Forget the “green hills of Earth.” Once upon a very long time ago, the hills on our planet– and many others– may have been purple. Or, more precisely, violet.
It has to do with the wavelengths of light and what our friends, the plants, do with them.
Most plants use chlorophyll, a pigment, to trap energy from the sun and use it to combine carbon dioxide and water into simple sugars. This process, called photosynthesis, also releases free oxygen. These two products, sugars and oxygen, are necessary for virtually all life on earth.
Plants appear green because the chlorophyll in their tissues absorbs light mostly in the blue and red portions of the spectrum, reflecting green. (Red-leafed plants also have chlorophyll, but in addition contain compounds called anthocyanins, which are red-to-purple in color and mask the usual green tones of plant material.)
But– purple?
A compound called retinal, an unrelated light-sensitive material, existed on the earth prior to the appearance of chlorophyll. Very primitive life-forms called halobacteria utilized retinal in much the same way that modern plants use chlorophyll, and their present-day descendants display a characteristic purple-violet color. Both life-forms co-existed for a time, and then chlorophyll-based forms gradually came to dominate.
If, however, the balance had tipped in favor of the retinal-based forms, our earth would today display a lovely violet cloak instead of its mantle of green. Other planets may have taken that evolutionary path, and our focus on finding a green planet like ours should perhaps be expanded to include eggplant hues as well.
The science-fiction artifice of purple aliens may not be so far off, after all.