Forces of Nature

by Professor Brian Cox

  The Earth’s blue skies are not the result of the selective absorption of sunlight, but of selective scattering. Here again, the demonstration cooked up by the television production team is rather instructive. On the crisp Monday morning of 28 September 2015, a total lunar eclipse was visible from the UK. Every two and a half years or so, the Sun, Moon and Earth align such that the Earth, positioned briefly between our star and satellite, casts a shadow across the face of the Moon. It is a beautiful and relatively common sight, and one that delivers a powerful component of the feeling I experienced when I watched a total solar eclipse from Varanasi, in India, in 2009. Both are a display of moving shadows, cast across the Solar System by orbiting balls of rock, and once you have that in your mind, the effect of an eclipse is all the more powerful. During a lunar eclipse, the shadow of our world is visible, and it is lonely and dark against the Moon. In Varanasi, by the banks of the Ganges on a dripping tropical July morning, heavy with sweet incense and sweat, a million voices fell silent as the shadow of the Moon darkened the magical old Ghats. In England, thousands of miles, seven long years and a great spiritual rift away, as I prepared to recount my feelings on a quiet English moor, two ‘witches’ decided to mark the occasion by singing the theme from Walt Disney’s Frozen.

  During a lunar eclipse, the shadow of our world is visible, and it is lonely and dark against the Moon.

  The Earth’s shadow completely covers the Moon during a total lunar eclipse, but the Moon doesn’t fall into absolute darkness. Instead, it glows a dim, deep red. The red illumination of the lunar surface is the result of sunlight being deflected onto the Moon by Earth’s atmosphere. The Moon is normally viewed in direct sunlight; it reflects 12 per cent of the visible spectrum – a little less from the dark basalt seas laid down by ancient volcanic eruptions, and a little more from the brighter anorthosite highlands. Asked to describe moonlight in a single word, you’d probably say white; not too different from sunlight. This is because Moon rocks reflect light reasonably democratically at all wavelengths; bright rainbow in – dimmer rainbow out. There are certainly no reds, greens and blues visible to the naked eye. During an eclipse, the illumination is very different. The Earth’s atmosphere acts as a filter, removing most of the solar spectrum other than the red light, which remains to illuminate the maria and highlands. This is why the Moon turns red during a lunar eclipse.

  The Super Moon during the lunar eclipse of 2015, gradually turning a beautiful, dramatic deep red.

  The same physical process turns the sky red at sunset. As the Sun falls, or should we say as the Earth rotates beneath the Sun, the sunlight has to travel through an increasing amount of atmosphere on the way to our eyes. The image of the Sun reddens, and as the Sun approaches the horizon, the sky itself turns from blue to red. To understand what is happening, we need to know how photons of different wavelengths, and therefore energies, interact with the molecules, dust and water vapour in the Earth’s atmosphere.

  The red illumination of the lunar surface occurs when sunlight is deflected onto the Moon by Earth’s atmosphere.

  The changing colours of Earth’s skies, and the deep red of the lunar surface during an eclipse, are caused by a process known as Rayleigh scattering, named after the British physicist Lord Rayleigh (John William Strutt). The process can be described as the elastic scattering of photons off the oxygen and nitrogen molecules that make up our atmosphere. Picture billiard balls bouncing off each other; this is a good image if the wavelength of the incoming light is significantly larger than the size of the molecules, which is the case for visible light making its way through the air. The wavelengths of visible photons are between 400 and 650nm, and oxygen and nitrogen molecules are over a thousand times smaller.

  From Earth’s orbit our atmosphere is rarely visible... The dominant atmospheric features that are visible from space are the bright white clouds.

  In modern language, Rayleigh’s formula shows that the probability for a photon to scatter is inversely proportional to the fourth power of its wavelength. This means that blue photons (450nm) are over three times more likely to scatter off gas molecules on their way through the atmosphere than longer wavelength red photons (650nm). The illustration opposite shows the percentage of sunlight that is scattered on its way through the atmosphere when the Sun is directly overhead. Around one in five blue photons scatter, whereas only one in twenty red photons will be deviated from a straight-line path from the Sun into your eye. This is why the sky appears blue and the Sun takes on a yellow tinge. As the Sun drops towards the horizon and the photons have to journey through more air, the chance of any photon scattering will increase, and in particular more of the blue light is scattered away. This is why the skies become increasingly orange and even red in the evening, leaving a fading, deepening disc of red as the Sun falls below the horizon.

