Did we start using “STD” instead of “VD” so as to placate a bunch of aliens?
You appeared to be disagreeing with the OP. You said:
While the OP had the details wrong about the evidence, the answer to the basic question posed, “Was Mars believed to support life in the 1950s?” is “Yes.” It wasn’t clear to me what you considered to be “wrong,” since it seemed to me that the details concerning color photography being the evidence were a quite trivial part of the question.
Not at all. Chlorophyll is a molecule that has some very special properties, which is why it is used by both photosynthetic procaryotes and eucaryotes. (Of course, the chloroplasts of eucaryotes are descended from procaryotes.) Those photosynthetic earth organisms that are not green mostly get their colors from supplemental pigments in addition to chlorophyll. One might expect Martian photosynthesizers to have convergently developed chlorophyll, or something very much like it. There is every reason to expect that Martian plants might be green. Why would you think that a process other than photosynthesis would be used on Mars?
What exactly is your evidence for this statement? I think that lots of scientist in the 1950s believed there might be organisms convergent to multicellular green plants on Mars. In any case, back in the 1950s the “Two Kingdom” classification scheme prevailed. “Plants” were considered to include blue-green “algae” - now known to be procaryotes and called cyanobacteria - , as well as unicellular eucaryotic algae and multicellular plants, and also fungi, now considered a separate kingdom. No strong distinction was made between any of these groups - the division was between plants on one hand, and animals on the other. Back in the 1950s, the term “plants” was essentially equivalent to “photosynthetic organisms” (plus sessile non-photosynthetic organisms with cell walls like fungi).
Good God, those are strange looking, uh, whatever.
It’s not surprising that Clarke would think of banyan trees, given that he lives in Sri Lanka, or at least did so for a long time.
Right. When we get to the stars (or perhaps, even somewhere in the Solar system), it won’t be suprising if there are “plants” there which use photosynthesis and something very like (if not the same as) Chlorophyll.
True, now some living thins that were called “plants” are no longer included in that Kingdom. But that doesn’t mean they were wrong then, it’s just that we have learned more, and made sharper and “better” divisions. It’s like the Lion was “Felis Leo” and now is called “Panthera Leo”- it’s doesn’t change what the Lion is.
I believe I read somewhere that the surface of a gaseous planet is considered to be the point at which the atmospheric pressure exceeds that of Earth at sea level.
Actually, I wouldn’t be surprised if alien plants used something other than chlorophyll.
Chlorophyll is pretty weird, when you think about it. The peak of our eye’s response is in the green, because, after sunlight (with its characteristic spectrum) has made it through the absorbing and scattering atmosphere, the most prevalent wavelengths are the visible rays, peaking in the green. Chlorophyll is green because it reflects green, thereby throwing away the strongest component of the sunlight in favor of absorbing lower-energy red photons and the higher-energy (but fewer in number) blue photons. Howcum?
A theory I read a few years ago was that the earliest plants used something very similar to rhodopsin – the “visual purple” in our eyes that responds well to green light. Competing plants were elbowed out, until some upstarts started using chorophyll , which utilized the red and blue wavelengths that these purple plants were leaving behind. They out-competed the purple plants, and chlorophyll plants took over, and got so good at it that they kept on using chlorophyll metabolism, even though it was less efficient.
If so, it wouldn’t be hard to imagine a world where chlorophyll didn’t take over. Maybe, if some disaster helped the chlorophyll get a foothold on earth didn’t happen on that alien world, it would be populated by predominantly purple plants.
The Martians of Known Space were pretty different from previous depictions of Martian life. He knew Mars had very little water and a CO2 atmosphere when he wrote them, and the Martians had a circulatory system based on carbonic acid and they reacted explosively when exposed to water. They also believed at the time that Mars would be covered with a very fine dust that had accumulated over the millions of years that flowed like thick oil (the atmosphere was thick enough to prevent vacuum cementing, but had no moisture or other factors to cause the dust to become sedimentary rock). Niven Martians swam in a sea of dust and rarely came to the surface. We know now that the surface of Mars is not like that, but it was good science at the time.
