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The more I think on it, it benefits us not to call the Emperor the Omnissiah because it very much is in our grasp to just become the Omnissiah ourselves (and that's why we made that vote earlier).
Terraformation tech is rare, but not unheard of. Cawl was able to transform the lifeless rock of Sotha back into a habitable green planet in a few decades. With the resources of Lucius, and a couple of ocean worlds to siphon from, we likely could bring (some) life back to the Red Planet. Just enough to make it rain in some places consistently, like a few valleys, to prove we're the real deal.
Mars is, after all as the ancient Magos Gol-Du-Lock said, "Middle Ursine" along with Terra. Not too close to Sol as to burn, not too far away as to freeze. Just right. And that's canon lore.