I'm using a spherical model for simplicity. Although I'm fetching image tiles and suchlike I reproject from mercator directly to spherical for simplicity rather

I'm using a spherical model for simplicity. Although I'm fetching image tiles and suchlike I reproject from mercator directly to spherical for simplicity rather than the slightly ellipsoidal model. And the image tile source and elevation tile source and radius of the sphere is a user settable attribute from the <a-terrain> tag. If you simply set the right radius and image tile source in html - you would have a working mars view... I believe. I can ask Cesium if they have some official sources for this.