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Mariner 4 was the first spacecraft to return photographs of Mars from near the planet, making its flyby of Mars in 1964. Those photographs showed what appeared to be a barren, desert surface. Mariner 4 also discovered that Mars had no magnetic field protecting it from ionizing solar radiation.

I've read many places that until Mariner 4, Mars was often assumed to host life of some kind, causing the seasonal color changes on the planet. Wikipedia's article on Mars sums it up well:

The seasonal changes (consisting of the diminishing of the polar caps and the dark areas formed during Martian summer) in combination with the canals lead to speculation about life on Mars, and it was a long-held belief that Mars contained vast seas and vegetation.

Was the general view of the scientific community at the time that Mars had life? Or did scientists generally believe Mars to be a lifeless place?

A few statements about life on Mars before Mariner 4:

NASA's Summer Study, a group of eminent scientists gathered at Stanford University, said this in 1964:

Given all the evidence presently available, we believe is entirely reasonable that Mars is inhabited with living organisms and that life independently originated there.

From Life magazine, 1944:

The photographs of Mars...present evidence that points to the existence of life on Mars. ... Given this environment, it is logical to conclude that the vast regions on Mars that change from green to brown in seasonal cycles are covered by vegetation.

From a letter published in New Scientist, 1959 by a Cambridge professor:

At present there seems to be rather convincing evidence for life on Mars, and it is this that lends support to the assumption that life will come into existence where conditions are suitable.

A biologist said this in Popular Science, 1962:

We've said already that the color change fits neatly into a spring-summer vegetation cycle. A second point in favor of life is that when the yellow clouds do cover the planet they don't permanently tinge the markings yellow. Within a couple of weeks, whatever is there shakes it off or grows up through the cover.

I've been able to find plenty of citations that argue for life on Mars, and plenty that argue against. What I haven't been able to get is any clear picture of the dominant scientific consensus of the time.

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  • $\begingroup$ I'm adding the "discoveries" tag somewhat sarcastically, because of what wasn't found: A habitable planet. $\endgroup$
    – HDE 226868 ♦
    Commented Nov 8, 2014 at 22:01
  • $\begingroup$ One should make a distinction between multicellular and unicellular life. The Viking probes were designed for search for unicellular life, and the results were somewhat ambiguous. $\endgroup$
    – user466
    Commented May 8, 2015 at 17:40

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I do not see how Mariner 4 mission could lead to any conclusions about the life on Mars.

The crucial mission to my understanding was Viking 2 (1976) which landed on Mars and made biochemical experiments trying to find any signs of life. At that time almost nobody seriously expected that there is life on Mars.

The presence and chemical composition of the atmosphere was found in 1926 by spectral analysis from the Earth. (The earlier attempts in 1894 did not show the presence of a substantial atmosphere. All speculations about the life before that time were totally groundless, I suppose).

These results showed that the presence of life (at least in the form we know it) is very unlikely. Exact composition of the atmosphere was found with the first probe that landed on Mars; this was the Soviet Mars 2 in 1969, if I remember correctly.

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    $\begingroup$ Mariner 4 made two significant contributions to the idea of Mars as a lifeless world: photos of an apparently barren surface, discovery that Mars has no magnetic field protecting it from radiation. (I've added these points to my question as well.) $\endgroup$
    – Joe
    Commented Nov 8, 2014 at 21:27
  • $\begingroup$ Magnetic field is essential, of course. How a photo of a barren surface from the space proves the absence of life, I am not sure. Did they expect to see highways and skyscrapers? :-) $\endgroup$
    – Alexandre Eremenko
    Commented Nov 9, 2014 at 0:05
  • $\begingroup$ A photo doesn't prove there's no life, sure. But from what I've read, those photographs were a blow to many who were expecting to see a world covered in vegetation. $\endgroup$
    – Joe
    Commented Nov 9, 2014 at 0:08
  • $\begingroup$ Analysis of the atmosphere in 1926 should have indicated that there is no reason to expect vegetation. Viking 2 was looking for microbes and viruses. $\endgroup$
    – Alexandre Eremenko
    Commented Nov 9, 2014 at 0:19
  • $\begingroup$ Take a look at the quotes I added above. Many scientists after 1926 were expecting vegetation. Today we look back and say that they should have known (and some of them did), but at the time, many people came to a different conclusion. $\endgroup$
    – Joe
    Commented Nov 9, 2014 at 0:38

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