Sands of Time
These hourglasses contain contain what scientists call regolith simulants, materials made to reproduce the appearance and behavior of real planetary soils (regolith) for use in laboratory and engineering experiments. The soils differ due to the composition of each body and the way that it was formed, as well as the types of which weathering processes that drive the evolution of its landscapes over time. As different worlds evolve in different ways and at different speeds, the resulting morphologies of their boulders and landforms tell their unique history, and the nature of the dust left to settle reflects how, in a way, the passage of time seems to vary across worlds.
The simulants in the hourglasses are for the Earth (white sand), the Moon (JSC-1A), Mars (MMS), and carbonaceous asteroids (Tagish Lake). Note how the martian soil does fall through the opening, but the flow is less smooth since there is a larger spread of grain sizes in the soil than in the white sand. The lunar regolith does not fall through the hole because the particles are too fine, causing electrostatic forces to help them stick together. This is the result of the fact that the Moon has been bombarded with impacts for so long it has reduced its entire surface to these fines. The asteroid simulant is similar, except that it has both very fine particles as well as larger chunks. This was chosen to reflect the nature of small “rubble pile” asteroids, which are loose conglomerates of dust and rock that accumulated in space and are only weakly bound by gravity.