The Rough Guide to the Earth

Planet Earth

The answer to the question «What is the Earth?» might seem obvious: it is a planet. But just what is a planet? How are planets formed? What distinguishes them from other objects in the universe? And, perhaps most importantly for us, how unusual a planet is Earth — what characteristics does it share with its cousins, and what makes it unique?

The Earth in space

As we saw in Chapter 1, a planet is almost by definition a subsidiary body. So when thinking about the Earth, it makes sense to start by thinking of it in its setting, in the context of the forces that surround it. That means everything from the Moon and passing meteorites to the star that keeps the whole thing going — the Sun.

Like every star, the Sun is really a pretty simple device — just a very hot centre where heat is being generated by nuclear fusion, surrounded by hot gas through which it is transmitted to the universe beyond. Astrophysicists can describe the whole thing in some detail in a few equations. As we shall see, there are magnetic and electrical effects going on that complicate matters and have severe effects here on Earth. But a complete description of a square kilometre of the Earth’s surface would be far longer than a detailed account of the workings of the entire Sun. The Sun’s effects on the Earth are, however, far from simple.

The solid Earth

Now for the hard stuff, literally. The Earth is a solid ball, 40,000km in circumference, which translates to being 12,732km across. In other words, whether you dive deep in the ocean or climb Everest, the centre of the Earth will always be about 6366km below your feet.

A quick sum shows that this gives the Earth a surface area of 509 million square kilometres of water, land and ice. This chapter will focus on the layer immediately below the Earths surface — the crust — and the forces that shape it. In the next chapter we shall come on to the deep Earth and its visible effects around us.

The deep Earth

Too few people read Jules Verne these days. But if you have read Journey To The Centre Of The Earth, or seen the film, do your best to forget it. Engineering knows no material strong enough to make caves to lead you to the centre of the planet. If any did open up for some reason, they would collapse again long before James Mason and his companions could make use of them.

Nor is pressure die only problem with day trips to the deep Earth. Anyone going to the centre of the planet would have the weight of the world on their shoulders, but they would also be disagreeably warm. Mines scratched just a few kilometres into the Earth’s crust already need substantial air-cooling equipment to allow people to work in them. As you get deeper, the heat carries on building.

The sheer difficulty of examining the deep interior of the Eardi means that while spacecraft have made objects billions of kilometres across the solar system familiar to us, we have yet to make any significant incursions into the ground below our feet.

Instead, the deep Earth is a showcase for what science can find out about something it can never see. And this is not mere scientific virtuosity for the sake of it. There are plenty of things going on thousands of kilometres below our feet that influence our everyday lives here at the Earths surface.

But what do we mean by the deep Earth? Perhaps the easiest way to define it is to regard it as the Earth below the crust. That makes more sense than adopting some arbitrary depth as the boundary. For the tidy-minded it has the disadvantage that the crust below the continents is a lot thicker than below the oceans. So the deep Earth in mid-ocean could start 10km below sea level, but in a hefty mountain range it could be 80km down.

However, nobody can argue that either of these is exactly shallow by human standards. The deepest mine in the world, East Rand gold mine in South Africa, extends to a mere 3585m below the surface. At the time of writing, the deepest well ever drilled is 12,262m, in the Kola Peninsula in Russia.

This is impressive technology, but the radius of the Earth is 6371km. So even the most ambitious digging and drilling have yet to make it even 0.2 percent of the way to the centre. Another way to think of it is that the mantle makes up 84 percent of the solid Earths volume, and the core another 15 percent, but our direct exploration is confined almost entirely to the outer 1 percent that we call the crust.

The airy Earth

You cannot feel it, but as you are reading this, a weight equivalent to a 10m column of water is pressing down on top of your head. You don’t notice it because you are used to it, but it is there alright. It is the atmosphere, our subject in this chapter.

Another way to think about atmospheric pressure is to say that, from sea level to the edge of the atmosphere, there is a column of air weighing almost exactly one kilogram tor every square centimetre of the Earths surface. This means that, if you are near sea level, the atmosphere is exerting a downward pressure on you of about 1 bar. This is not the simplest measurement to take in. A bar is 100,000 pascals, which in turn is a pres sure of a newton exerted over an area of a square metre.

