OK, OK, I was only half serious yesterday, and 2100 may be too soon, but it will happen.
A spirit of adventure. The challenge of the unknown. Scientific curiosity. A common project to unite humankind. A high-tech alternative for scientists and engineers who have hitherto made their living producing the instruments of war. Entertainment. The inspiration of young people. A hedge on our survival in case the human race becomes extinct on Earth through self- annihilation or cosmic catastrophe.
It's inevitable that sooner or later humans will colonize Mars.
Early in its history, Mars was warmer, wetter and had a denser atmosphere than it does today; except for the composition of the atmosphere, the planet was altogether more congenial for life. It can be made that way again. And the atmosphere can be terraformed too.
If my whimsical scheme for financing Martian colonization by selling real estate is implemented, I have my Martian acres all picked out. Near latitude 30 degrees south, longitude 280 degrees, on the rim of a vast circular depression called Hellas Planitia. This is one of the few lowlands in the mid-latitudes of the red planet's southern hemisphere. I figure this will become one of the first artificial seas on Mars, when the climate has been warmed sufficiently that water can exist as a liquid. If I'm calling it correctly, my great-great-great-great-grandchildren will be owners of enviable beachfront property.
Latitudes on Mars are of course defined, as on Earth, by the poles and equator (the axis of rotation). But what determines the prime meridian -- the Martian Greenwich? I know, Tom. Do you?