How to Destroy the Universe (6 page)

Temperature increases markedly as you dig down into the crust, sometimes by as much as 2–3°C (4–5°F) for every 100 m (33 ft) down you go. And so the temperature in a mine 3 km (2 miles) below the surface can rise
to nearly 100°C (212°F). This is mainly due to the compression effect caused by the weight of the overlying rock and soil—just as the compressed air in a bicycle pump feels hot to the touch. Radioactivity in rocks can also contribute. The rate of increase of temperature with depth slackens off in the lower regions of the crust, reaching a maximum of about 400°C (750°F) at the boundary with the next layer down, known as the mantle.

The mantle is a soft layer of semi-molten rock. At almost 3,000 km (1,900 miles) thick, it accounts for more than 80 percent of the volume of the entire planet. It is broadly split into upper and lower mantle. No one has ever visited the mantle, but scientists have been able to investigate its properties by measuring the speed at which seismic waves—sound waves in Earth—pass through it. Seismic waves in the lower mantle are found to be faster than they are in the upper mantle, suggesting that the lower mantle is denser. In fact, while the upper mantle is soft and pliable, the high density and pressure in the lower mantle seems to squash it back into solid form. Both these major mantle layers are heavier than the overlying crust, which is formed from low-density rock that melted and then rose up to float at the surface like a cork in water.

The upper mantle divides into a number of sub-zones. The lithosphere is made up of the crust together with
the stony outer shell of the upper mantle; the astheno-sphere is a softer shell of mantle directly below this; and finally the Mohorovicic discontinuity, or “Moho layer,” is the boundary that separates the mantle from the crust. The upper mantle reaches down to a depth beneath the planet's surface of around 400 km (250 miles). Between its upper and lower zones is a layer known as the transition zone. From there, the lower mantle extends down to a depth of 2,900 km (1,800 miles), at which point the temperature has soared to a blistering 4,000°C (7,000°F). This is the beginning of Earth's core.

The core is the densest, hottest part of Earth's interior. It's made mainly from the metals iron and nickel, and, like the mantle, it is divided into an inner and an outer layer. The outer core is mostly liquid and reaches down from the bottom of the mantle to 5,150 km (3,200 miles) beneath the planet's surface. The inner core is a ball of solid metal within this, roughly 2,440 km (1,500 miles) in diameter. The outer core is thought to be responsible for generating Earth's magnetic field. As electric currents in the conducting liquid metal are swished round by the planet's rotation, they create a magnetic field through the dynamo effect—exactly the same principle by which electrical generators work. Although the field is very weak—about one thousandth
the strength of a fridge magnet—it's strong enough to bat away electrically charged cosmic ray particles that wander in from space. A single cosmic ray can contain as much energy as a fast tennis serve—all packed into a tiny subatomic particle. If this much energy crashes into a DNA molecule in your body it can cause mutations that could lead to cancer and death—indeed, life on Earth would have had a hard time getting going were it not for the core and its magnetism. We owe our very existence to it.

Some scientists have speculated, however, that Earth's magnetic field may fail us yet. Every few tens of thousands of years, the field undergoes a complete reversal—where north and south poles literally trade places. There has been concern that as the field reverses its strength could temporarily diminish, allowing harmful cosmic rays to reach the planet's surface. The next reversal is due to begin in the next few thousand years.

The deepest people have ventured below Earth's surface is 3.9 km (2.4 miles), down the TauTona gold mine in South Africa. Getting to this depth in the mine shaft's elevator takes an hour and the temperature of the rock face once you get there reaches 60°C (140°F). The only way human beings can function in this heat is by fitting the mine with a sophisticated cooling system that
pumps refrigerated air into the tunnels. But humans have managed to go deeper than this, albeit indirectly—that is, by drilling down into the planet from the surface. The deepest borehole ever dug into Earth went down 12 km (7 miles). The hole was drilled by Soviet scientists working in the Kola Peninsula east of Finland, and penetrated about a third of Earth's crust at that point. It took 19 years to drill and was completed in 1989. The scientists had hoped to continue drilling deeper still, but at a depth of 12 km (7 miles) they found the temperature was rising above the threshold at which their drill could operate.

The Kola Borehole gave scientists access to rock from a period of Earth's history known as the Archaean Aeon, 2.7 billion years ago. Studying rock from such an ancient era has helped to refine ideas about how Earth was formed and how it evolved into the world we see today. It is also shedding light on modern-day climate change. In 2004, samples of seafloor rock that had been retrieved by drilling into the Lomonosov Ridge beneath the Arctic ice sheet revealed that 55 million years ago the region had been so warm that it had no sea ice—the North Pole was located amid a liquid water ocean. Studies of how the climate responded to such periods of warmth in the past may be the key to understanding the consequences of future global warming.

The wealth of knowledge that can be gained from exploring Earth's interior has led to a renewed interest in drilling deeper. An international collaboration of geologists known as the Integrated Ocean Drilling Program (IODP) is trying to punch a hole all the way through Earth's crust and down into the Moho layer, which marks the start of the semi-molten mantle underneath. Geologists have long been on a quest to reach the elusive Moho layer. In the 1950s, the US Project Mohole was proposed to drill down to the Moho to help work out how movements of the mantle influence the crust above.

