When I was young (and maple syrup had just been invented) it was established that Mars had seasons, the planet changing color depending on the time of the Martian year. In the spring, the planet turned visibly greener to earth telescopes, while in its autumn it turned redder.
What were we seeing? Was this just human projection, like canals? Was this merely sandstorms? Was it totally wrong? What is the best explanation, today, for such observations at the time?
There is some color change with the seasons, due to polar caps growing and shrinking, and dust storms.
You’d need a pretty good telescope to see any difference, though, because like Earth, Mars has opposite seasons in the northern and southern hemispheres, and so you’d need enough resolution to clearly see the hemispheres separately.
I don’t remember the details, but it was said to coincide with the seasons. They didn’t claim that they were seeing “green” in the northern hemisphere, just “green” in springtime. What is the most likely thing they were actually seeing?
(What is the most likely thing that people were actually seeing when they saw “canals?”)
The canals were an optical illusion created by poor telescopes that made surface features appear to line up in a straight line. The green/red season observations have been chalked up to the difference between clear Martian skies and reddish dust storms occasionally coinciding with the growing and shrinking of the icecaps.
Mars really has two seasons: red and redder.
Which could be Earth-ified by science popularizers, ordinary news media, and common everyman knowledge into seasons browner and greener.
So the basic answer is dust storms. The seasonal association? Just coincidence, or are dust storms more common in some seasons? Anyway, all kinda sad, because seasons (and canals!) would have been cool!