The stratification of planetary regolith: Investigating the relation between turnover pressure and grain size

In the gravitational surface field of airless Solar System bodies, regolith should show densification when moving from the surface to greater depths. We aim to validate and constrain basic parameters of an existing regolith stratification model by using literature data on the compression of granular media. We analysed granular-matter compression curves and determined the turnover pressure from the loose to the compact state and the logarithmic transition width between the two states. In addition, we analysed laboratory measurements of the average packing fraction of granular samples as a function of grain size with respect to the turnover pressure. We were able to derive the turnover pressure for grain radii between ∼ 0.5 and ∼ 100 . The relation between turnover pressure and particle size follows a power law with slope -2, as suggested by a dust aggregate compression model. ̆pmu m ̆pmu m We present a ready-to-use regolith stratification model and the most relevant free parameters we have determined. The model is valid over a wide range of grain sizes from sub-micrometres to millimetres and should cover most of the regolith occurring in the Solar System.