Now imagine yourself on a distant planet with similarly abundant plant life. What would it look like? It might well be scarlet or vermilion, rather than verdant.
Most Earthly plants are green because contain the pigment chlorophyll, which reflects green light and absorbs red and blue light. The energy from the light absorbed by chlorophyll is used for photosynthesis – the conversion of carbon dioxide into sugars and other organic compounds.
Along with green light, chlorophyll also reflects near-infrared light – called the “red edge” – which is invisible to the human eye, but can be detected remotely using near-infrared sensitive cameras. Currently satellites use such systems to remotely monitor the health of vegetation on Earth. That ability makes chlorophyll detection a reasonable potential molecular signature of extraterrestrial life.
But it may wrong to assume that plant life (or the equivalent) on other planets will necessarily be green.
Once the light from our Sun is filtered through the ultraviolet light-absorbing ozone layer, more photons at the red end of the spectrum reach the Earth’s surface than at other wavelengths. It makes sense, then, that Earthly plants primarily use red light.
A planet that orbits a star hotter than our own sun or that has an atmosphere that absorbs a different range of wavelengths than Earth’s, might have a greater abundance of blue photons than red photons on its surface. Orange or red plants might dominate there.
And Washington University chemist Robert Blankenship has suggested that alien plants might use black pigments that absorb all visible wavelengths of light. That might be necessary for plants on planets orbiting cool red dwarf stars.
The only plant color that is considered to be unlikely is bright blue, since that would mean that high energy blue light is being reflected from the leaves, rather than utilized. But I consider that to mean that blue plants are unlikely, not impossible.
So how are plants portrayed in science fiction?
H.G. Wells’ invaders in the War of the Worlds carried invasive red-colored weeds to Earth. That fits nicely with the notion of Mars as the “red planet”, but isn’t really based on biology – not too surprising, since photosynthesis was not well understood in the early 20th century.
Other SF novels do include strange and alien plants, but to the best of my recollection they generally have green foliage. It seems like a missed opportunity to increase the strangeness of alien worlds.
I’d love suggestions for SF stories that do include alien non-green plants in the comments.
- Cosmic Variance: “The Red Edge“
- Seager S. et al. “Vegetation’s Red Edge: A Possible Spectroscopic Biosignature of Exraterrestrial Plants” Astrobiology 5: 372-390 (2005) DOI: 10.1089/ast.2005.5.372 (subscription required to access full article)
- Goddard Space Flight Center News: NASA Predicts Non-Green Plants on Other Planets
- NASA GISS: Science Briefs: The Color of Life, on Earth and on Extrasolar Planets
- Astrobiology News: Pondering Alien Plants
- Astrobiology News: Fall Colors on Alien Worlds