The rainbow connection

22 November 2013 by Malcolm Campbell, posted in Science

Rainbows are visions, but only illusion, and rainbows have nothing to hide.” From The Rainbow Connection, by Kenneth Ascher (1944- ) and Paul Williams (1940- )

From a distance, it’s just an ordinary puddle.

Worse, it’s a puddle positioned in the gutter - one of countless millions that must pool, curbside, on urban streets the world over.

But, on closer inspection, it is so much more than that.

On closer inspection, it is a long sinuous rainbow. It glistens, snake-like – basking in the subdued, filtered, post-rain sunlight. It is predominantly blues, indigos and violets from one angle, with green, yellow, orange and only the slightest tinge of red straining to be seen from another.

A splendid serpent it is, winding horizontally along the contours of the gutter.

Despite its superficial resemblance to something living, the rainbow is inanimate. But the absence of animation makes it no less remarkable. This rainbow is woven of the finest of chemistry and physics. A trinity – comprising two liquids and light – dance together to create a dazzling vision.

The first ingredient is no more, no less than water. Water is so prevalent on this planet, so integrated into our very being, that it’s easy to forget what a wonder it is.

Water’s origins harken to the very earliest days of our universe, 13.8 billion years ago.  Within three minutes of unleashing its incredible energy, the Big Bang created the conditions that yielded huge quantities of simple atoms, hydrogen, helium and lithium. Of course, hydrogen is the key ingredient of water.

By another billion years after the Big Bang, active stars had forged more complex atoms, like carbon, nitrogen and oxygen. The oxygen forged in those celestial furnaces was flung into space when those same stars lived out their creative lives, and exploded as supernova. As they whisked at phenomenal rates across space, oxygen atoms collided with hydrogen, to make water.

Now, water in space does not a watery planet make. Even though water would have been plentiful as cosmic matter congealed to make our sun and its planets some 9 billion years after the Big Bang, the fiery conditions on early Earth would have boiled water right off its surface. What’s more, the absence of an atmosphere would have resulted in that boiled water drifting right back into space.

A cooled Earth, with a thin atmosphere could contain water, but by the time those conditions had arrived, most, if not all, water would have been a thing long past. In fact, the atmosphere would have prevented water in any substantial quantity from just drifting onto the planet’s surface. Instead, our planet was reliant on another source of water, a different kind of extraterrestrial source.

Multiple hypotheses have been invoked to explain the preponderance of water on Earth’s surface. Some of these involve delivery by either meteors or asteroids, through impact with the planet. Asteroids are cooled hunks of icy rock that share our solar system. Every now and then, some cross paths with Earth. On even rarer occasions, they collide. Earth continues on its way, scarred but undaunted, while the asteroid is integrated into the fabric of the planet. In earlier days of our solar system, such collisions were more frequent – those that could collide have had billions of years to do so. Water may also be delivered by smaller collisions, involving meteors, likely the derived from the tails of comets. Hypothetically, these collisions peppered our planet with different kinds of minerals, and created its oceans.

Recent evidence poses some problems for the collision-origin-of-terrestrial-water hypothesis. The signature for Earth’s water does not match those of known asteroids or comets. In light of this, an alternative hypothesis posits that water may, in fact, have been established within the confines of Earth as the planet was formed from bits of cosmic matter – from the “accretion disk” that congealed to make our planet. In the absence of time travel, or new evidence that unequivocally rules out one or the other hypothesis, both remain plausible, alternatives to water our world.

With a new home on Earth, water played crucial role in not only shaping, but founding life here.  Now, some four billion years later, life here proceeds with the expectancy of water – it is essential to life. Water courses through all life, travelling through us all like we were tiny rivulets – sustaining the chemical reactions and comprising building blocks that create all manners of living organisms as it does. Water that was pumped through the hearts of dinosaurs continues to flow through the blood of humans today.

Water circulates on Earth through a giant loop that includes evaporation from oceans, lakes, rivers and streams, as well as exhalation from the breath of animals and the transpiration of plants. Water vapour is cooled in the upper reaches of our atmosphere, and falls to Earth again to replenish terrestrial aquifers and aquatic ecosystems alike.

When it falls to Earth, water creates puddles – like the one where the rainbow resides.

The second element of the rainbow is also a liquid. Unlike its odourless counterpart, the second component of the rainbow has the distinctive fragrance of a petroleum product. It is oil – shed from the tarmac surface of the road, enveloping the puddle with a metallic sheen.

The history of the oil is intertwined with the history of the water. The two are stitched together through the activity of aquatic life.

Oil originates from life. Oil is a fossil fuel. As that name implies, oil is the fragmented imprint of lives that once were.

Millions of years ago, oil began its genesis through the workings of aquatic organisms – algae and the tiny creatures that feed on that plant matter, and each other, the zooplankton. These organisms lived all those millions of years, as they do today, in vast ecosystems. The algae, or phytoplankton, are like tiny aquatic plants – using the energy from sunlight to combine water and carbon dioxide to make sugar, through the process of photosynthesis.

Sugar from photosynthesis is used as both an energy source and a construction material – building organisms composed of complex assemblages of molecules. Proteins, complex carbohydrates, nucleic acids, and more – all are built and used so that organisms could survive in their environment and reproduce. In doing so, they also serve as a food source for other organisms, like the zooplankton.