Heat circulates in the mantle by rolling convection cycles—the same process that makes warm air rise and cool air sink. These churning currents of molten magma drag on the crust, causing it to move. This effect makes the crust heave and crack, leading to volcanic eruptions. It is directly responsible for the motion of Earth's tectonic plates, which is the cause of earthquakes (see
How to survive an earthquake
). But the details of exactly how this all happens are poorly understood. Project Mohole was canceled before it ever got going. But now the IODP has picked up the baton, using two specially equipped drilling ships to try to penetrate the crust beneath the ocean—where it's thinnest. The goal is to open a window on the deep Earth that will give scientists fresh new insights into
the physics responsible for some of the planet's most destructive outbursts.

On a planet many thousands of kilometers across, boreholes just a few kilometers deep barely prick the skin. One physicist, however, has come up with a plan by which we could send a scientific probe all the way down to Earth's core. In a paper published in 2003 in the respected science journal
Nature
, US planetary scientist David Stevenson proposed using a multi-megaton nuclear bomb to open up a vast crack in the planet's crust. Into this enormous crater would be poured 100,000 tons of molten iron. It sounds like a lot but that's the same amount that's turned out by all of the world's foundries in the space of about a week. The sheer weight of the iron—it's about twice as dense as the rock in Earth's crust—would then make it sink, causing the crack in the ground to spread downward. As the iron sinks the pressure within Earth closes the crack behind it.

Stevenson calculated that a grapefruit-sized probe thrown in with the iron would drop down to the core in about a week, where it would be able to measure details such as the precise temperature, pressure and chemical composition. The probe would transmit its findings back to the surface via seismic waves—the
same sound waves that geologists use to gauge Earth's interior structure remotely. Stevenson's plan could be put into action for a total price tag of around $10 billion. This is much less than the total amount the United States spends on space exploration every year (NASA's total budget for 2010 was nearly $19 billion), and could help plug a massive gap in our knowledge—we know more about some faraway planets than we do about the ground beneath our very feet.

But where human explorers have often followed in the footsteps of their robot counterparts in space, it seems unlikely there will ever be any attempts to visit Earth's core in person. The temperature there rises to around 7,000°C (13,000°F)—hotter than the surface of the Sun. The pressure is over 3 million times the planet's atmospheric pressure. These conditions are far too extreme for the center of Earth to be a haven for the extinct species Verne envisaged—or indeed any known life forms whatsoever.

• The greenhouse effect

• Air pollution

• The tipping point

• Fixing a broken planet

• Sun blockers

• Terraforming

Earth is getting hotter. Estimates in 2009 suggest that if current trends continue global temperatures could rise by as much 5°C (9°F) this century, bringing droughts, extreme weather and sea-level rises of several meters that will threaten coastal cities around the world. Growing evidence suggests that the current acceleration in climate change is caused by the chemical by-products of our industrial civilization. Now some scientists think this same civilization might be able to use its technological know-how to undo the damage.

The principal contributor to climate change is the greenhouse effect, where the atmosphere traps some
of the heat that arrives from the Sun. It happens because the atmosphere is partially opaque to infrared radiation, which is the type of radiation by which most heat is transmitted. How radiation travels through a material is determined by the material's atomic and molecular structure. Atoms are made of a central, positively charged nucleus with electron particles orbiting around it. The electrons can each occupy one of a well-defined set of orbits around the nucleus, and each orbit has an energy associated with it. The difference between two orbits forms an energy gap. Radiation with energy equal to the size of the gap can be absorbed, causing the electron to jump from the lower-energy to the higher-energy orbit. Different atoms have their own characteristic set of energy levels enabling them to absorb radiation at particular energies. Similarly molecules—made from atoms bolted together—have their characteristic energy levels.

The chemicals in Earth's atmosphere absorb radiation at many different wavelengths. The primary absorbers of infrared are water vapor (H
₂
O) and carbon dioxide (CO
₂
). You might think that if the atmosphere is opaque to the Sun's heat then, if anything, the planet should be getting cooler. However, the Sun doesn't just bombard Earth with infrared, but radiation from right across the electromagnetic spectrum—including radio waves, visible light and ultraviolet. Much of this can pass through the atmosphere and makes its way down
to the ground. Here, it is absorbed by the soil, rocks, oceans and buildings, which then re-emit it as heat. And it's this extra heat that gets trapped and warms up the planet.

The principal man-made gas contributing to the greenhouse effect on our planet is carbon dioxide (CO
₂
). Levels of atmospheric CO
₂
are now believed to be at the highest they've ever been in the last 15 million years. Scientists know this from studies of ice cores gathered from deep beneath Earth's polar caps, and from marine sediments, which together serve as a kind of fossil record for the planet's atmospheric composition. Scientists have also managed to trace Earth's temperature back through time. This is possible because the thickness of ancient tree rings is linked to the length of each year's growing season, which is in turn coupled to how warm the climate is. Plotting the temperature over the last thousand years leads to a graph known as the “hockey stick” because of its shape.

The hockey stick is a plot of temperature against time and it shows a flat line with a steep upturn around the late 19th century—as industrialization started worldwide. Even so, it wasn't until the 1970s that better understanding of the climate led scientists to realize that rising temperatures could have catastrophic
results. If all of the ice on Earth melted, it would be enough to raise the sea by 70 m (230 ft). That scenario is unlikely, but even a rise of 1–2 m (3–6 ft)would be enough flood major cities—including London, New York and Tokyo. Meanwhile the damage to crops and freshwater supplies would threaten billions of people.