The orange, orange grass of home …
Published on May 1, 2007 at 11:04 pm.
3 Comments.
Filed under astrobiology.
There’s an old Tom Jones song called the Green Green Grass of Home. But, that song might not work on other worlds. Actually, it isn’t all that true at my home, either. I am firmly convinced that plants are brown and crunchy, and if they aren’t, then they are heading that was after I am given charge of taking care of them. I can even kill a cactus!
But, plants other than in my care are normally green. Yes, I know that there are some that have other colored pigments in them, but most plantlife on Earth is some shade of green. But, why are plants green? Scientists at the Virtual Planetary Laboratory (VPL) and at the Goddard Institute for Space Studies (GISS) think that they may have an answer. Plants are able to make us of light as an energy source in a process known as photosynthesis.
Plants use light to provide the energy needed to perform these chemical reactions. But, they don’t use all colors of light equally. Particles of light are called photons. Photons of different colors of light carry different amounts of energy. Blue photons carry more energy than red photons. So, you might think that plants would selectively absorb blue light. But, as it turns out, the red light carries enough energy to perform these reactions, and excess energy doesn’t help. So, naturally plants absorb red light. But, they also absorb blue because of the way that the Earth’s atmosphere scatters the blue light. The blue light is absorbed, but the pigments absorbing the shorter wavelengths funnel the energy in a cascade manner to the longer wavelengths, where the pigments that use those wavelengths actually perform the photosynthesis. Because of the way that the atmosphere scatters the light, it turns out to be good for plants to absorb it. Green turns out to be the least useful, and the red and blue light provides plenty of energy, so plants then reflect the green light. Plants therefore are green. The graph below shows the absorption of light for typical chlorophyll pigments.
But, chlorophyll isn’t the only photoreactive chemicals used in biology. Our own eyes use a much older technology, and more to the point, different cone receptors in the eyes have pigments that respond to different colors of light. Chlorophyll is optimised for the Earth’s current atmosphere. However, in the distant past, Earth had a different atmosphere. The Sun, also peaked at a slightly different wavelenght. These effects change the way that light is tranmitted to the surface of the Earth. So, it is no surprise that cyanoplankton and other ancient organisms have pigments that collect different wavelenghts of light than the typical green land plants, which are more modern.
But VPL and GISS scientists, including astronomers, meteorologists, and biologists, have studied photosynthesis and have developed computer models that show what wavelengths of light would be most favorable for plants to use. They find that the answer varies wildly. Some stars emit more blue and UV light than the Sun. Others emit more red light than the Sun. But, even more important, atmospheres can radically change what kind of light gets to the surface of a planet. According to their models, plants on other worlds might actually work better if they were reddish, orange, or yellow, rather than green. Of course, a planet with an oxygen and nitrogen rich atmosphere orbiting an early G type star like the Sun turn out to do best if they, too, are green. But, that is not necessarily true for hotter or cooler stars. Recent findings seem to show that planets may be found around far more types of stars that astronomers had first thought. So, this means that plants on those planets (if those planets have any life) may be predominantly some color other than green. So, the scifi stories about all the wild colored vegetation on other worlds might actually have been right after all!
-Astroprof
Artwork courtesy of NASA, Caltech
Graph courtesy of NASA
Photosynthesis diagram courtesy of DOE
Melissa Dowd on June 8, 2007 at 5:45 am: 1
This is a fascinating area of study. I’m particularly interested in red dwarf stars, since they persist so much longer than hotter stars. Since m-class stars radiate so much more infrared, I’m thinking that foliage on a planet with an oxygen atmosphere in orbit around a red dwarf would likely be black or purple.
Cody Frovarp on March 6, 2009 at 2:52 pm: 2
What color would plants be that orbit a bright blue star if the atmosphere was fairly similar to ours but with slightly less oxygen? Did they release information on what types of stars would possibly have what colors of plant life?
Konrad on January 19, 2012 at 11:11 pm: 3
Assuming a sunlight star what kind of atmospere would be condusive to orange plants?
I’m toying with the idea of Fictional version of Staturn’s Moon Titan where the orange color is the result of plant life rather then clouds.
Getting a plausable explanation of how this could come about would be very handy.