The Book of the Moon: A Guide to Our Closest Neighbor

ISBN-13: 9781419738494

Media Type: Hardcover

Publisher: Abrams Image

Publication Date: 04-09-2019

Pages: 240

Product Dimensions: 5.20(w) x 7.80(h) x 0.90(d)

Dr. Maggie Aderin-Pocock is an Honorary Research Associate in University College London’s Department of Physics and Astronomy. She is widely recognized as the BBC’s “face of space,” presenting the The Sky at Night to nearly 500,000 weekly viewers. With a degree in physics from Imperial College and a PhD in mechanical engineering, she has worked as a space scientist for many years and now tours the UK speaking to inner-city schools and inspiring the next generation of physicists. In 2006, she was one of six “Women of Outstanding Achievement” winners with GetSET Women.

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CHAPTER 1

MOON 101:

THE BASICS

THE MOON has fascinated humankind since the beginning of history and has long captured our imagination. After years of investigation and scientific progress, we now know much more about the science of the Moon – such as what it is made of and, at least in theory, how it was formed.

So let's begin by getting to grips with the scientific fundamentals of the Moon. The term '101' is used in some universities and colleges around the world to indicate that a course is an introduction to the subject and requires no previous knowledge, which is precisely the case here. There will be times when the science gets complex but I can assure you, by the end of this chapter, you'll have a firm foundation for everything you need to know. And what better place to start than with its name?

WHY IS THE MOON CALLED THE MOON?

It seems like an odd question; what else would it be called? But in the past our partner through the solar system has had a number of different names. To the Romans it was Luna (or 'lunar' in English); she was the goddess that personified the Moon and this term is still used today. The Greek version, Selena, was the name of their Moon goddess. You still hear it around as a girl's name although it is less associated with the Moon these days.

It might come as a surprise but giving the Moon its official name was actually one of the first things that was done by the International Astronomical Union (IAU) when it came into existence in 1919. The members did this because they wanted 'to standardise the multiple, confusing systems of nomenclature for the Moon that were then in use'.

The reason why they went for the somewhat basic name 'Moon' rather than something more exotic was because the name had already been in use for millennia, and in a range of different languages. Given the IAU was a newly formed organisation, it probably seemed like a good idea not to rock the boat too much on its first outing.

But, of course, it's a slightly confusing choice because ours is not the only moon. In the past, as we looked out into the solar system, we realised that there were many planets with moons orbiting them. In fact, we are still discovering more moons out there as we explore further into space. Although they are, fundamentally, all just called moons, we came up with more interesting naming systems for these satellites, often related to the name of the planet they orbit.

The planet Jupiter, for example, is named after the head honcho of the Roman deities. Jupiter was the god of sky and thunder and by all accounts a bit of a lad. Jupiter's largest moons – Io, Europa, Ganymede and Callisto – are named after his Greek counterpart Zeus's sexual conquests, which is one way of achieving immortality, I suppose. But names have been given to just 53 of Jupiter's 69 moons.

For the planet Mars, named after the Roman god of war, things were kept simple: it only has two moons so they were named after Deimos and Phobos, the sons of the Greek equivalent of the god of war, Ares.

The planet Saturn, named after the Roman god of agriculture, has many, varied moons. To date, 62 have been confirmed but only 53 have been named. The Greek theme continued here with the moons being named after Greek mythological figures. But by the time moons were spotted around Uranus the classical naming system had been dropped and characters from Shakespeare's plays and a poem by Alexander Pope were chosen instead.

Interestingly, the word 'moon' seems to stem from an old English word derived from the Germanic word menon, which in turn is thought to come from an Indo-European word, menses, meaning 'month' or 'moon'. So Moon seems like an appropriate term that has origins from across the world. Well, in English, it works for me.

THE MOON'S PHYSICAL CHARACTERISTICS

The Moon is an almost spherical lump of rock, gravitationally tethered to the Earth with an elliptical orbit (which means its path around the Earth is oval-shaped rather than a round circle). I say an almost spherical object because the Moon, like the Earth and many of the planets, is oblated, which means it is a slightly squashed sphere with the pole-to-pole distance being shorter (in this case only about 2km shorter) than the equatorial diameter.

