Suppose you could come with me to a place that is typical of the Universe. A location that experiences the average conditions found throughout the whole of space. Where would we be? On the surface of an alien planet, perhaps, luxuriating among exotic plants and strange, colourful creatures? Or close to the brilliant churning atmosphere of a hot star, with tortuous magnetic fields funnelling lethal bursts of plasma towards us? Falling into a black hole? Or just – nowhere?
It’s the last of these that is closest to the truth. A typical place in the Universe is empty, cold and dark. And nothing in our experience can quantify just how empty, cold and dark it is. If you’re lucky, you might find one atom of hydrogen in the volume of space normally taken up by 15 adults – a cubic metre. The temperature you’d experience is 2.7 degrees above absolute zero, or –270 °C. That’s cold. And, to your unaided eyes, the darkness is complete.
But don’t worry – I’m not going to leave you here. From this typical spot, we can move at the speed of light towards a place that, after 100 million years or so of travel, will reveal itself to our eyes as a gigantic disc of stars, dust and glowing gas in space. It’s set among a handful of other swirls of light now becoming visible, but this one is special enough to have a name, and is known as the Milky Way Galaxy. As we approach it at light speed, another 100 000 years brings us into its suburbs, now visible as a shimmering haze of stars and pink clouds of hydrogen, with dusty patches between them. And setting our sights on one unassuming star brings us to a curious collection of planets – four small rocky ones and four big gassy ones, with a lot of small debris roaming between them. The third planet out from the star looks a bit unusual, with blue and white colouring interrupted by occasional patches of reddish-brown. Mind you, it’s nothing compared to the weird one with the rings around it.
As we finally touch down on solid ground, we find there could hardly be a better place in which to check out our home planet. We’re in the wilderness of southern Darmaraland in Namibia, surrounded by house-sized granite boulders flushed pink as the rising Sun adds its own hue to the iron-rich stone. There’s precious little vegetation in this desert landscape, and the restless history of the Earth’s surface is clearly revealed in the tumbling spine of mountains before us. They speak of a time 130 million years ago, when molten rock spilled from gigantic fractures in the supercontinent of Gondwana as it broke apart. Its remnants are present-day Africa, South America, Antarctica, Australia and the Indian subcontinent.
The realisation that plate-like segments of the Earth’s crust, or lithosphere – which ranges from 60 to 250 kilometres thick – are in a state of constant movement was one of the great triumphs of mid 20th-century geophysics. It was a theory whose time had come, and half a century of scepticism was ending when I was a pimply teenager at school in the 1960s. New mathematical modelling of heat flow in the Earth’s mantle (the underlying layer of soft rock that extends some 2900 kilometres below the surface) had shown that upwelling plumes of viscous rock could, indeed, drive breakneck motion in continental plates. Think lava lamps, and you’ll see what I mean. And yes, I know – ‘breakneck’ is an adjective seldom used in geology, but it’s justified in this case: the African and South American plates separate at 2 to 3 centimetres per year, roughly the speed at which your fingernails grow.
It’s the ever-widening boundary between these two plates that forms the Mid-Atlantic Ridge, a submarine feature that extends almost from pole to pole, and breaks the ocean surface only in the youthful volcanic landscape of Iceland. While we’re used to hearing about tectonic activity in places like Japan, Sumatra and New Zealand – where plates converge, often with disastrous seismic consequences – it is in Iceland where the dynamics of our planet are perhaps at their most visible. As the island is unrelentingly torn in half, volcanic activity is commonplace.
To the best of our knowledge, Earth is unique in the Solar System in having plate tectonics – at least in the present era. And its vigorous geology has spawned a rich chemistry on and near the surface, stimulating a wealth of pre-biotic reactions – and, some three billion years ago, the emergence of living organisms. Today, life blazes forth in all its myriad forms: even here in Darmaraland, where noble desert elephants epitomise its ability to adapt to the most adverse conditions. And we all know the ultimate consequence of biological adaptation. It has produced the most complex entity known in the Universe – the extraordinary brain of Homo sapiens.
JUST AFTER SUNSET TONIGHT, THE CLEAR NAMIBIAN SKY will bring a feast of Solar System celebrities. Deep in the western twilight, the planet Venus will herald giant Jupiter high above, while Saturn vies for prominence in the north-east. But it’s the slender crescent close to Jupiter that will grab everyone’s attention. At this phase, the Moon’s disc is bathed in Earthshine (sunlight reflected from the full Earth in the lunar sky), and its dusky surface is faintly visible between the sunlit horns of the crescent. Earthshine has a practical scientific use, explored a few years ago by Canadian and French astronomers. It is the sum total of daylight from the whole Earth – oceans, landmasses, clouds and ice-caps. Cities, towns, golf courses and beer gardens. Everything – and, by analysing it using the rainbow spectroscopy described in chapter 15, astronomers can look for signs of life on our own planet in a trial of the technique’s effectiveness for observing the planets of other stars in the future.
