No. 111: May-Jun 1997
Photosynthesis is the ultimate source of energy for most of the life forms we recognize here on earth. Sure, there are also a few creatures that derive their energy by oxidizing the sulfides dissolved in the 400° water gushing forth from deep-sea vents. We will call them "geochemical creatures" to separate them from the "photosynthetic creatures" we are more familiar with. But, in principle at least, there could also be "Carnot creatures", whose metabolisms depend upon temperature differences like almost all human-built engines. Some bizarre animal, such as a meter-long tube worm, could plant one end on a hot rock surface and dangle the other in cold seawater to reject waste heat from its Carnot engine. Since thermodynamic-cycle efficiencies can approach 60% compared with only 10% for photosynthesis, evolution would have been remiss if it had not tried to evolve "Carnot creatures." For, as D. Jones comments below, Carnot creatures would be adaptable to many more habitats in the universe than photosynthetic creatures, which must have a sun with a very specific electromagnetic spectrum.
"Many worlds, from distant 'brown dwarf' stars to the satellites of giant planets, may have internal heating but no effective 'Sun'. If Carnot life is possible, it may well have evolved in such dark and distant places -- making life abundant throughout the Universe. Indeed, our distant descendants may be able to harness Carnot biochemistry to sustain themselves on geothermal or residual browndwarf warmth when the Sun finally grows dim."
(Jones, David; "The Dark Is Light Enough," Nature, 385:301, 1997.)
Comments. To our knowledge, those who search for extraterrestrial life do not consider the possibility of Carnot creatures and wouldn't recognize them if they stumbled across them or their signals. For example, Carnot creatures might emit infrared signals rather than radio waves; and they might be immense in size. (See related item under ASTRONOMY in this issue.)