Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization
© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm
10.1 Finding the Energy to Live
Active life requires a flow of energy between a source (a region of high energy) and a sink (a region of lower energy). To use heat energy, for example, a difference in temperature between two points in space must be maintained. A steam engine works not because it is hot, but rather because the boiler is hotter than the condenser.
How does plant life fit into this scheme? It has been said that the only reason photosynthesis works at all is that the surface of the sun is at 6000 K, whereas the surface of the Earth (and its plant life) is only at 300 K. Photons emitted at the higher solar temperature travel through space to this planet, enter the chlorophyll molecule and power the plant’s metabolism. Later, photons of waste heat, a form of degraded energy, are radiated off at the far lower planetary surface temperature.
Animals too need sources and sinks. The food they eat is burned by the oxygen they breathe, and this constitutes a useful source of energy. The external environment, by accepting bodily waste heat, serves as a sink.
All life on Earth ultimately depends upon one of two sources: Photons from Sol (photosynthetic organisms) and chemical energy (all nonphotosynthetic terran lifeforms). However, various other possibilities have been suggested for hypothetical alien beings on other worlds.
One widely discussed alternative involves the evolution of life on so-called "starless planets."128,816 Such worlds, if they exist, lie in the dark plumbless abyss of interstellar space far from the coddling embrace of any friendly star. Were the object large enough, say, a massive jovian or super-jovian, it might be warmly self-heating with a tepid surface crust.
Of course, we know that heat alone will not power a living organism. And it is difficult to imagine how to establish a flow of energy in an environment heated to some relatively fixed, unvarying temperature. Most writers have ruled out life on starless planets on this basis.18,22,714
Dr. Thomas Gold at Cornell University disagrees. If we consider the surface of a starless planet as a source at 300 K, he points out, then all we need to do is find a sink somewhere at a lower temperature to establish a life-giving flow.
Space is very cold, only about 2.7 K. If this or something very close to it could be used for the energy sink, then biological thermodynamic efficiencies approaching those of terrestrial photosynthesis might in principle be possible.22
Extending this idea just a bit further, Gold suggests that some alien lifeforms may base their processes on a thermal gradient in time rather than in space. Imagine a uniformly heated environment in which there was a slow but regular diurnal temperature variation. Usable bioenergy could be extracted through the use of a chemical system which coupled only to the equilibrium state established at each extreme.
At the hottest extreme, certain reactions might take place which stored energy in chemical form. This energy would then be released only when the temperature swung down to the coolest extreme. In this scheme, the source and sink are no longer coextensive in time. As temperature fluctuates, the surroundings would be first the source, later the sink, and so on.
Many other imaginative and exotic energy systems have been postulated by various writers, including geothermal heat and volcanism, piezoelectricity, solar wind ions, planetary magnetic fields, atmospheric electricity (e.g. lightning), and radioactive decay (fission power).
J.W. Ycas has come up with a novel form of energy transduction, to which he has given the formidable appellation "palirrhotrophy."2379 His organisms, should they exist, are powered by chemical osmosis. A flow of bioenergy -- an "osmotic current" -- is established "by exploiting the rhythmic variations in salinity which occur in the estuarine environment." As the palirrhotrophic lifeform is periodically flushed, first with salty seawater and later with upriver freshwater, energy is pumped into its system osmotically.
Such creatures might exist on a predominantly watery world, one with a large moon or moons and a fast rotation to make the tides frequent but brief. A tropical climate would ensure plenty of rainfall and a bountiful source of freshwater, and high gravity would cause mountain water runoff to cut deep channels and fjords to the sea -- a viable niche for palirrhotrophic ETs.
Another distant possibility is the use of mechanical energy. The waves, winds or tides might be harnessed to power a shore-dwelling alien creature. A slowly rotating planet with a massive moon in a fast orbit would have plenty of mechanical wave energy available at the surface. Yet organisms would find themselves without sunshine for such long night-time stretches that they might find it useful to evolve a biomechanical energy system as an auxiliary power supply.
A similar proposal is that extraterrestrial lifeforms might be able to use the internal heat flowing up though the surface of a terrestrial world. Unfortunately, even on a world as far removed from the stellar campfire as cold, distant Pluto, the sun out radiates internal planetary sources by nearly two orders of magnitude. A more viable proposition, perhaps, is the concept of the thermoelectric organism.607 On a planet with thin air, located close to its star, the temperature differential between direct sunlight and shade might be sufficient to adequately power an alien biochemistry.
A few hardy souls have even suggested thermonuclear lifeforms. At a meeting of the British Interplanetary Society back in 1948, Olaf Stapledon proposed that the fusion power of the sun might conceivably be harnessed as an alternative to biochemical processes.556 Although Isaac Asimov has used this idea in the context of a small, planetbound animal,94 such a power supply might be more apropos for electromorphs akin to Hoyle’s Black Cloud.*
* It is interesting to note that Sol, the only nearby entity we know of that uses fusion power, has an overall energy output of only about 0.0002 watts/kg. The human body, on the other hand, operates at a whopping 2.0 watts/kg, about four orders of magnitude higher than the sun!
Last updated on 6 December 2008