Hello, SFIA audio listeners. In this month's Nebula exclusive, big alien theory,

we're asking the reason alien civilizations might be rare is because most aliens are huge.

To hear it and every episode early and ad-free, plus hours of bonus content,

check out go.nebola.tv slash Isaac Arthur and use my code, Isaac Arthur.

One day we will journey to the red planet and perhaps settle there, but what great civilizations

and cities will emerge on the red sands?

The earliest description of a city of Mars I know of are the cities of Helium at the end

of Edgar Rice Burroughs classic novel, A Princess of Mars, in his Barsoom series featuring

John Carter. The name itself evokes a high, light, and lofty image, and what little description

we get of the city seems

to confirm that. The nation of helium is home to roughly 10 million Martians, it has two

primary cities, greater and lesser helium, 75 miles apart, and dominated by an enormous

mile-high tower at their center. Travel between them is done by

airship or monorail. We also see the city of Zodanga, which in the books is an enormous

city with an equally enormous wall, 75 feet high and 50 thick, though the 2012 film

John Carter instead shows us a mobile city, that's one-part fortress and one-part

factory and refinery, and we discussed it in similar options in last year's episode on

mobile cities.

The Barsoom series began in 1912 and Burroughs kept riding it all the way up to the beginning

of World War II in his late 60s, and was living in Hawaii at the time

of Pearl Harbor and became a war correspondent. Needless to say, the science on Mars was

pretty limited at the time, so much of what he describes won't apply today as we dip into

...