Experimental and theoretical porosity determination for ices of astrophysical interest: CH4, C2H4, C2H6, CH3OH, N2, NH3, CO, and CO2

Abstract

In a previous work of this group, the effective medium approximations (EMAs), mainly Lorentz-Lorenz, Maxwell-Garnett, and Bruggeman models, were experimentally tested for the case of CO₂ for determination of porosity. The present work extends the porosity calculation for a set of astrophysical relevant molecules from their experimental density and refractive index. Important discrepancies between theoretical and experimental values are not detected except in case of CO₂, already reported. In addition, the porosity acquired by this set of molecules becomes quite clear.

The theoretical values of porosity predicted by EMAs models show similar trends for all the molecules subject of our study. Porosity vs. temperature behavior allows us to group molecules in two sets, and each molecule displays characteristics of its own possibly caused by phase changes.

A given set of molecules no longer appears to exhibit density variation in the temperature range from the accretion to desorption one, and as a consequence porosity remains unchanged, which does not mean an absolute lack of porosity. All the molecules show a temperature range in which the porosity is constant (plateau). In this work, a relation between the desorption energy and the temperature at which the porosity vs temperature shows a plateau is obtained. This could be explained as a possible consequence of desorption and diffusion energy dependence. On this basis, it can be speculated the existence of a limit temperature for this set of molecules, upon which, the porosity stops evolving and becomes constant.