Why not that railgun they are building/have on the Olympus Mons? That would be a good start since it can probably fire from planet to planet, the damn tanks can engage space targets. The full might of the empire comes crashing in and these guys are known for destroying moons and planets. They set Phobos out of orbit with the Psion Flayers. the skyburners assaulted a super Dreadnaught with a troop transport and were more successful than the Vex were when they attacked. The Cabal are beating out the Vex on Mars with worn out troops and equipment. These guys would make for a hell of a raid. (Railgun info is from some idle chatter from the bounty master. Tanks firing into space are from Grimore. Phobos out of orbit is from Grimore. Vex attacking the Dreadnaught is from the Book of Sorrows grimore.)