From ancient civilizations to modern day, the colors and symbols of the four elements have represented the different aspects of nature and the forces of energy in our world. The idea that these four elements—Earth, Water, Air, and Fire—made up all matter was the cornerstone of philosophy, science, and medicine for two-thousand years.
Historians believe that as early as the 8th century BCE, ancient Greek philosophers of the Archaic period began formulating theories of the four classical elements. Although the Greeks believed that the four elements were unchanging in nature, everything was made up of these elements, held together or pushed apart by forces of attraction and repulsion, causing substances to appear to change. This is similar to what really happens with elements and all molecules at an atomic level.
To the ancient Greeks, the four elements described not only physical manifestations of the material world but essential qualities of human nature as well. For instance, the Earth, solid and substantial, was associated with the physical and sensual aspects of life. Water, flowing and ever-changing, denoted emotion and empathy. Air was not only the air we breathe and the atmosphere, but signified the mind, intelligence, and inspiration. Fire meant the Sun and flame; it also indicated creative passion and destructive zeal.
Before digging into the myriad of aspects of how the Four Elements in fact shape and affect our life, this is the back-story of how they originated.
The Birth of Mother Earth
Ten billion years before the Earth was born, the universe started out with only two elements: hydrogen and helium, which formed stars that burned these elements in nuclear fusion reactions. Generations of stars were born in gas clouds and died in explosive novas that produced the heavier elements we have today.
Some 5 billion years ago, a supernova exploded, pushing a lot of its heavy-element wreckage into a cloud of hydrogen gas and interstellar dust. The mixture grew hot and compressed under its own gravity; at its center, a new star began to form, around which swirled a disk of the same material that grew white-hot from the great compressive forces. The new star became our Sun, and the glowing disk gave rise to Earth and its sister planets.
While the Sun grew in size and energy, the hot disk slowly cooled. This took millions of years, during which the components of the disk began to freeze out into small dust-size grains. Iron metal and compounds of silicon, magnesium, aluminum, and oxygen came out first in that fiery setting; bits of these are preserved in chondrite meteorites*. Slowly, these grains settled together into clusters, then lumps, then boulders, and finally bodies large enough to exert their own gravity.
As time went by, these bodies grew by collision with other bodies, producing a lot of melting and vaporization. Materials, which we can call rocks and iron metal, began to sort themselves out; the dense iron settled in the center, while lighter rock separated into a mantle around it, in a miniature of Earth and the other inner planets today.
At some point, the Sun ignited; although the Sun was only about two-thirds as bright as it is today, the process of ignition was energetic enough to blow away most of the gaseous part of the protoplanetary disk. The chunks, boulders, and mini-planets left behind continued to collect into large, stable bodies in well-spaced orbits.
At one point early in this process a very large mini-planet struck Earth an off-center blow, spraying much of Earth's rocky mantle into space. The planet got most of it back after a period of time, but some of it collected into a second mini-planet circling Earth, the Moon. Since this theory took center stage in the mid-1980s, it has become everyone's favorite, as geophysicist Don Anderson once explained, “The objection that such an event would be extremely rare is actually a point in its favor, since the Moon is unique”.
The oldest rocks, dated by the uranium-lead method at about 3.96 billion years old, show that there were volcanoes, continents, oceans, crustal plates, and life on Earth in those days. While the eons that followed were full of strange stories and far-reaching changes, the Earth had taken on its basic structure long before.
Where Water Came From
The exact origin of our planet's water, which covers about 70% of Earth's surface, is still a mystery to scientists. Many researchers think that, instead of water forming at the same time as Earth, objects in the outer solar system delivered water to Earth in violent collisions shortly after its formation.
Researchers speculate that any water conglomerating on the surface of the planet as it formed would have most likely been evaporated away by the young, blazing Sun, which means that water probably came here from somewhere else. The inner planets—Mars, Mercury, and Venus—were probably too hot to house water during the Solar System's formation, so our water did not come from them either; on the other hand, outer planetary bodies, such as the moons of Jupiter and comets, were far enough away from the Sun to retain ice.
During a period around 4 billion years ago called the “Late Heavy Bombardment”, massive objects, probably from the outer solar system, hit Earth and the inner planets. It is possible that these objects were filled with water, and that these collisions could have delivered gigantic reservoirs of water that filled Earth.
For a long time, astronomers thought that comets were the likely culprit. However, remote measurements of the water evaporating off of several major existing comets—Halley, Hyakutake, and Hale-Bopp—revealed that their water ice was made of a different type of H2O, containing a heavier isotope of hydrogen than Earth's, suggesting that these comets could not be the source of our water.
With major comets crossed off the list, astronomers began to wonder if clues to our water's past may lie in the asteroid belt. This region of hundreds of thousands of asteroids orbiting between the inner and outer planets was believed by astronomers, to be too close to the Sun to house water, but astronomers found the first evidence of ice on the asteroid 24 Themis.
This discovery and others of ice on asteroids suggest that there might be far more ice in the asteroid belt than originally thought and provide another possibility for the origin of ocean water. Probes sent to explore asteroids, such as the DAWN spacecraft, in the coming years will reveal more about their mysterious water ice, potentially help us understand the beginnings of Earth's water.
On the other hand, evidence that water came to Earth during its formation from cosmic dust, rather than following later in asteroids, has been shown by a group of international scientists. Nora de Leeuw at University College London, UK, and colleagues used molecular-level calculations to prove that when mineral dust particles came together during Earth formation, gas-solid interactions could have resulted in water being adsorbed onto the surface of the dust particles, meaning water could have been part of the Earth from the very beginning.
Glossary
*Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids.
References
geology.about.com
lifeslittlemysteries.com
rsc.org
teachertech.rice.edu
csep10.phys.utk.edu
hearth.com