Mars - Red Planet, Penal Colony.
Mars is Hell off Earth! Although terraforming has been begun here it still has several hundred years to go before showing anything like a noticeable result. In the meantime infrastructure has been established, and this costs money. Nothing reduces costs like slave labour.
Mars is formerly a frontier world, but everyone knows it is simply a penal colony where the worst of the worst are sent. Less well known is that the worst of those are sent to Diemos Rehabilitation Centre and are never seen again.
A volunteer settler to Mars is in for a hard time, only slightly moderated by access to free labour in the form of transported variants still serving time. Of course such labour comes with a risk, many settlers have been slaughtered in their beds by their workers if treated too harshly.
Mars is cold, windy, dusty, dry. It has a weak atmosphere that at least keeps the affects of vacuum at bay, but it has little oxygen so you must constantly be sure to carry sufficient with you at all times. Gravity on Mars is 38% that of earth (lets say 1/3rd for simplicity).
What Mars does have is plenty of raw materials not that common on earth or the moon. Mars is rich in Crystals, Rare and Super Rare Earths and Silicates.
The other factor about Mars is that it is NOT Earth, or close to Earth. Because of this nearly all the things that will never get approved on Earth end up on Mars. Mars is literally one big experimental laboratory where the worst, and the best, of humanities excesses are realised.
The canals of Mars are real, and they are hiding buried under the ground where they were thought to have been all along. The canals are giant underground aqueducts that transfer polar melt water to the great Martian cities, which are underground also. Humanity has not yet discovered the Martians, although they have begun discovering the ruins they left behind on the surface.
The Martians themselves are not the ones with secret bases on the Moon. The Martians of Mars are not in the slightest bit interested in the outside world beyond their underground cities. They dont even know that humanity exists on the 3rd planet, although they may have noticed a few of them wandering around on Mars recently.
If you didnt use the Hollow World scenario for the moon, then you can use it here on Mars. Even if you did then you can use a similar setup. The Mars beneath Mars is more like John Carters Mars, which leads us into the RPG "MARS" from Adamant Entertainment, which has a Savage Worlds edition. The world beneath Mars is savage and brutal, with hints of past wonders. Better yet it has a real atmosphere.