When was the martian dichotomy first observed? 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. (&quot;Hemispheres Apart: The Crustal Dichotomy on Mars&quot;):  &quot;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.&quot;  Other papers cite a 1973 paper by Hartmann (&quot;Martian cratering 4, Mariner 9 initial analysis of cratering chronology&quot;) 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).