Planetary regolith in vacuum: Current issues with thermal conductivity and packing

Kurzfassung

Airless bodies in the solar system are usually covered by regolith, i.e. a dry granular medium ("packed bed") consisting of a polydisperse mixture of angular silicate particles. The density here is in the range of 200- 2000 kg/m³ covering porosities from 30-90%. Remote sensing in the mid-infrared allows to derive the thermal conductivity of the surface layer (~ 1 thermal skindepth); if the porosity, polydispersity, typical shape can be "guesstimated", inferences about the (median) grain size can be made. We are currently refining a continuum model for the effective thermal conductivity k, which rests on an analytic description of the solid conduction through the random packing of particles as well as the radiative heat transfer through the pores. k is a function of temperature, (median) grain size, porosity, grain material, cohesion (surface energy or Hamaker constant) and (overburden) pressure. Major issues are encountered for the adhesive contact radii if the particles are not spheres and smooth (=JKR) but angular and rough; with the value of the Hamaker constant or surface energy itself; for the coupling of solid and radiative conduction leading to deviations of the usual T³ term; porosity dependence of the radiative transfer; description of contact network including average coordination numbers and the effect of distributions of "averages" to the effective k; description of the average forces on each particle resulting from overburden pressure; enhancement of radiative heat tranfer if particle distances are << thermal wavelength; prediction of the gravitational "loose" settling porosity for the general case and many more. The aim is to understand the physics really well: an accurate theory (even DEM) is needed, proven by accurate experiments over a sufficient range of parameters. This might probably be re-cast it into a simpler, but sufficiently accurate analytical description. The author is also interested in the mechanical properties of packed beds under quasistatic compressive loads and for slow impacts (1-100 cm/s).