Date: December 28, 2009
Title: Life in Technicolor
Podcaster: Maria Pereira
Organization: Columbia University Astronomy
Description: What will plants on other worlds look like? Will they be green and leafy as on Earth? Or could there be planets with purple trees? Black grassy plains? Orange marshes? These questions might seem purely speculative, something out of a technicolor daydream, but, in reality, their answers are bringing us closer to finding the first signs of extraterrestrial life. Unfortunately, they are not particularly easy questions to answer. In fact… Why are plants on Earth green?
Bio: Maria Pereira is a postdoctoral researcher at Steward Observatory in Tucson, Az. She received her PhD from Columbia University in 2009, with a thesis on the dynamics of galaxies in clusters.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Joseph Brimacombe, a mega-enthusiastic amateur astronomer based at the Coral Towers and Macedon Ranges Observatories in Australia, and the New Mexico Skies observatory in the United States. and is dedicated to: The other great podcasts and their hyper-smart, ultra-devoted and endlessly eloquent podcasters – Astronomy Cast, Slacker Astronomy, the Jodcast, Skeptoid and the Skeptics Guide to the Universe. PS Pamela, Megan, and Rebecca – will you marry me! Please google “Joseph Brimacombe, photostream” for further information.
Hello, and welcome to the final installment for 2009 of Columbia Mondays! My name is Maria Pereira and I obtained my PhD this year from Columbia University in New York. The title of the podcast today is Life in Technicolor: Astrobiology through Newton’s prism.
Life In Technicolor
In the science fiction masterpiece “War of the Worlds,” H. G. Wells imagined the surface of Mars covered by “red creepers”, an invasive plant that is accidentally brought back to Earth and takes over our own plant life. While the existence of plants on Mars is now all but ruled out, astrobiologists are nevertheless pondering the existence of plants on other planets outside our solar system. What will alien plants look like? Will they be green and leafy as on Earth? Red and leathery as in Wells’ Red Planet? Blood Sucking Monsters like in the Little Shop of Horrors?
These are questions that have until now been quite firmly in the realm of science fiction, but with the rapid advances in telescope and detector technology, it is conceivable that they may be answered scientifically by astronomers within our lifetimes.
Once astronomers started trying to predict what plants might look like on other planets,though, it became apparent that no-one had quite understood why plants on earth look the way they do.
More specifically, why are plants on Earth (mostly) green? If you ask a biologist, she might give the following answer: “Plants are green because they contain chlorophyll, a green pigment, responsible for photosynthesis.”
But, why is chlorophyll green? To which a physicist might reply: “Why is anything green? What does it mean to be green, or red, or blue?”
In 1666, the father of modern physics, Isaac Newton, sat in his study at Woolsthorpe Manor pondering just such questions. Forced out of Cambridge University by the threat of an advancing plague that would sweep the nation and kill over 75,000 people, he retreated to his childhood home and there spent a fantastic- ally productive year, his annus mirabilis, during which he would tackle (and solve) many problems of optics, mechanics, gravitation and calculus.
It was at Woolsthorpe that Newton reportedly observed (or, according to some accounts, was hit on the head by) an apple falling from its tree, and was struck by the idea of Universal Gravitation.
When Newton was not contemplating his orchard and the fruit within it, he spent a lot of time wondering about light, colors, and their composition. With a simple prism, he proved that white light was in fact composed of many different colors, a so-called spectrum, and furthermore, that once a color had been “extracted” from white light by a prism, it could not be split further by another prism, but remained “pure”.
The prism is said to refract the light and decompose it into individual colors, or more accurately, wavelengths of light. Monochromatic light is “pure” light, light that has only one wavelength, one color.
In his spectrum, Newton thought he could identify 7 pure colors, and he devised a color circle where he related these to notes on a diatonic scale. In fact, we know now that a prism1 disperses white light into an infinity of wavelengths, a continuous spectrum, in which there are no separate colors but just a continuous gradation from Red to Violet.
