The contributions of co-authors J.D. Goguen and R.A. West were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We are indebted to Rocio Marquez from the Scientific Instrumentation center of the University of Granada for providing part of the SEM images. This work has been supported by the Plan Nacional de Astronomia y Astrofisica contracts AYA2015-67152-R and AYA2015-71975-REDT.