Purple Earth hypothesis
The Purple Earth hypothesis is an astrobiological hypothesis that life forms of early Earth were retinal-based rather than chlorophyll-based thus making Earth appear purple rather than green. An example of a retinal-based organism today is the photosynthetic microbes called halobacteria.
- LiveScience.com: Early Earth Was Purple, Study Suggests Accessed May 9, 2007
- Sparks, William B.; DasSarma, S.; Reid, I. N. "Evolutionary Competition Between Primitive Photosynthetic Systems: Existence of an early purple Earth?". Bulletin of the American Astronomical Society, Vol. 38, p.901.
- Colorful Worlds: Plants on Other Planets Might Not Be Green
-  2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #06.05; Bulletin of the American Astronomical Society, Vol. 38, p. 901
-  BR Green, E Gantt - J Phycol, 2000 - interscience.wiley.com
-  Softpedia article
-  Fox News article
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