To Chronos and CalMeacham: I’m not saying that the biochemistry found in Earth life is the only one possible. It may not even be the best one. However, life on Earth has been tinkering with its biochemistry for four billion years. It may have taken a billion years or more just to come up with chlorophyll. While certain solutions to problems may have persisted just because they were the first ones chanced upon, we do have reason to suppose that life on Earth has had enough time to experiment with a lot of variations and select the best of them.
Certainly the biochemistry found on Earth has been a very suitable one to produce a large diversity of life under the conditions found on Earth-like planets. We have fairly good reason to suppose that life on other Earth-like planets (such as Mars) might use the same general classes of compounds for the same functions, and no particularly compelling reason to think they might be different.
On planets that are not Earth-like, where different conditions of temperature, radiation, and atmospheric composition prevail, all bets are off. While life is still likely to be carbon-based, the compounds used on Earth may not be so suitable or stable, and others could be used. If life exists in the atmosphere of Jupiter (which it might) I would not venture to guess what its biochemistry would be.
It would be no particular surprise (as I have been saying) that plant life on Mars would use DNA, since it is a good molecule on which to base a genetic code. However, there is no necessary relationship (that we know of) that determines which base triplets code for which amino acids. (An alien code may not even use triplets, though it would probably have to use more than two - unless it involved more than four bases - in order to code for enough amino acids.) If the code were the same - that is, we found that the same base triplets coded for the same amino acids - THAT would be an extremely strong indication that life on Earth and that on Mars shared a common ancestor, since that relationship is based purely on chance (as far as we know).
As a related comment to Chronos: it might be expected that alien DNA (or other ribonucleic acid) could differ in the actual base pairs used, since DNA and RNA do anyway. Also, proteins are likely to differ to some extent in the selection of amino acids used. (And it has been obscure why life on Earth has chosen to used only the levo isomers of amino acids.) But it certainly would not be extraodinary to find that life elsewhere uses the same tools for the same jobs.
If I recall correctly (I apologize, but I doubt I can track my memory back to a cite) one of the issues is that colors are hard to judge against a contrasting background and all pertinent observations were fleeting.
Earth-based telescopes were subject to all sorts of atmospheric wobbles; in consequence, as I understand it, much serious observation of Mars depended on using the human eye to try to sort out fleeting glimpses a photograph wouldn’t capture. So you’re looking at shimmering glimpses of dark areas, against a reddish background; perceiving them as dark-greenish-black isn’t unlikely, and my impression is that earth-based photographs weren’t good enough to overrule that perception with any confidence.
One intriguing issue that I haven’t noticed yet in the thread is the persistence of imaginations of the solar system which were rooted in pre-nuclear-energy assumptions. Much of what the late 19th and early 20th century thought about the solar system connected with: what makes the sun hot?
Subtract fusion from the picture, and what most scientists seem to have come up with was that the sun was slowly contracting under the pressure of its own gravity, and releasing heat by that contraction. Which I guess would work; but not for nearly as long as we now assume the solar system has lasted. One consequence would be that, within a comparatively short time frame, the habitable zone around our sun would have been out near Mars, and might one of these days move in past Earth to Venus. On the assumption that each planet would follow somewhat the same development, this would give you a picture of old-worn-out-Mars, vigorous Earth, and prehistoric Venus.
Yes, it’s not as if anyone had scopes with adaptive optics back in the 50’s. Even Valles Marineris was unknown until Mariner 9 snapped its picture in 1971.
[nitpick]Mariner 4 was a flyby, not an orbiter.[/nitpick] The next two Mars missions (Mariners 6 & 7) were also flybys. Mariner 9 (1971) was the first orbiter sent to Mars.
As I recall, Mariner 4 returned only 22 images, almost all of cratered terrain. That gave everyone the wrong impression that Mars was a lot like the Moon – mostly craters. This impression was not dispelled until Mariner 9, since the other two mission also mostly imaged cratered terrain.