The table below sets out some facts about the Moon to get us started:

Parameter (unit) Value

Average distance to Earth (km) 385,000 (239,000 miles)
Axial tilt (degrees) 1.5
Rotational period (Earth days) 27.32
Orbital period (Earth days) 27.32
Maximum Temp (°C) 120 (248°F)
Minimum Temp (°C) –247(–413°F)

These facts are all very well and give us a good starting point for understanding the Moon. Yet I think it is easier to describe its characteristics relative to something with which we're already very familiar: the planet we live on. So the table below highlights some other characteristics of the Moon and gives an indication of how these key measurements compare to the Earth:

Compared Parameter (unit) Value to Earth

Average diameter (km) 3,474 (2,159 miles) 27%
Volume (km3) 21.9 billion 2%
Surface area (km2) 37.9 million 7.4%
Mass (kg) 7.35 x 1022 1.2%
Density (kg/m3), 344 60%
Gravity (m/s2) 1.6 16.7%

Now let's look at some of these characteristics in more detail.

Size Matters

Compared with the Earth, the Moon is actually very small, having around a quarter of the diameter of our planet. This small diameter means that the Earth has a volume nearly 50 times that of the Moon.

The surface area of the Moon is also surprisingly small, just 7 per cent of the area that the Earth has. This means that the continent of Asia, which has a surface area of 44.4 million km2, is actually larger than the Moon's surface. When you also take into account that only around 30 per cent of the Earth's surface is land, the Moon's surface area really is pint-sized compared with the Earth.

Although all these statistics make the Moon sound tiny, no other moon in our solar system is bigger in comparison to the size of the planet it orbits. Its large size and relatively close proximity means that it has a strong influence on our planet in many different ways (see page 108).

Weighing It Up

As the Moon is so much smaller in volume than the Earth, it will come as no surprise that its mass is, as expected, minor compared with that of the Earth, weighing in at just 1.2 per cent of our planet's mass. But even though the Moon is significantly smaller than the Earth, it has a curious property: it is lighter than it should be if both bodies were made out of the same material. In fact, the Moon's density is just 60 per cent of that of our planet, even though the two bodies have a similar chemical composition. This is because the Moon's internal structure is significantly different to that of the Earth's (see pages 30–3). That said, the Moon is no lightweight: it's actually the second densest moon in the solar system, coming in a close second to Io, one of Jupiter's moons.

THE LUNAR L ANDSCAPE

The surface of the Moon is rather lumpy, with its top elevations about 8km higher than the mean level of the surface, and lowest depths about 9km below the mean. This closely matches the range that we have on Earth from the highest mountains to the lowest part of the sea floor, but on the Moon this range is on a much smaller body.

On Earth, this 'dynamic range' in topography is caused by plate tectonics. As the plates collide they throw up mountain ranges, and, as other plates are forced below each other and sink down, trenches are formed. On the Moon, however, there is a very different process at work. Here the range in elevation is due to the craters that have been formed on the Moon's surface over billions of years. These craters vary in depth from hundreds of metres (the deepest crater on the Moon is the Aitken basin, which is about 12km (7.5 miles) deep from its raised rim), to micrometres, i.e. one-thousandth of a millimetre. And the Moon's surface is absolutely covered in them.

What's more, the Moon's atmosphere is so thin (see page 43) that very little erosion takes place. In fact, the rate of erosion is just 1cm every 20 million years. So craters that were formed billions of years ago can still be evident today. Indeed, a crater will stay on the lunar surface virtually for ever unless its presence is eroded by the arrival of fresh impact craters.

Luckily for us, on a clear, still night it is possible to see quite a bit of detail on the Moon's surface, if its phase is right for observation (see pages 139–42 for an explanation of phases). Bright and dim areas can be seen and its pocked and cratered surface can be observed with just the naked eye or, more clearly, with binoculars or a telescope. The dark areas are called maria (pronounced 'mar-ee-a') and the light areas are called highlands or terrae, and they mark the two distinct types of the lunar terrain. Now we know how to spot them, let's look at these in more detail.