Newton aimed this decomposed light at different objects and concluded that, the reason an apple is green is that, when it is illuminated with white light, it absorbs light at all wavelengths of the spectrum except green. The green light is not absorbed, but is reflected back in the direction of the observer, who therefore sees – a green apple! . If you shine monochromatic red light on an green apple, it will be pretty nearly invisible, because it will absorb all the incident red light and reflect none. Newton added these observaions to his new theory of colors, and concluded that color is a property of light and not of the object itself.
When Newton examined the dispersed spectrum closely, however, he noticed that some naturally occurring colors were absent, such as brown or pink. This led him to the conclusion that color must not be an intrinsic property of light after all but is instead a quality that is manipulated and maybe even defined by the observer.
He therefore abandoned his prisms and slits and turned his experiments on himself, determined to understand the human role in the theory of colors. According to his journals, he probed his sense of vision by sticking needles in his eye socket, between eyeball and bone, and in his own words “…pressed my eye [with the] end of it (soe as to make [the] curvature [..] in my eye) there appeared several white darke & coloured cir- cles…”. He concluded that colors were a function of how much pressure he applied to the back of his eye. (and please, kids don’t try this at home…) On another occasion, he looked at the sun for as long as he could bear and spent the next few days in a dark room until his eyesight recovered.
Two hundred years later, Thomas Young, in an amazingly prescient work, proposed that the entire range of human color perception could be achieved by combining three monochromatic colors in varying degrees of intensity.
“As it is almost impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation, it becomes necessary to suppose the number limited; for instance to the three principal colours red, yellow and blue, and that each of the particles is capable of being put in motion more or less forcibly by undulations differing less or more from perfect unison. Each sensitive filament of the nerve may consist of three portions, one for each principal colour.”
Shortly thereafter, the first microscopic dissections of retinas were performed, confirming the existence of three distinct types of photoreceptor cells, called cones, in the eyes that were sensitive to long (red), medium (yellow/green) and short (blue) wavelengths of the visible spectrum. Each cone acts as a collecting bucket, counting photons it receives in a specific wavelength range. After a short time interval (less than a tenth of a second) the cells total the counts received and communicate this value to the brain, which then computes a unique color for each RGB combination.
Normal human color vision is therefore simply a mental construct based on the outputs of these cells. For color blind peo- ple, one, two or all three types of cone do not function,
normally due to some genetic anomaly, and they are therefore unable to distinguish between certain hues.
Ok, but why these colors? We now know that light comes in a vast range of wavelengths, from the most energetic Gamma rays down to radio waves. Why are our eyes limited to such a narrow range? Why can’t we see microwaves in our oven, or the radio waves that permeate air space?
That question is mostly answered by the theory of evolution – given enough time, organisms will go through enough mutations that natural selection will eventually create species that are optimally adapted to their environment. Our eyes are most efficient when they are sensitive to the most commonly occuring wavelengths of light.
The solar spectrum spans a wide range of wavelengths, from radio to X-ray, but it peaks right in the middle of the visible range, around green, and therefore our eyes are tuned to this wavelength range. Color vision and trichromacy probably followed due to the evolutionary advantage in picking out mature fruit from trees and prey and predators from shrubs with enhanced contrast.
Not all animals have the same type of vision. Most nocturnal animals are monochromats, sacrificing color vision for more sensitive eyes that can detect very small quantities of light. Vipers are known to have infrared (i.e. heat) detectors, withwhich they can see warm prey on cold nights with added contrast. Honeybees are trichromats, but instead of a red cone they have a cone sensitive to ultraviolet photons. The reason for this was unclear until UV images were taken of certain types of flowers, revealing colorful UV patters, circular targets, beckoning bees to land and pollinate. Fruits change to more contrasting colors when they mature to attract birds and animal gatherers that will spread their seeds. In this harmonious picture of life on Earth, every color in nature appears to have a purpose, a reason.