Green plants representing life is so useful a device in fiction that the slightest possibility of their existence would be jumped on.
Not what I said. I said Zelazny finished off the genre artistically. (And in his first couple of major stories! He never gets enough credit for this.) It was still used but no longer taken seriously. And if Blanche is correct - it’s been 30 years since I read Known Space stories - Niven is a bad example for your case.
Colibri, I disagree. I’d be amazed if convergence of that sort worked on Mars. There was no reason at the time - except that we had only one data point to extrapolate from - to think that Martian plants would be in any way like earth plants. Some form of life, perhaps some form of vegetation analogous to plants, yes, but green plants were mostly wishful thinking. Scientists can fall prey to wishful thinking as much as anyone but chlorophyll convergence on Mars? That’s pushing reality.
I hauled out some of my older books to see what they have to say. The first two are the first books on natural history I ever read, which I received in grade school. Although they are children’s books, they are serious ones and I think sum up the “popular science” understanding of life on Mars in the 1950s.
From The Golden Treasury of Natural History, by Bertha Morris Parker (1952 edition):
From The World We Live In, Life Magazine Special Edition for Young Readers, 1956:
Even as late as 1974, the Encyclopedia Britannica (15th edition) had passages such as these:
So at least one a expert observer reported seeing an “olive” tone (although photometry later showed this to be incorrect).
Also:
After discussion of the evidence, the article concludes:
So even in 1974, a reference like the Encyclopedia Britannica was willing to entertain the possibility that primitive life forms - perhaps even lichen-like - could exist on Mars.
That was a wonderful book in its time.
A kid could learn a lot from it, or just look at the pictures of dinosaurs etc, and still learn something.
Who’s disagreeing? The Clarke piece I quoted said in 1951 that “the evidence for the growth of vegetation is overwhelming.” That was indeed the scientific consensus. And they thought that lichens, which are not plants and do not use chlorophyll as far as I know, were the most likely analog for earth-type vegetation. Which is what I’ve been saying.
I’d love to argue with you about this, but I can’t think of anything to argue about, except possibly that they saw green because they wanted to see green rather than because it was truly there. But with the telescopes of the day even that was a borderline point. We’re all saying the same thing.
This is EXACTLY why one might suppose that chlorophyll might be a general solution for photosynthetic organisms, wherever they might be found. The fact that chlorophyll is so prevalent DESPITE the fact that it reflects green light indicates that it must have some other very strong advantage in order to have have been selected as the almost universal pigment for photosynthesis on Earth. As you mention, other pigments have better absorption of green light. It is not due to chance, or to some random disaster, that they were supplanted, but because chlorophyll is competively superior. Chlorophyll’s deficiency in absorbing green light is compensated for by carotenoids and other pigments, which pick up this part of the spectrum and pass the energy on to chlorophyll.
Chlorophyll contains a porphyrin ring, around which electrons are free to migrate. Because of this the ring has the potential to gain or lose electrons easily, and thus can provide energized electrons to other molecules. This is the basis of chlorophyll’s utility in capturing the energy of sunlight. Other available molecules don’t do it as well. It is fairly likely that other biotic systems may well have hit on this same solution also.
While it is possible that other pigments such as rhodopsin could be used, it would most likely only in a world where chlorophyll had never been invented.
With all due respect, Exapno, I think that your opinion here may not be a well-informed one with respect to the basic biological issues involved. If Martian organisms exist, they almost certainly will use DNA or RNA as their genetic code (although the details of the code will be different) and will be based on proteins. If they are photosynthetic, the chances are pretty good that they will have hit on the same solution as life on Earth has, that is, chlorophyll. As the quotes I cited above show, it was widely assumed at the time that if plants existed on Mars, they would be green; or if there were green areas on Mars, they represented plant life.
In the 1950s lichens most assuredly were considered plants. They are actually symbiotic organisms composed of fungi associated with green algae (usually). Both of these were considered to belong to the Plant Kingdom at the time. (Now fungi are classified in their own kingdom; algae may be placed either in the Kingdom Protista or Plantae depending on the author. The Five-Kingdom system didn’t really become accepted until the 1970s, but the whole concept is kingdom idea is becoming old-hat anyway today). Green algae use the same types of chlorophyll as multicellular plants, and are very closely related to them.