The Highlands

It is thought that the lunar crust was created about 4.5 billion years ago, soon after the Moon's formation, out of a sea of lava called a 'magma ocean'. While it was in liquid form, the lighter, lower density materials in the magma, mainly aluminium and silicates, floated to the surface and, as things cooled down, the whole outer crust solidified. A while later, around 4 billion years ago, the solar system entered a period called the 'Late Heavy Bombardment', when the Moon and other solar system bodies were peppered with asteroids and meteors. This bombardment caused the intense cratering all over the Moon's surface today.

The highlands are what are left of the original crust after this heavy bombardment. These areas are highly cratered and can be dated back to a time close to the Moon's formation. The highlands are pale in colour as they are made of the lighter materials that floated up through the magma. These regions are older than the Moon's other distinctive terrain, the maria.

The Maria

The maria (singular: 'mare') are the Moon's lunar planes, dark and relatively featureless compared with the highland regions. There were originally called maria – Latin for 'seas' – because early astronomers, looking at them from Earth, thought they were full of water.

During the period of heavy bombardment, the Moon, with no significant atmospheric protection, was hit hard. Immense asteroid impacts fractured the previously formed crust, allowing lava from the layers below to erupt to the surface through the deep cracks. This activity left huge pools of basalt lava on the lunar surface, which later solidified to form the maria. Basalt is one of the most common volcanic (igneous) rocks found here on Earth. It can be found on our ocean floors and around volcanic activity, such as on the islands of Hawaii. It is dark in colour to look at, hence the dark colouring of the maria, and is made up of about 50 per cent silica (silicon oxide). It may seem strange that a volcanic rock that is abundant here on Earth can also be found on the Moon nearly 400,000km away, but this gives us a clue to their probable common ancestry (see pages 47–9).

Radioactive dating has estimated the age of the maria at about 3 to 3.5 billion years old, so these plains are younger than the highland areas. The maria cover just over 15 per cent of the Moon's surface. Over the years, 23 maria have been identified and named. Most of these sit on the 'near side' of the Moon (the side that constantly points towards Earth, see page 53).

The variation in the Moon's surface, which I mentioned earlier, is not uniform. The highlands show evidence of older craters that have remained undisturbed, apart from the impingement of other craters over their surfaces. The maria, due to their more recent volcanic activity, tend to have less topographical variation.

Other Lunar Features

Rilles

The word 'rille' stems from the German for 'groove' or 'furrow'. Rilles were first identified and named by the German scientist Johann Hieronymus Schröter at the end of the eighteenth century. Rilles are cracks that can be seen on the lunar surface and they are thought to be produced by past volcanic activity on the Moon. They can often be tracked back along the Moon's surface to old volcanic vents and they may have been caused by collapsed lava tubes.

Domes

These are rounded, circular features that have gentle slopes and rise to an elevation of a few hundred metres. They are thought to have been formed by the flow of relatively thick lava erupting from vents. Being thick, the lava would have solidified before it travelled far, and formed the dome as it hardened. Domes typically have a diameter of about 10km, but can be as big as 20km across.

Wrinkle ridges

These are features found within the maria, caused by tectonic activity. When released, the basalt lava cooled and contracted, and in some places it did this at different rates, for example if there was a mix of different types of lava erupting from the same hole. This different rate of cooling caused a buckling of the surface and the formation of long ridges.

Grabens

'Graben' stems from the German word for 'trench' or 'ditch', and these too were formed from tectonic activity. They are essentially troughs, which are created when two cracks or faults that are roughly parallel to each other are stretched. The area that sits between the cracks subsides to form the graben. Most grabens are found within lunar maria near the edges of large impact basins.

Regolith: The Moon's 'Soil'

The surface of the Moon has had a tough time, having been pummelled by asteroids and meteors over much of its lifetime. This continuous pounding has broken down the top layers of its crusted surface, creating a lunar covering called regolith.

The term 'regolith' is used to describe a layer of unconsolidated material that sits on top of the bedrock of a planet or body. In the case of the Earth, the regolith is comprised of soil (organic plant remains in which new plants can grow), rock fragments, sand, volcanic ash and glacial drifts (material transported by the activities of glaciers). Yet the Moon's regolith has an entirely different composition, mainly made up of the Moon's own crust that was broken up by the asteroid and meteor bombardment.

