The specific heat of regolith material

Kurzfassung

Specific heat cP(T) is one of the pa- rameters which determine a surface’s temperature re- sponse to heating. Remote sensing in the mid-infrared is often used to estimate a parameter termed the ther- mal inertia of the surface material, which is defined as  = k(T )cp (T ) , where T is absolute temperature in K, k is thermal conductivity in W m-1 K-1, ρ is bulk density in kg m-3, and cP is specific heat at constant pressure in units of J kg-1 K-1. Knowledge or an esti- mate of cP(T) is required to extract information on, e.g. thermal conductivity k from the data, which in turn allows for an estimation of important surface properties like grain size [1-4] and porosity [5] . Furthermore knowledge of thermophysical surface properties is es- sential to model the Yarkovsky [6-8] and YORP [8, 9] effects as well as the response of planetary surfaces to impact cratering. Only a handful of meteorite heat capacities have been published, virtually all of them measured at tem- peratures at or above 300 K or at a ~175 K (Consol- magno et al., 2013). The only other extraterrestrial material with known cP over a limited temperature range is lunar samples from the Apollo missions, and many studies have used these values as a “standard” cP(T) curve. Heat capacity, however, strongly depends not only on temperature but also on composition, thus the use of lunar data for, e.g., C- or M-class asteroids or objects containing frozen volatiles may give rise to large systematic errors. Missing cP(T) data for rocks (in general, “regolith material”, any solid material found on the surface of solar system bodies) can be calculat- ed from the contributions of the constituent minerals (and mineraloids, i.e. amorphous substances): linear mixing model, neglecting (except for olivine Fo-Fa) the non-ideal “excess heat capacity”. Except at very low temperatures, the model specific heat is accurate to ~1% in general.