I was looking at an old National Geographic article about Mars. It showed color photographs of Mars, taken through a large telescope. Indeed, parts of Mars looked green. Was this an artifact of the film used then? Did astronomers assume that Mars had plant life then? Also, how long did it take before astronomers knew that Mars was actually a dead planet?
Short answer: Yes.
Long answer: The dark areas (believed to be green by contrast with the ruddy surface color) seemed to expand and contract with the seasons, as observed in the telescopes of the time. This was probably either an illusion (probably) or the effect of sandstorms. The movement to a dead Mars took place over the next 20 years, very gradually, as increasingly clearer observations slowly and inexorably destroyed the “living Mars” of 1950s theory.
Larry Niven’s tongue-in-cheek fantasy Rainbow Mars of course suggests that it was a near miss – that Mars was running out of water and air, etc., in the years between Schapiarelli’s observtions and the Viking lander.
I don’t know if they did that, but they probably did think that it was possible that Mars had plant life.
Mariner 4 went to Mars in 1964-65 (orbited, but didn’t land), and showed that Mars didn’t have widespread life that was visible and obvious from an orbiting spacecraft (as Earth, of course, does). I’m not sure, but I think Mariner 4 might also have been the first time we knew for sure that the atmosphere of Mars was as thin as it is.
Comics and fictional books had all our solar sytem populated, until the the 1960’s. They moved on to other star systems at that time. It’s likely because of better observational equipment by that time.
I have to say that the Solar System would be a much cooler place if there were life on Mars, even if it was just plant life. Of course, intelligent life, provided it wasn’t interested in taking over Earth, would be even cooler.
There would be some reasons it might not be- Mars’ surface gravity is only 38% of Earth’s, and any Martian life would presumably have evolved to cope with that gravity. Their coming to Earth would be like our trying to work in Jupiter’s “surface” gravity, minus the whole difficulty of Jupiter not having a solid surface. Come to think of it, I’m really not sure what they’re using for a “surface” when they make that calculation, anyway…
We still don’t know that Mars is a dead planet. I doubt there are Martians but many still consider the chances are reasonably good that Mars still hosts something like our bacteria. Mars missions are still looking for it.
Nonsense. There are no bacteria on Mars. That’s why the invasion of 1900 failed – no immunities.
Speculation about Mars from telescopic observations, was about higher life forms than bacteria. They were looking for something that had enough intelligence to dig canals. Be it six limbed bipeds, or winged angels, they were expected to be intellegent at one point in their evolution. Many expected degenerated beings on a dying world, sunk to barbarism, or super intellegent beings producing all their needs by advanced science.
Some were, some weren’t. Not all observers agreed that the canals were there, and among those who did think there were canals, some of them thought that the canals were natural features, not necessarily made by intelligent life.
When I was a kid, I read nothing from any serious scientist whcih even showed a possibility of ther being anything much "higher’ than simple mosses/lichens. Some did speculate there could have been a long dead civilization, sure- but even that came mostly from SF writers .
And the fact is- given what they knew then- “simple” forms of plant life made sense.
Semi-hijack
I have the same question as the OP, but regarding Venus.
As a veteran of 1950s elementary school science classes, I recall that the prevailing opinion was the green canals certainly suggested plant life, even if it couldn’t be proved.
Like DrDeth the green patches weren’t supposed to be more than moss or lichens (and maybe something even simpler, like mold or algae) but their presence indicated water, and water is necessary for life…so anything could be possible.
But even by the 1950s, the prevailing opinion was that the “canals” were, if anything, dried up river beds, certainly not anything that had been constructed.
We couldn’t see any surface features at all on Venus, because of the clouds. Before radio observations of Venus began in the 1960s, the clouds were assumed to hide an Earthlike surface, probably wetter than Earth (because there were more clouds, there must be more water).
The radio observations and later spacecraft missions told us that conditions on Venus aren’t suitable at all for life.
That’d be Mariner 2 and Venera 4 (1962 and 1967) which revealed a venerian surface temperature of nearly 500°C.
Contemporary evidence says that most of the speculations above are wrong.
My source is 1951’s The Exploration of Space by Arthur C. Clarke. It doesn’t excerpt very well for our purposes and he includes a great deal of frank guesswork and hope about what we might find there. But his scientific evidence is completely modern for the time.
On Venus:
On Mars:
No green areas of any kind, despite what was said in the OP.
Small nitpick it’s Venusian.
The correct Latin adjectival form would be Venereal, not Venusian, but obviously if one were to speak of the Venereal atmosphere some people might get the wrong idea. 
How wide was the gulf between scientific consensus of the time and public knowledge?
IMO The average person today thinks only Saturn has rings, doesn’t know what a gas giant is, or that Titan has an atmosphere.
The Clarke quote you gave seems to say that the average person at the time did think Venus was covered by clouds of water and that life was probable below.
That you were speaking of Bill The Galactic Hero by Harry Harrison? IIRC The Venereal atmosphere is torrid and you should never sit down without a knife.
Yes, but even scientists believed that the dark areas represented vegetation, so that although the OP got the details wrong, the OP’s conclusion that it was believed that Mars had plant life is correct