  Sunset spreads an orangey-red hue across the sky as the yellow and red photons bounce around in the air.

  Thanks to the Apollo astronauts, we can see what the Sun looks like in a sky with little or no atmosphere. The photographs overleaf were taken on 19 November 1969 by the team of astronauts on board Apollo 12. The Sun is bright white over the ‘Ocean of Storms’ because none of the colours of the rainbow have been scattered away and the sky is deep black.

  The percentage of sunlight scattered by the Earth’s atmosphere when the Sun is directly overhead, as a function of wavelength.; diagram based on image created by Robert A. Rohde as licensed under the GFDL

  From Earth’s orbit our atmosphere is rarely visible, although photographs of the limb of the Earth from the International Space Station provide a dramatic view of the thin blue line that separates us from the vacuum of space. The dominant atmospheric features that are visible from space are the bright white clouds. Clouds are white because they are composed of water droplets, which are typically of comparable size to the wavelength of visible light. Rayleigh’s calculation does not apply here, and the dominant scattering process is known as Mie scattering, after the German physicist Gustav Mie. Larger particles, such as water droplets, scatter light with a probability that is almost independent of wavelength, and this democratic deflection is the reason why clouds on Earth are bright white.

  The thin blue line that separates our planet from the vacuum of space, as seen from the ISS.

  Thanks to the Apollo astronauts, we can see what the Sun looks like in a sky with little or no atmosphere.

  The bright white light of Sun in the lunar sky illuminates astronaut Alan Bean and the craters he encounters on his moonwalk.

  Pale blue green planet

  Part 3: The Land

  Beneath the white clouds, lined by the blue oceans, is the land. The polar regions are white, the equatorial belts a dusty Mars-red, but the temperate north on the White Marble image is green. As I write, looking at that photograph (see here), I’m taken aback by just how green Europe and northern Asia are. There is no sign of concrete or highways or cities. The surfaces of Britain, France, Germany, the lowlands of Norway and Finland and out across the eastern planes of Russia, halfway around the globe to the North Pacific coast, are uniformly verdant. The ring of green is completed by North America, just visible through the clouds off the upper limb. These are the places where we know there will be abundant food, shelter and rain, because we recognise green as the colour of life. But why are plants green?

  Chlorophyll in plants is the primary collector of photons, and because they absorb blue and red photons, the green photons are reflected back out, which is why plants appear green to our eyes.

  As with many of the simply phrased questions we’ve asked in this book, there are multiple answers to this of increasing depth, and at the end of the thread lies that most wonderful answer for a scientist: we don’t quite know yet. With that exciting and tantalising admission of ignorance, a most magnificent and humble thing, let’s start with what we do know.

  A very simple answer is that green is the colour that life throws away. Just as the oceans
are blue because water molecules do not readily absorb blue photons, so plants are green because chlorophyll, the pigment contained within all green plants, absorbs blue and red photons and the green photons are reflected back out into our eyes. The diagram, left, shows the absorption spectra for the two most common forms of chlorophyll, labelled a and b. They absorb wavelengths at both ends of the visible spectrum, but leave the green centre well alone. To make more progress in understanding why this might be the case, we need to know a little about the complex biological magic of photosynthesis.

  ‘A teardrop of green.’

