Long story short, those faces had been carved in big asteroids in the asteroid field, then brought to planet's orbit. I'm writing a story to this one too, so stay tuned, you will get your answers soon.
This piece belongs to The Empire universe I'm developing. Probably it's the first piece in the series The Monuments of the Empire.
Photoshop CS4. File was that big that it had to be saved as .psb file (large document format, files above 2GB) instead of normal .psd file (not more than 2GB).
This piece is also in the new Attrius artpack .