II. August 13, 1672.

The south polar cap was discovered by Dutch astronomer Christiaan Huygens. He was also credited to have drawn mars using an advanced telescope. He also wrote a document detailing the essential conditions necessary for a planet to support life.

Some key Mars events in the 1600s: https://mars.nasa.gov/allaboutmars/mystique/history/1600/
Here’s his page: https://en.wikipedia.org/wiki/Christiaan_Huygens