An adventure for MARS: A Savage Setting of Planetary Romance.
From the pdf's introduction:
Rebels of Mars concerns a Confederate unit that is plucked from the American Civil War and transported to the Green Wastes of Mars. Stranded on the red planet, the Rebels manage to rally the Red Nomad tribes and fend off a group of slavers, freeing a key slave in the process.
Meanwhile, a noble of the Red Martians has ventured out into the wastes in an attempt to recover one of his escaped slaves (now with the Rebels), but ends up lost himself. The noble’s wife hires the player-characters to find her husband, throwing them on a collision course with the Confederates.
In the midst of this, the Grey Martian responsible for transporting the Earthmen in the first place has decided to rectify his mistake. He is hunts the Green Wastes in a powerful Tripod, looking to cleanse them from the surface of Mars with his heat ray.