So, why is grass green? In a physical sense, grass is green because the clorophyll it contains absorbs more light with wavelengths in the red and blue ranges of the visible spec- trum, and reflects mainly green light. This reflected light is then detected by the cones in our eyes, mostly by the medium wavelength (yellow-green) cone, but also in varyingly small amounts by the long and short wavelength cones, variations that our brain then processes as wonderful gradations in hue, with mint green competing with asparagus, olive, moss and jade to complete the full range of colors our eyes can distinguish.
But why green? Why does chlorophyll reflect green and absorb red instead of reflecting red and absorbing green? It is this question that baffled scientists. Chlorophyll’s function is to absorb light from the sun, [..] but it seems to be performing this function suboptimally, since it is not absorbing the wavelengths at which the sun emits most of its light, i.e. green.
Scientists spent years analyzing data collected on photosynthesis from around the globe, and they think they have understood the reason behind this discrepancy. The chemical reaction which forms the basis of photosynthesis requires photons to have a minimum energy or wavelength, around the red part of the spectrum. More energetic photons (i.e. greener) can be used, but do not speed up the reaction, or make it more effi- cient, since the rate of the reaction only depends on the number of photons. In addition, oxygen (a product of photosynthesis) and water vapour in our atmosphere tend to absorb and reflect preferentially in the green parts of the spectrum, and let red and blue wavelengths through more easily.
With this understanding, astronomers are now able to identify, a posteriori, what color Earth plants should be, based solely on the spectrum of light emitted by the sun, our parent star and the composition of our atmosphere, and they can confidently generalize this model to predict the colors of plants around stars that are very different from our own.
As of December 2009, 415 planets have been found orbiting distant stars, and this number is likely to increase exponen-tially with new space facilities such as Corot and Kepler going online in the next few years. The main question astrobiologists are asking these days is, now that weÕve found these planets, how can we tell if they harbor life?
Ideally, the biologist would take a field trip, but space travel is still limited to much smaller distances. The Voyager probe, which was launched in 1977 and passed Jupiter and Neptune 2 years later, is only now, in 2007, exiting the solar system. It is the farthest human-made object from Earth, at a staggering distance of 9.6 billion miles, but it is still 6000 times closer to us than the nearest extrasolar planet. If we are to discover life on these planets, we must do it from afar, by looking for signatures of life in the light they emit.
It is tricky to look for evidence of lifeforms that we have never seen. The diversity of organisms that evolved on Earth is astonishing, and it is a challenge to entertain the myriad other types of life a Universe as vast as ours could harbor. Limited, as we are, to our own experience of life on Earth, it is diffi- cult to speculate about aliens without some a priori expectation as to what they should look like. While martians in science fic- tion tend to be green and slimey, they are also anthropomorphic, with legs, arms, eyes and mouths, and sometimes even a sense of humor.
Nevertheless, some reasonable inductions about life at large can be made by studying life on our own planet, albeit with varying degrees of confidence. Even though Earth’s life is entirely based on carbon, research shows that certain alterna- tive biochemistries might be viable, such as ones based on ni- trogen or silicon. On the other hand, scientists are convinced that most forms of life in the Universe will depend on photosyn- thesis, given that stellar light is the most ubiquitous source of energy available, and photosynthesis appears to be the most efficient way of transforming luminous energy into a storable, chemical form.
When we observe our planet from space, far enough away so that even the Great Wall of China dissolves into the great Eura- sian mass, the most obvious indicator of life is the green light reflected from the plants that carpet the sur- face. If we are able to predict what color plants might have on other planets, we will be able to design more efficient instruments to look for them, and bring the aliens a little closer to reach.
This has been a podcast of Columbia University here in the City of New York. For more information about our public events at Columbia Astronomy visit outreach.astro.columbia.edu.
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365 Days of Astronomy
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