When I was studying Planetary Atmospheres as an undergrad we were given the task of critiquing an article by Sagan in a respected journal, not a fluff piece for a magazine or newspaper, but a peer reviewed article. He argued that Venus might have liquid water on the surface near its poles, despite its high average temperature, which was known pretty accurately at the time (1970). The problem is that he did not take into consideration geochemistry, even if he was right that the poles of Venus were cold enough to allow ice to form and water to collect there from the atmosphere (ignore whether the atmosphere circulation would allow such a temperature differential, which seemed unlikely to me, though he thought the clouds and glancing sun rays would act to reduce solar radiation capture near the poles -- it happens on Earth). He thought the glaciers would head towards the equator, and would melt as the heat rose, leaving a band of liquid water before it evaporated further away from the poles. This, he said, could support life. I argued that given the high CO2 content of Venus' atmosphere, any liquid water would become very acidic (we see that on a small scale increasing here on Earth due to increasing CO2 in our air). It would be maximal on Venus. This would dissolve calcium and magnesium in the rocks (mostly the former), which would precipitate out in the evaporation zone, leaving an increasing wall of limestone and marble. This would be helped by the grinding effect of the glaciers, which would carry and push crushed rock into the melt zone, giving a lot of surface for chemical reactions, even if the bedrock was mostly igneous. Even allowing that life could form in a highly acidic environment (perhaps, we don't really know even now), the wall would be detectable by gravitational studies that had been done, as it would have grown quite large over the eons. Note that my critique required the sort of knowledge of chemical, physical and detection methods available that most specialists don't have, but arguably Sagan did, but didn't use his critical faculties (it must be said that I had the benefit of several extra years of scientific research). Why not? Well, he was a big proponent of the ubiquity and naturalness of life appearing, so I suspect he was blinded by his wishful thinking. Not just him, but the referees and the editors of the journal. I suspect a many people are similarly blinded by the performance claims (health claims are another big area for wonky ideas propagating) because they want the claims to be true, and their critical faculties are dulled. In general, unlike Sagan, they don't even have access to the relevant science, and their motive to investigate is weak. It is a typical human failing, to let our will extend beyond our reason, as Descartes put it. It is worthy of study by cognitive scientists, anthropologists and evolutionary theorists. Of course they too need to watch their biases. I was just at a meeting in Poland where such people investigating the myriad religious beliefs were mostly warning of their own tendency to do the same thing themselves. Fortunately, if we are aware of the tendency, then we can control it, but reaction to it can lead to too strong a reaction, and tilt the other way. Besides, pet ideas that give hope can be fun, even if they do cost. Perhaps they can even be a rational investment. See the 1963 video on YouTube: http://www.google.co.za/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CGgQtwIwAA&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DA6A-Vfy2cNY&ei=94kRUMv2PIKDhQfc6YDADQ&usg=AFQjCNEDnOT93bHgBmmw7DDX9RLJ2mKsgQ&sig2=hjXWYAVFPpI4dp3YQ1-FOg
One 1967 Nature article is here: http://www.nature.com/nature/journal/v215/n5107/abs/2151259a0.html
I have had some intellectual run-ins with Sagan's co-author during my professional career. I suspect that Sagan's son, Dorion is more in the line of his recently and sadly deceased mother, Lynn Margulis, from my few interactions with him, and many more with one of his co-authors. Enthusiasms can be exciting, but they need to be balanced. Margulis turned out to be right about her central passion for symbioisis as an valid and additional alternative to competition in evolution. Sagan was, of course, a genius, but even a genius can be swayed by wishful thinking. It is very human, and we all do it at times.