Rebels of the Red Planet by Charles L. Fontenay

Brute Hennessey plodded westward through the Xanthe Desert, naked, wearing no marsuit, his head bare to the thin, oxygen-poor Martian air. The two small moons shone in the star-spangled sky above the lone figure, casting fantastic shadows on the sands.

But this was not the stupid, shambling Brute Hennessey of a few months past. He walked surely and proudly, and the light of intelligence shone in his eyes.