Like oceanic herds, herbivorous zooplankton grazed on phytoplankton, while other zooplankton made meals of their fellow animals. Together, the phytoplankton and zooplankton made up, and still make up, an exquisitely interconnected aquatic ecosystem. So productive is this ecosystem that it generates phenomenal quantities of biomass – biomass that is contained in organisms that ultimately die – their tiny corpses falling to the sea floor. There, they are covered with sediment – mass burial sites blanketing the ocean’s depths. The rates of deposition in these sites can exceed the rate of decomposition, particularly as they are formed at such depths as to be depleted in oxygen, anoxic.

The accumulations of plankton detritus over the course of millennia, millions of years ago, were vast. There they sat, gathering the fallen bodies of microscopic organisms, covered with silty sediment. With the shifting of the great continental plates, and, with it, the rise and fall of oceans, these large plankton graveyards were themselves covered over, pushed well beneath the surface. With the weight of earth and ocean above them, the accumulated corpses were compressed under intense pressures, and heated with extreme temperatures.

Over millions of years, the pockets of plankton biomass were subjected to conditions that utterly transformed the biological materials of which they were made. The polymeric proteins and carbohydrates are at first degraded, broken down into constituent parts. Under intense pressure and temperature these constituent parts are reassembled into new polymers, forming a waxy substance known as kerogen. With even greater geothermal pressures, and longer geological time, kerogen undergoes as process known as catagenesis.

Categenesis involves the reconfiguration of the molecules found in kerogen into the hydrocarbon mixture that makes up oil. As their name implies, oil hydrocarbons are molecules rich in carbon and hydrogen. In fact, oil is greater than 80% carbon, with most of the remainder made up of bonded hydrogen. Oxygen only accounts for less then 1.5%, and sometimes as low as 0.05%, of the fossil liquid.

Oil itself is a mixture of compounds. These include the linear alkanes – strings of carbons spiked along their backbone with hydrogen – as well as cycloalkanes and aromatic hydrocarbons – rings of carbons, festooned with hydrogen, like benzene. Together they comprise an incredible assortment of high energy, combustible molecules. It is precisely this property that has made them the ubiquitous fuel source they are today.

Another marked property of the molecules that make up oil is that they are, generally speaking, water loathing. That is, they do not readily mix with water. Instead, due to their density, they sit atop the surface of water, forming a filmy coat. It is this feature of oil that contributes to the rainbow in the puddle.

The final component of the rainbow is sunlight.

The light that illuminates the roadside rainbow was born in the thermonuclear reactions of the nearest star, our Sun. Energy from the atomic explosions that power our sun is released as electromagnetic radiation. Some of that radiation is in the form of visible light – photons that travel with wavelengths that our eyes can see. These photons are formed in the sun, and are battered around the solar environment for literally thousands of years, up to 170000 years in fact, before they escape and make their 8.3 minute trip to Earth.

The photons that reach Earth are a mixed lot – they each have different wavelengths. Each wavelength is seen as a different colour. Some photons have shorter wavelengths, in the 380-495 nm range, and appear violet through to blue in colour. Others have intermediate wavelengths, from 495 nm to 590 nm, and cover the colours from green through to yellow. Yet others have longer wavelengths, in the 590 nm to 750 nm range, and make colours from orange to deep red. As sunlight contains a mix of these different photons we generally see it as white light.

But the oil-coated puddle reveals the colours in sunlight – it shows the rainbow hidden in the shafts of light.

The oily film over the water unweaves white light into its component parts. Some of the light is reflected off the surface of the oil, and will appear white. Other light, however, will travel into the oil. Light normally travels at more than 299 million metres per second. Oil impedes this speed.

As it travels into the oil, light is slowed in a manner that causes it to turn, to bend. This slightly bent light will be reflected back by either molecules within the oil, or ultimately, by the water that lies beneath. When it is reflected back, sometimes the light will be on the same angle as the incident light, but more often this will not be the case. When the light is reflected back on the same angle, there is positive interference, and the light is seen as more intense; whereas, when the light is reflected back at different angles, there is negative interference, and the light is seen as less intense. The extent to which this occurs is contingent on the wavelength of the incident light, as well as the angle that it strikes the surface of the oil.

Consequently, the layers of oil molecules selectively enable some wavelengths of light to be seen, and others not. They have extracted a rainbow from the incident white light. What’s more, as you move your head around the oily puddle, looking at the light from different angles, a different rainbow is observed. The oil-coated puddle is like a dynamic prism – yielding a new visual experience depending on where you stand.

The oil puddle rainbow is a marvel. The vision orchestrated by the oil, the water, and the sunlight all share a common origin in the heart of stars. The water from stars at the early days of our universe, the oil from photosynthesis-supporting solar power of millions of years ago, and the sunlight that emerged from our nearby star just minutes ago – they provide a connection, a rainbow connection, from the dawn of time to today.

The rainbow connection is an important reminder of the surprising connectivity between all things. Actions distantly separated in time can have a profound effect on things happening right now, this very instant. Events and actions, now but forgotten memories, can have long-lasting resonance. They can emerge again, and create instances of overwhelming beauty and wonder.

Images: All photographs by Malcolm M. Campbell.

References:

Alexander COD, Bowden R, Fogel ML, Howard KT, Herd CDK, & Nittler LR (2012) The provenances of asteroids, and their contributions to the volatile inventories of the terrestrial planets. Science 337: 721-723

Drake MJ (2005) Origin of water in the terrestrial planets. Meteoritics & Planetary Science 40: 519-527

Izidoro A, de Souza Torres K, Winter OC, & Haghighipour N (2013) A Compound model for the origin of Earth's water. The Astrophysical Journal 767: 54

Vattuone L, Smerieri M, Savio L, Asaduzzaman AM, Muralidharan K, Drake MJ, & Rocca M (2013) Accretion disc origin of the Earth's water. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 371: 1994

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