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The North and South hemispheres of Mars are very different one from another. They have different elevations, different crust thickness, different surface ages. This is known as the martian dichotomy.
When was this striking difference between the two hemispheres first observed?
It looks like the Mariner 9 era.
Based on a review paper by Watters et al. ("Hemispheres Apart: The Crustal Dichotomy on Mars"): "The north-south asymmetry ... was clear from the first global image mosaics of Mars returned in the 1970s by Mariner 9 (Mutch et al., 1976) and the Viking Orbiters." Other papers cite a 1973 paper by Hartmann ("Martian cratering 4, Mariner 9 initial analysis of cratering chronology") as one of the first papers to describe it, but it’s still based on Mariner 9 images.
I suppose in hindsight one could look at earlier flyby imagery and see that there is a dichotomy, but with 23 images from the first flyby, that's hardly representative of the surface and not enough to draw such conclusions. Similarly, one could probably look at Earth-based telescopic observations from before the Space Age and retrofit observations into a north-south dichotomy paradigm, but I don't think one could legitimately claim earlier discovery of it.
So, 1971-1972 (Mariner 9 entered orbit in late 1971).
In "The Planet of Doubt", Stanley G. Weinbaum, Astounding Stories, October 1935, there is a scene where Hamilton Hammond, leader of a expedition to the north pole of Uranus, explains his descision to set a southeastern course while searching for land:
"I'll tell you. Did you ever look at a globe of the Earth, Pat? Then maybe you've noticed that all continents, all large islands, and all important peninsulas are narrowed to points toward the south. In other words, the northern hemisphere is more favorable for land formation, and as a matter of fact, by far the greater part of the Earth's land is north of the equator.
"The Arctic Ocean is nearly surrounded by a ring of land, but the Antarctic's wide open. And that same thing is true of Mars, assuming that the dark, swampy plains are old ocean beds, and also true of the frozen oceans on the night side of Venus.
"So I assume that if all planets had a common origin, and all of them solidified under the same conditions, Uranus must have the same sort of land distribution. What Young found was the land that corresponds to our Antarctica; what I'm looking for is the land that ought to surround this north polar sea."
From http://gutenberg.net.au/ebooks14/1401921h.html
Of course science marches on, and we now know the giant planets down't have surfaces of sea or land, and that plate tectonics moves the continents of Earth around so that they weren't always where they are now, and we know a lot more about the surfaces of Mars and Venus than we did in 1935.
But it is possible that some people would claim that the idea that the northern and southern hemispheres of Mars are different goes back at least as far as this story in 1935.
