SUGGESTION: Rain and Slipery!
Add rain in different strengths (water or other liquids) to the planets. :-)
During and shortly after the rains when having been heavy, the slippery effect could be increased, means, that inclinations of hills attract a stronger downward-movement of unanchored objects than otherwise. Also affecting a player standing or walking on a mountain. easier now to slide down unintentionally (or intentionally).