Question: I'm trying to simulate this effect in cycles with materials

[![The opposition effect seen on earth](https://i.stack.imgur.com/SGw9V.jpg)](https://i.stack.imgur.com/SGw9V.jpg)

This phenomena is called "Opposition Surge/effect", and is a result of the phase angle between a light source (sun) and the observer approaches zero. The effect is mostly seen on rough surfaces, as the shadows of said surface disappear when the observer is directly between the surface and the light source

This is something I've been trying to simulate in cycles with materials. Now this effect should in theory be achieved simply by creating rough physical geometry, however, it is not very practical for large scale scenes as the physical geometry takes up a lot of memory. Can anyone think of a way to simulate this effect within a material?

Answer: 
You can create dense geometry only where needed with [Adaptive Subdivision](https://docs.blender.org/manual/en/2.80/render/cycles/object_settings/adaptive_subdiv.html) and displacement in Blender. It's still an experimental feature, but seems to work quite well.
Or you can create gravel with a particle system or use linked duplicates.