I think you’ll falling for the extrapolation from one data point fallacy. Unless you have good reason to suspect an interaction of life between the planets due to spores on rocks or the like, the “almost certainly use DNA or RNA” is more wishful thinking than science until we find an actual second example of life. And even the quotes you gave seen to be aware of a distinction between plants and lichens. (Thanks for the correction about the green algae, I had the cyanobacterium stuck in my head.)
I’ve been reading accounts of exobiology as long as you have, starting from the same base, although not reaching the expert depths that you have. These fallacies are common as dirt in the popular literature and whenever scientists speak to the public. Without any evidence to back up any claim, why should we still assume that life must be like us? That’s a serious question. The honest answer I keep getting is that it’s easier to think that way, but is there anything more to it than that?
Yes, basic organic chemistry and the fundamental properties of certain complex organic molecules. DNA and RNa, because they form templates which are easy to replicate, are particularly apt for being used for a genetic code. Proteins exhibit very complex folding which makes them apt for enzymes and other critical biological compounds. Porphyrin rings are very good at providing energized electrons for energy transfer. We have created a large number of exotic organic compounds in the lab, but they don’t have these kinds of properties. It is no accident, no random chance, that life on Earth hit on these particular classes of compounds to use for these particular functions. While other solutions may be possible, there is every reason to think that life elsewhere may use these same ones as well.
The DETAILS of the biology of course will be different. Organisms may use RNA instead of DNA; the genetic code - which sequences of nucleotides code for which amino acids - will probably have no relation to ours. The details of which proteins are used for which functions will differ. However, we may expect that versions of certain biological compounds may also occur on other planets - things like cellulose, chitin, collagen, etc or variants of them.
Porphyrin rings are, indeed, well-suited to tasks like harnessing energy from sunlight. But there are many, many molecules built around porphyrin rings, in Earth life alone. And there are many, many more which could hypothetically exist. I’d be reluctant to say that chlorophyll is the best one possible, or that it’s so good that no other molecule would stand a chance.
They may well use some sort of ladder-structure molecule, since I’ve never heard any better way to produce a self-replication information-storing molecule. But why our particular set of base pairs? We already know that there are bases other than the standard ACTG, since in fact RNA uses U instead of A. Why could you not have some other molecule, which used Q (hypothetical label for whatever that base might be) instead of G? Why not substitute for all of them? Or make them sufficiently different that there’s not even a correspondence between the Martian base-pairs and the Earthly ones? Why not use a base-6 system instead of base-4?
And for proteins, well, that depends on how you define “protein”. We already know that there are some amino acids different from the standard ones… Maybe Martian life uses a completely different set? Maybe they use monamers which we wouldn’t even label as “amino acids”, unless we saw them used in Martian protein-analogues?
I haven’t seen any arguments about why chlorophyll won out, but it’s not clear to me that its prevalence on earth proves that it’s better adapted. T.H. White to the contrary notwithstanding, everything not forbidden is not compulsory – I can give examples. The supplanting of purple pigment by chlorophyll might be the result of prehistorical asccident that chancve has not found an opportunity to “correct”. Yet.
Just wondering…suppose there is some martian plant life, and suppose we find it has the same DNA structure as earth plants? Would this be proof that life originated on mars 9and was propagated to the earth via meteorites)? Or is it more probable that some primative life evolved on mars, and died out long ago (when the atmosphere went away). Either way, i’d argu that a Mars sample-return mission ought to be a top priority-but those "trees’ that A.C. Clark mentioned-I’d love to see the Martian rover photos of just what those things are-fossilized trees?
Actually, the invasion took place sometime between 1897 and 1900:
Now, even if you’re one of those annoying people who consider “00” years to be part of the previous century, the fact is Mr. Wells clearly says "the last years" indicating that the events depicted take place before 1900…
Wanders off looking for his coat whilst whistling “The Eve Of War” 
and an invasion twenty years later was stopped by some style of death rays.