Where was it said the Greater Ark was destroyed? Then perhaps this is another Forerunner installation. Geographically, they seem to keep similar themes. Unlikely that it is a shield world, since there wouldn't be planets visible in the "sky." We didn't see enough of its skybox to get a good idea of if this is even a spherical object that the Chief is on. I highly doubt it's a Halo ring, as the ring is incredibly hard to miss.