I am amazed at how frequently I encounter an argument that involves the fallacy known as "special pleading." Such an argument utilizes only whatever evidence supports one's view, and ignores whatever evidence is contrary to that view. I see examples of this fallacy in a variety of philosophical and scientific arguments, but one area of science in which it seems especially prevalent is the search for extraterrestrial life. Indeed, it could be argued that continuing the search for life on other planets necessitates turning a blind eye to a host of evidence regarding the requirements for life.
For example, according to this article, NASA scientists think one of Saturn's moons, Enceladus, has real potential for life. This belief comes from evidence sent back by the Cassini spacecraft, which found both "organic materials" and temperatures as high as minus 135 degrees F. According to the University of Colorado's Larry Espositi, "We see on Enceladus the three basic ingredients for the origin of life," by which he means energy, water, and organic compounds.
I find two big problems with this claim (which serves as an example of many of the claims that come out of this research area). The first is that other scientists have identified a whole host of other ingredients and attributes (of a planet or moon) that are critical for any possibility of life support.
This recent article indicates that scientists have hopes that somewhere below Enceladus' surface there might be temperatures high enough to allow for liquid water. But most serious researchers admit that life actually requires an abundance of water in all three phases, ice, water, and water vapour, and that for a moon or planet to support advanced life, all three forms of water would have to have existed for long periods of time. In light of these realizations, the finding that Enceladus surface temperatures may be minus 135 (rather than, as formerly believed, minus 225) degrees F seems to caution against the optimism of these researchers.
Other characteristics (of a planet or moon) that have been identified as critical for life support include just-right surface gravity, inclination and eccentricity of orbit, axial tilt, rotation period, albedo, magnetic field, crust thickness, seismic activity, and a host of atmospheric characteristics. In addition, researchers believe that even the rate of asteroidal and cometary collision must be just right to allow for life support. In this regard, Saturn and (especially) Jupiter serve a critical function on Earth's behalf, shielding us from most of the comets and ateroids that pass through our solar system. By contrast, Enceladus would be expected to take many more life-destroying "hits" (because of its proximity to such a large planet).
But the other problem routinely ignored by scientists searching for extraterrestrial life is that life support is only part of the equation and that the other part--life's origin--is if anything more improbable (naturalistically) than is life support. Some of that evidence can wait for a later post. In the meantime, don't count on NASA actually finding life on Enceladus, since those hopeful of so doing must ignore a vast array of evidence disqualifying this moon from consideration.