottom of the Hellas crater, 8,200 metres down.
Armed with this bundle of information, we’re ready to write the first chapter of Mars’s glorious history.