Stepping Stones is about where we came from, in its fullest sense. It all started with the formation of a small, dominantly rocky planet that eventually settled into an orbit around a ‘common-or-garden’ star at a sort of ‘Goldilocks’ distance. By that I mean surface temperatures never left the range within which liquid water was stable once our planet had achieved a heat balance with its surroundings; that is probably the main prerequisite for life to form, survive and evolve on any planet in the universe.
Earth initially formed more than 4.5 billion years ago along with the rest of the Solar System from a tenuous gas cloud that also contained rare, dusty solids. This interstellar cloud became dense enough to collapse under its own gravity, possibly after compression by a nearby supernova. A rocky planet like ours mainly accumulated dusty material, which became ever hotter at the planet grew. The early development of a system of planets around the Sun was chaotic as gravity fields around the early Sun continually changed as the planets’ masses increased. These proto-planets interacted so that their orbits repeatedly changed. Much the most influential were giant planets, principally Jupiter and Saturn, that were rapidly accumulating from gas. Astronomers working with data about newly discovered worlds that orbit other stars are beginning to realise how unpredictable the early motions and gravitational effects of these monsters were. Their changing gravitational influence created perturbations throughout the disk as they grew, as to a much lesser extent did such influences from all the other planets. Each giant planet may have migrated to different orbits while it accreted matter. During this chaotic phase lesser planets would have been at the mercy of gas giants that may have flung them in the manner of slingshot towards or away from the central star, and even into its massive grasp.
At an early stage the proto-Earth collided with another growing planet in a one-off process that produced hellish conditions that changed Earth’s chemistry, and left it with an unusually large Moon ripped from the Earth. But the Earth-Moon duo escaped plummeting into the Sun or being flung into interstellar space during the early chaos that stabilised to form the Solar System. As Earth cooled from a partially molten state rocky materials crystallised, while denser iron and other metals with similar affinities remained liquid. Gravity separated our world into two main components: a rocky, solid mantle above a denser liquid-metal core. The core grew large enough to retain enough of its heat so that its outer part still hasn’t solidified completely and remains in constant motion; the reason Earth continues to have a magnetic field sufficiently powerful to deflect deadly charged particles emitted by the Sun.
In its youth our home world was fortunate enough to acquire and retain a lot more water than its other rocky companion planets, Mercury, Venus and Mars and likewise an atmosphere, some of whose gases helped it to retain sufficient warmth – but not too much – to remain in its ‘Goldilocks’ state. Along with water and gases, it received complex volatile chemicals based on hydrogen, carbon, oxygen and nitrogen that are among the most common elements in the cosmos. All the ingredients for life were present here from very early on; their self-assembly into living forms was just a matter of time, given suitable environmental conditions. Even on a boring, entirely watery world, chances are that some chemicals capable of self-replication must eventually form. Indeed, it may soon be made to happen in a test tube; plenty of scientists are keen to do that. Yet what events and living beings such a spark might ignite on a planetary scale depend entirely on the overall conditions and how they changed, and therefore on the workings of the home world.
Earth is the most exciting and changeable body we know of, by a very long way indeed. Its liveliness is down to the rocky mantle and the water and sources of energy within it, as well as to energy received from the Sun. Today the Earth’s constituents engage in unique motion and change. Matter and energy shift in great circulation systems that extend from its centre to the outermost wisps of its atmosphere. The rocks we can now examine prove beyond doubt that it has always been so, but what goes on now was a lot more vigorous further back in time. Yet the rocks also reveal that, as well as Earth processes that we can observe in action today, there have been others that no human has ever witnessed, with rates, magnitudes and scales that we can barely imagine. Some of the most enormous and rapid have been mediated by life itself, while others have arrived unannounced from elsewhere in interplanetary space, or from the deepest part of the Earth’s mantle. On the other hand, subtle, tiny processes of all kinds have engaged with larger ones, eventually to culminate in mighty consequences.
It has only become possible in the last two or three decades to draw these and many other factors together with a large and growing variety of scientific tools, deployed with insight and imagination that have arisen with two or three centuries of experience that spans every aspect of science. It is now possible to begin to relate the story of life’s evolution interwoven with Physical and chemical development of the only planet on which it exists; as far as we know. Humans, their consciousness, activities and societies are a very recent outcome of these interwoven threads. We are at centre stage by reflecting on them, as well as on ourselves and the future of our world. I don’t know of any other story that is more important. The title I chose, Stepping Stones: The Making of Our Home World, is a metaphor for addressing as many of those foundations and their connections as I can. This is not a textbook and nor is it just about geology, for it draws on aspects of all the sciences that bear on our planetary existence. In its seven Parts it works through the fundamental steps, starting with the most basic and leading up to the most complex. The adopted style acknowledges that we learn far more from the weaving of stories than we do from unadorned facts.
Inevitably, the project’s scope involves introducing many terms and concepts with which not all readers will be familiar. Their first appearance shows them in a bold font. Traditionally, science books have included a glossary, but Web browsers now provide means of searching for definitions and expanded accounts of virtually any of them. Using these terms in any browser allows you to expand and clarify your understanding. Likewise, indexing extensive web-based text is a difficult task, so each chapter has an internal search engine at the end, so that you can check each occurrence of a term or concept throughout the whole project to discover different contexts or expansions of their first usage.
The most conventional aspect of Stepping Stones is an assumption that readers will begin at the beginning and proceed towards the conclusion; not forgetting what are known as ‘preliminaries’ – Dedication, Acknowledgements, Preface and Introduction. But, if you have discovered this site through a Web search you will start with whichever Chapter your search has directed you to. The Chapters are fairly ‘standalone’, but they may direct you to other Chapters, and by using the internal search engine you can clarify the back references that you meet. But Stepping Stones is also available as an eBook . So you can study all the ramification in the order I intended.
A project with such a wide scope as that of Stepping Stones relies on a great many sources in the scientific literature. Rather than referencing each of them in the text to which they are relevant I have collated the most important material to which I referred during writing each Part, and later in Earth-Pages in Further Reading, accessed through each chapter’s menu bar. Most of the references may be downloaded free of charge as PDFs, using the highlighted links. Also, I have linked many references to short summaries at Earth-Pages
To begin reading go to Contents
©2016 Stephen A. Drury
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