Most of us take the Moon for granted, but its gravitational influence has probably been pivotal in the evolution of life on Earth. For example, ocean tides may have been important in animal life gradually migrating from a water environment to dry land, as the twice-daily flooding of the coastline provided a conducive environment. And, more fundamentally, the ‘flywheel’ effect of a large moon orbiting Earth is believed to have stabilised our planet’s axial tilt, keeping it within a whisker or so of its current value of 23.5 degrees. That has promoted stable climatic seasons favourable to biological evolution, and contrasts with a planet such as Mars, which is known to have experienced large changes of tilt (up to 20 degrees) over relatively short timescales (approximately 100 000 years).
Our planet has two other attributes that have assisted in the evolution of life. One is its nickel-iron core, whose diameter of 6970 kilometres is rather more than half that of the planet. At the centre of the molten outer core is a 2440-kilometre-diameter solid metal sphere under extreme pressure, and at a temperature recently estimated to be 6500 °C. Convection currents in the liquid core give rise to the Earth’s magnetic field and generate the magnetosphere, a protective barrier that effectively shields the planet’s surface and atmosphere from destructive bombardment by the solar wind. This is no benign zephyr, but an energetic stream of electrically charged subatomic particles from the Sun.
At irregular intervals, the dynamo-like interaction between the Earth’s solid and liquid cores causes the geomagnetic field to fall in intensity, and occasionally to reverse. It’s possible this might occur again within the next couple of thousand years, given the 10 to 15 per cent decline in magnetic field strength that has been observed since measurements began in the mid-19th century. So – no geomagnetism equals no magnetosphere, and a threat to life? Not quite – the interaction of the Sun’s magnetic field with the Earth’s metallic core induces magnetism that will at least partially protect our fragile environment.
And finally, the Earth’s atmosphere provides more than just the air we breathe. A large fraction of the 50 tonnes of meteoritic material that bombards our planet daily (at velocities between 11 and 72 kilometres per second) is harmlessly vaporised 95 kilometres or so above the Earth’s surface. Objects that make it into the lower atmosphere or survive long enough to hit the ground as meteorites are relatively rare. You’ll read all about them in chapter 4. And, back on the subatomic scale of inbound material, the atmosphere substantially reduces the radiation dose of galactic cosmic rays at the Earth’s surface. Dangerous particles again. Clearly, without our planet’s blanket of air, we would be at the mercy of a decidedly hostile environment.
But the atmosphere is constantly in a delicate balancing act between monumental geophysical forces. Crucial to its long-term stability is the greenhouse effect of carbon dioxide, and its circulation between the mantle and the air we breathe. This provides a natural thermostat that depends on plate tectonics. When converging tectonic plates collide, the oceanic plate slides under its continental neighbour in an action known as subduction. But with it goes a layer of carbon that has fallen out of the atmosphere onto the ocean floor. The lubricating effect of seawater allows the subducting plate to descend a long way into the mantle, enriching its carbon content. Volcanic eruptions along the line of convergence then push that carbon back up into the atmosphere as carbon dioxide, from where it eventually falls again to the ocean floor. Given the fine balance of this complex process, there’s little wonder that the additional atmospheric carbon dioxide from a century of fossil-fuel burning has a significant impact on global temperature.
Over geological time, the carbon dioxide content of Earth’s atmosphere is regulated by plate tectonics. CO2 enters oceanic water directly or through rainfall to form carbonaceous rocks on the ocean floor. These are recycled into the atmosphere via subduction and volcanism.
Author, after USGS
HERE, IN THE PARCHED UPLANDS OF THE DARMARALAND wilderness, the atmosphere is thin, and the sunlight intense. It highlights a constant struggle taking place among the deep shadows of those giant granite blocks – the struggle of myriad species of African wildlife to survive. And it brings home a message to visitors like you and me. There is absolutely nothing typical about our planet. It is an extraordinary world, and caring for its atmosphere is something we could do better – much better. But, as an admirer of humankind’s resilience, and an inveterate optimist, I’m willing to bet that like the tenacious flora and fauna of Namibia, we will fix it. A grassroots movement towards renewable energy was foreshadowed more than a decade ago by the late Hermann Scheer, a German politician and solar power advocate, and it’s happening today. Hopefully, it will take effect soon enough to avert the peril of a runaway greenhouse effect like the one our next-door planet suffered some three billion years ago. With a surface temperature hovering around 470 °C and an atmosphere that drizzles sulphuric acid in its upper layers, Venus is not the kind of wilderness you’d ever want to visit. No matter how atypical it might be.