  – Ron McNair, physicist and NASA astronaut, on viewing the Earth from the Space Shuttle, Newsweek magazine, 10 February 1986

  The biochemist Albert Szent-Gyorgi once observed, ‘Life is nothing but an electron looking for a place to rest.’ Photosynthesis is the process by which plants use energy from the Sun to move electrons around, and today it lies at the heart of the entire food chain. You’ll probably remember this basic equation from school:

  6CO2 + 6H2O -> C6H12O6 + 6O2

  Strictly speaking, we should refer to this as oxygenic photosynthesis, because the source of the electrons in this case is water, which falls apart, releasing oxygen into the atmosphere as a waste product. Ripping electrons off water is extremely difficult to do. There is perhaps another corner of your mind where, amongst the windmills, you’ve filed away a school science experiment; the electrolysis of water. Water can be split into hydrogen and oxygen by passing electricity through it, but it’s not easy because water is a very stable and tightly bound molecule. If there was an energy-efficient way of splitting water using our current technology, the world economy would be based on hydrogen rather than oil.

  The absorption spectra of the two closely related photosynthetic pigments, chlorophyll a and b.

  Photosynthesis has been around for a very long time, and probably dates back at least 3.5 billion years to some of the oldest organisms, known as cyanobacteria. These early forms of life would not have possessed the advanced biochemical machinery necessary to split water, and would have grabbed their electrons off less-stable molecules such as hydrogen sulphide, readily available in the oceans of the young Earth. Just as in plants today, they would have forced those electrons onto carbon dioxide to make sugars, the building blocks of living things. They also had the ability to use the electrons liberated by sunlight to manufacture ATP, life’s universal battery. At some point earlier than 2.5 billion years ago, an evolutionary innovation known as the oxygen evolving complex allowed organisms to replace hydrogen sulphide with the more readily available water, and the whole lot was linked together to form the Z-scheme, which is present in all green plants today.

  The Z-scheme is one of the wonders of evolutionary biology. The sugar-manufacturing piece alone, known as photosystem 1, consists of 46,630 atoms. The ATP piece is known as photosystem 2. The oxygen-evolving complex has such an intricate structure that it was not fully understood until 2006.

  The power source for all this machinery is the plentiful stream of photons from the Sun, and chlorophyll is the primary collector of photons. There are several types of chlorophyll, which perform different functions that depend on their molecular structure and their surrounding proteins. At the reactive heart of photosystem 2, chlorophyll absorbs light most strongly at a wavelength of 680nm, which is in the red part of the spectrum. The energy absorbed reconfigures the distribution of electrons in the molecular structure, resulting in one being made available to the first electron transport chain of the Z-scheme, which whisks it away to manufacture ATP. This leaves the chlorophyll with a voracious appetite to regain its lost electron, which it grabs from water with the help of the oxygen evolving complex. The structure that contains the chlorophyll molecules is known as the P680 reaction centre, and when it has absorbed a photon it is the strongest-known biological oxidising agent. This is why it has the power to split water, delivering the oxygen we breathe into the Earth’s atmosphere in the process.

  After progressing through photosystem 2, the electron is ready to enter photosystem 1, the business end of which contains another set of chlorophyll molecules inside a different structure called the P700 reaction centre. It absorbs light most strongly at the slightly higher wavelength of 700nm, deeper into the red. In this guise, chlorophyll absorbs light just as before, but with a different result. It now becomes the most powerful known biological reducing agent, which means that its appetite is focused on getting rid of its energised electron onto anything it can – in this case, via a few more pieces of molecular machinery, onto carbon dioxide. The result, with the addition of a few protons, is to turn CO2 into sugars. The missing electron is replaced by the spare one that popped out of photosystem 2.

  This may seem unnecessarily complicated, but it probably isn’t. If you gave a chemical engineer the job of pulling electrons off water and putting them onto carbon dioxide, she’d probably laugh in your face. Water doesn’t want to give up electrons, and carbon dioxide doesn’t want to receive them. The job of pulling electrons off a stable thing is very different to the job of putting electrons onto a stable thing, and this is why there are two separate reaction centres allowing the chlorophyll pigments to perform these different tasks.

  Z-scheme

  The Z-scheme is an awesome thing, which is probably why every organism on the planet that carries out oxygenic photosynthesis does it in precisely the same way. It almost certainly only evolved once, probably in a cyanobacterium somewhere in a primordial ocean. These clever cyanobacteria somehow found their way into the cells of other organisms and became the chloroplasts – the seat of photosynthesis in all the green plants on the planet today. This may give you pause for thought, because without the Z-scheme there would be very little oxygen in our atmosphere and complex life on Earth wouldn’t exist.

