Resolves to the number of people (in millions) that will watch a "live" stream or "live" broadcast of the first person walking on Mars.
Live is in quotes, because of the obvious ~30-minute delay of the video getting to Earth.
Similar, but for next moon landing: https://manifold.markets/chris/how-many-millions-of-people-will-wa
Assuming it happens, I think it depends on when they time the stream more than anything else. See this question on Reddit for instance about when most people on the planet are awake: https://www.reddit.com/r/geography/comments/189lvyd/at_what_time_are_there_the_most_people/
Then it’s the product of that with other factors such as number of humans with streaming devices, publicity, country / organization making the landing, etc.
Not sure if a upper bound of 1.6bn is high enough.