Old Mars

Mars has lost most of its air and water because it is old. Martians direct water from the poles along canals and maintain their atmosphere with oxygen-generating machinery in large buildings called "atmosphere plants," as far as I remember. My source is A Princess Of Mars by Edgar Rice Burroughs. 

But surely Mars should be no older than Earth? They were formed at the same time together with the rest of the Solar System.

Mars has not developed a thick atmosphere yet because it is small, according to Yasmin in Poul Anderson's "A Tragedy of Errors." See "Yasmin's Moment Of Realization," here.

I like Yasmin's additional point that any star is nearly all hydrogen and helium so that:

"'Variations in other elements...'" (p. 532)

- cannot affect it much. This helps her to deduce that Nike and its sun are old, not young, which in turn has immediate and important practical consequences.

Anderson's characters are problem-solvers, Yasmin and Roan Tom par excellence.