  This coloured electron micrograph shows two chloroplasts in the leaf of a pea plant (Pisum sativum). Chloroplasts convert light and carbon dioxide into carbohydrates.

  If the two reaction centres absorb light most strongly in the red, then why are all plants green? The answer is that the P700 and P680 reaction centres don’t absorb sunlight directly. This is done by a complex array of different chlorophyll pigments, and other pigments called accessory pigments, which channel the light into the reaction centres in a cascade that gradually increases the wavelength towards the red part of the spectrum, allowing the chemical business to begin. The accessory pigments are revealed in the autumn, when the chlorophyll decays away, as the reds, oranges and golden yellows of autumn leaves. The two most common chlorophyll pigments outside of the reaction centres absorb light in both the red and blue parts of the spectrum. Together with the accessory pigments, they harvest over 90 per cent of the Sun’s light, leaving only a very small band of green to be reflected away.

  Photosynthesis is complicated and wonderful. It uses almost all of the sunlight falling on the surface of the Earth to power the plants that lie at the base of our planet’s food chain, and oxygenates the atmosphere in the process. Why don’t plants use 100 per cent of the visible spectrum and have black leaves, rather than reflecting 10 per cent of the light away? Nobody knows. The answer is probably an important lesson in evolutionary biology. Evolution by natural selection doesn’t find optimal engineering solutions to problems. If an engineer designed a plant, it would have black leaves. Rather, organisms are a bit of a bodge job, the result of 4 billion years of mutations, selection pressures and genetic and physical mergers. The greens that dominate the temperate regions of planet Earth could well be a frozen evolutionary accident.

  The molecular structure of chlorophyll A, which has the molecular formula C55H72O5N4Mg.

  Pale coloured dots

  Having taken a wander through the origin of Earth’s defining colours, we can now return to the beginning and cast our minds out towards the stars. Is there any way we can use what we know about the reflection and absorption of sunlight on Earth to explore other worlds, and to search fo
r the signatures of life beyond our Solar System? The answer is yes, and astronomers are doing just that.

  The first planet to be discovered outside the Solar System is known as PSR B1257+12 B. The discovery was announced in January 1992. PSR stands for pulsar – a rapidly rotating neutron star around 1.5 times the mass of the Sun but with the radius of a city. Pulsars rotate extremely fast – the parent star of the first planet spins around once every 0.006219 seconds. The timing accuracy is important, because it is by measuring wobbles in the spin rate that the existence of planets can be inferred.

  There are three known planets in the PSR B1257 system, which have been named Draugr, Poltergeist and Phobetor. Poltergeist was the first to be discovered. I know; I was curious about their names as well. Poltergeist means pounding ghost, the Draugr are the undead in Norse legend who live in their graves, and Phobetor is the personification of nightmares and the son of Nyx, Greek goddess of the night. Astronomers are such Goths. Mind you, the PSR B1257 system wouldn’t be a very nice place to live – the planets are bathed in radiation from their violent host. Draugr is the closest in, orbiting once every 25.262 Earth days. It is the lowest-mass planet yet to be discovered, at only twice the mass of our moon.

  The Kepler Space Telescope was launched on 7 March 2009 and has revolutionised the search for extra-solar planets. Kepler looks for periodic dips in the light of stars as planets pass across their face as seen from Earth. By studying the details of the light drop, and with additional data from supporting observations by ground-based telescopes, a great deal of information about the planets can be deduced. I write on 11 May 2016, a day after the discovery of 1284 new planets was announced by the Kepler team. In this new sample alone, there are 550 rocky Earth-like planetary candidates, and nine of these orbit in the so-called habitable zone around their parent stars, which allows them to have surface conditions compatible with the existence of lakes and oceans. The 21 rocky planets less than twice the size of Earth discovered by Kepler are shown in the illustration, left.