wow, 1,000,000 polys in 4.838 cm (at least as I load it into mine). The UV’s are kind of goofed too I applied a UV test grid and it’s not all nice and regular like I would expect (largely but not all). Not all the image is being used with that original UVW tag (perhaps this is just a portion of the original landscape?)
I upscaled some (because it makes the next steps easier) and removed the UVW tag then made the material Flat Projection – offset and sized it to match the landscape then generated a new UVW tag from that so the texture fits better.
It gives the PlanetUVTest.c4d… the original (with the same UVgrid image applied) gives the marsplanetv015 included.
files here https://dl.dropboxusercontent.com/u/52769499/PlanetUVTest.zip
the UV test image is only 1024×1024 and those pixels are spread across the surface but if you are zooming in at all the individual pixels are visible and if you can see the pixels you can’t put any more detail into the area it’s covering on the object. Generally you want a 1 to 1 or better pixels ratio between the pixels in a texture and the pixels on your screen (monitor) (that’s rule from 72/96 dpi monitors that’s not so true in the Retina display era but you should not be able to see individual pixels in your texture when the camera is at it’s closest to the object for a scene).
How close does the camera get to the surface? If I use the supplied soil image in the PlanetUVTest file it doesn’t take much zooming to see individual pixels (particularly with image Sampling set to None). Either image needs to be higher resolution (10000×10000 rather 1010×1020) or it has to be tiled many times over the same area (which always shows the repetitions unless it’s a really well made texture for that purpose) or the image needs to be used (scaled down) into a tiny area.
What is the soil image supposed to cover? The whole object or just a tiny portion? I don’t know whether you mean “adding some rocks and similar things” to be that texture over everything or only on small areas adding individual rocks…