Atmospheric pressure varies for a number of reasons. For example, the heat of the Sun makes air expand, and become less dense. As we shall see, these pressure differences drive the Earths weather. But they are imperceptibly small to us, and the variations are measured in millibars, thousandths of a bar. To be precise, the average pressure of the atmosphere at sea level is 1013 millibar or 1.013 bar.

These variations in atmospheric pressure are measured using a barometer. As we have already established, air at sea level exerts about as much pressure as a 10m column of water. If the liquid were mercury, the column would be 76cm high, since mercury is much denser than water. This is the principle behind the mercury barometer. It consists of a vertical tube of mercury connected to a small reservoir of the metal which is exposed to the air. The height of the mercury column adjusts until the weight of the column balances the atmospheric force exerted on the reservoir.

Mercury is used for barometers because it is the densest liquid we have, so the barometer can be a sensible size. With a glass tube 10m or more high, you can make a water barometer, and they do exist, but you will need an impressive house to display it. The little barometers you see on walls are called aneroid, or no-liquid, barometers. They contain a very thin-walled metal box which expands or contracts as the air pressure falls or rises respectively.

The liquid Earth

No popular description of the Earth can get far without using the term «blue planet». Pedants might object that the solar system contains another planet — Neptune — that is also blue and is four times the size of this one. But the Earth has no plans to relinquish the title. The Earths distinctive blue colour is to do with water. Throughout its history, most of the Earth’s surface has been under water. Look at it from space and the main thing you see is water, which reflects blue light while absorbing the redder light at the other end of the visible spectrum. Pictures of the Earth from orbit or from further afield also show land, which looks mainly brown, and clouds and ice, which are white. But it is water that dominates the scene because it adds up to over 70 percent of the total surface area of the Earth.

In this chapter, we shall look at water in several different ways. It is essential to life. It is a natural resource that people are using in increasing amounts. And it is arguably the most significant player in shaping the Earths surface and driving the processes that make the Earth the place it is.

The icy Earth

The Earth’s frozen areas are many. They currently cover about 10 percent of its surface. At the height of the most recent ice age the figure was about 32 percent. In this chapter we will explore the great icy zones of the planet, before looking into the Earth’s icier past and the current threats to the Earth’s ice cover.

The Earth and us

The future Earth

Scottish geologist James Hutton, one of the founders of modern Earth science, said in the 1790s that geological investigation showed «no vestige of a beginning, no sign of an end». But knowledge has moved on since then. The beginning is now clear enough. We know how the Earth formed and when, about 4.54 billion years ago. And we even have a fair idea of its life expectancy.

Although there are many possible disasters that could affect conditions on the Earth, there are few we know of that could affect its very existence. It is likely that the end of the world as we know it will be brought about by the future activity of the Sun. As we have seen, the Sun is already hotter than it was a few hundred million years ago. In perhaps 5 billion years, it will be so much hotter that it will have boiled away the oceans, making the Earth essentially as comfortable for life as Mars is today, albeit a lot warmer. As we have seen, the Gaia theory holds that the Earth can cope with major disruptions provided they are gradual enough. But it may be that this increase in the Earths temperature is beyond even Gaias powers of adaptation. Even if it can cope, there is worse to come.

In about 6.5 billion years, the Sun will have finished the hydrogen burning that provides most of its luminosity today. Instead, it will stop being a “main sequence” star and will run through a series of comparatively rapid changes over a few’ hundred million years.

In the first of these, the Sun will swell to a size which will engulf Mercury, and raise the Earths surface temperature to perhaps 750°C. Later, it will swell to about the size of the Earths orbit today. This sounds terminal, but in fact it will not be. Because the Sun will have blown off large amounts oа its mass into space, the Earths orbit will have expanded as the Sun got lighter. Finally, the Sun will erupt to produce a gas cloud called a planetary nebula (because it can look like a planet when seen in a telescope) and cease almost all heat production. At this stage the ashy leftover of the Earth will orbit the remains of the Sun for ever, lit only by distant starlight and with its temperature far below freezing.

Even further in the future lie such possible fates as the heat death of the universe, in which it has “run down” to a state where there is no free energy to sustain motion or life, or the Big Crunch, when galaxies merge back together as the expansion of the universe goes into reverse.