Although some lunar regolith can be the size of boulders, 90 per cent of it has a grain size of less than 1mm. This means that most of the lunar surface material has a consistency that is finer than granulated sugar. The full mix would actually be closer to a combination of granulated, caster and icing sugar – but the chemical composition of the lunar regolith means that it would not taste as good!

Lunar soil is mainly made up of oxygen, with silicon, iron, calcium, aluminium and magnesium making up the bulk of the rest. It also has a sprinkle of more exotic elements like titanium, thorium and manganese. The proportions of these chemicals relative to each other are fairly similar to the proportions found on Earth. There is, however, a difference in chemical concentrations occurring in the lunar highlands and maria (see pages 26–8).

WHAT LIES BENEATH

Like the Earth, the interior of the Moon can be simply broken down into three main zones: the crust, the mantle and the core. Our knowledge of the interior structure of the Moon is still limited but the knowledge we do have has been obtained from missions that have been sent to the Moon over the last 50 years. The Apollo missions used a device called the Apollo Passive Seismic Experiment, made up of four seismometers, which were used to measure earthquakes and volcanic eruptions, and were deployed on the Moon's surface between 1969 and 1972. These seismometers took continuous measurements till 1977, generating a wealth of data on moonquakes (see pages 33–5) and other activities. Monitoring how seismic waves, generated by lunar activity, travel through the body has enabled scientists to calculate the probable structure of the interior of the Moon. If we were to use a common everyday item to describe the Earth's internal structure, the item that would work best would be an egg. The eggshell represents the Earth's crust, a very thin layer covering the surface. The egg white represents the mantle, a deep volume that surrounds the egg yolk, which in this analogy would represent the Earth's core. This analogy works as the proportions of the egg layers approximately match the proportions of the Earth's interior zones.

However, the egg analogy does not work well for the Moon's structure. Here, a better analogy would be a rather boring sugar-coated chocolate-chip muffin. I say boring because the muffin has just a single chocolate chip at its centre, definitely a disappointing result for chocolate lovers everywhere. But when it comes to the Moon, it makes for an apt illustration. In this scenario, the sugar coating represents the crust, which in the Moon's case is the regolith that was mentioned earlier. Under the thin crust would sit the mantle, the fluffy cakey bit, and at roughly the centre would sit a relatively small core represented by the chocolate chip.

(Continues…)

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Excerpted from "The Book of the Moon"
by .
Copyright © 2018 Maggie Aderin-Pocock.
Excerpted by permission of Abrams Books.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents

Introduction: Why Be a Lunatic?, 1,
My Father, the Moon and Me, 3,
Beyond Borders, 12,
An Iconic Influence, 15,
Moon 101: The Basics, 19,
Why Is the Moon Called the Moon?, 20,
The Moon's Physical Characteristics, 22,
The Lunar Landscape, 25,
What Lies Beneath, 30,
An Alien Environment, 35,
How Was the Moon Formed?, 47,
Libration: Seeing What Lies Behind, 55,
Moon Past: The Moon in Our Culture, 59,
A Brief History of Moongazing, 60,
Five People, 64,
Five Places, 73,
Five Artefacts, 78,
Five Poems, 83,
Five Folktales and Science Fiction Stories, 89,
Five Works of Art, 101,
Moon Present: A Sharper Focus, 107,
The Raw Power of the Moon, 108,
Observing the Moon, 135,
The Moon with the Naked Eye, 139,
Lunar Highlights with Binoculars, 161,
Lunar Highlights with an Amateur Telescope, 164,
Reaching for the Moon: the Moon Landings and the Space Race, 169,
Current Missions to the Moon: the Last Ten Years, 182,
Moon Future: What Lies Ahead?, 189,
When Will We Return to the Moon?, 190,
The Future of Science on the Moon, 192,
The Future of Commerce on the Moon, 199,
Proposed Moon Bases, 206,
Who Owns the Moon?, 214,
Moon, Mars and Beyond?, 217,
Conclusion: Looking Outwards, 219,
Acknowledgements, 221,
Index, 223,