Aristotle regarded the four fundamental elements of the universe to be air, water, earth and fire. This could easily be a first approximation made by extrasolar alien explorers visiting the solar system for the first time. The only difference is that they might call our planet Water, rather than Earth: whereas the sun and the other planets have plenty of fire, gas, dust and rock between them, Earth is distinguished by having the majority of its surface covered by water – and much of the biomass is made of the stuff too. The development of the science of chemistry has, of course, been able to pin down more precisely the presence and nature of fundamental substances. The basic chemical elements were found to be those that could not be broken down into further constituent elements: each of these elements has its own kind of atom. Which substance has its own kind of atom is not intuitively knowable in advance – neither humans nor spacefaring aliens without a grounding in chemistry could readily tell that neon and mercury and aluminium are elements, but air and water aren’t.
In fact, as the science of chemistry developed, it came to be understood that although the chemical elements are in some way fundamental, these are not disparate unrelated substances, but are all related to each other, since each atom is composed of the same constituent particles: protons, neutrons and electrons. So although we intuitively recognise different elements as distinct chemical substances, the differences are down to having sub-atomic particles in slightly different proportions and configurations. So despite the tangible qualitiative difference between glittering gold and base lead, the chemical distinction could be intepreted as ‘merely’ a quantitative difference, the small matter of things like three electrons here or three protons there.
This quantitative aspect is nevertheless significant as it allows all elements to be defined and ordered in a unique sequence, which can be arranged in two-dimensional tabular form. Dmitri Mendeleyev’s invention of the Periodic Table – or his discovery of the relationships underpinning it – brought a satisfying new order to chemistry, echoing the kinds of order found in other sciences. And just as other sciences can use the lawful order of nature to make predictions about the world, so too could chemistry. Mendeleyev was able not only to tabulate all known elements, but could predict the existence and properties of unknown ones. In his book Crucibles, Bernard Jaffee described the astonishing prophetic effect by which Mendeleyev was able to predict the existence of previously undiscovered elements, and their chemical properties: elements and properties that were duly discovered in the likes of Gallium, Germanium and Scandium. This showed the potency of the predictive capacity of chemistry – as effectively as if predicting unseen planets in astronomy – whereby things once unknown and unimagined, became known and understood.
Because of the order found in the chemical elements on Earth – and because of the discovery of the same elements existing as far as we can detect beyond Earth – we can be reasonably sure that if we were to find and land on a new planet, we could reasonably confidently predict that it would contain elements already in the Periodic Table, or compound substances that were some combination of them. So, we should not be too surprised to find planets with rain of molten iron, rivers of quicksilver, or lakes of brimstone – since these are made from elements already found naturally on Earth. Even the most fantastic imaginable planetary chemistries – deserts of gold-dust sparkling under neon-lit skies, or candy-floss clouds floating over rivers of alcohol, plains of sugar-grains and deserts of monosodium glutamate – would still be based on the same chemistry as found on Earth.
Conversely, we would expect an alien chemist visiting Earth to be quite comfortable with our own deserts of salts and silicates, rivers of hydrogen oxide and atmospheric cocktail of greenhouse gases. And so an itinerant alien chemist would be likely to make use of the same kind of classification of chemical elements as us, and bring their periodic tables on their travels, whereas an alien biologist would not expect their taxonomy of homeland life-forms to be much use on Earth.
Of course, we would not be able to anticipate the value an alien would attach to the different elements. Perhaps an alien from a planet with gold-dust from horizon to horizon might consider lead – a soft, dense, magically X-ray resistant material – more valuable than boring old gold. What is considered precious may be as much to do with rarity as utility or beauty. On a planet of crystalline carbon, the sparkling diamond may be no more or less precious than the sparkling water on Earth.
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