Estimation of the porosity in ice of astrophysical interest by means of ultrasonic techniques. IceUS

Ice porosity is intimately linked to the history of formation and evolution of the ice. A high porosity of water ice allows mixing with other molecules, permitting chemical reactions in the ice bulk and on its surface. Porosity also influences the spectroscopic infrared observations of ices in space. One of the major achievements of the ESA-Rosetta mission was the precise measurement of the porosity in the nucleus of comet 67P/Churyumov-Gerasimenko.
In the laboratory, the measurement of ice porosity is usually performed indirectly by means of spectroscopy, obtaining non-quantitative information on the porosity which is altered by the roughness of the ice surface. In astrophysics, in addition to water, interstellar and Solar System ices are composed of species such as CO, CO2, CH3OH, CH4, NH3, etc.
We propose the study and development of a methodology to estimate the porosity in astrophysical ice analogs. The system is based on the emission and reception of an ultrasound pulse through the ice layer. For that purpose, it would be necessary to determine which ultrasonic parameters are sensitive to porosity establishing a model that relates the ultrasonic parameters with the porosity. It will be necessary to study how to propagate the ultrasonic waves at temperatures below 10 K in a bilayer structure formed by a substrate in which a 10 μm-thick ice layer is deposited. An additional risk for ultrasonic applications is that the process of ice formation occurs in a ultra-high-vacuum chamber that allows to characterize and monitor the ice during its formation and subsequent irradiation or heating. Several experimental tests will be performed to validate the system with water ice. Subsequently, the study of porosity will be extended to ices whose porosity has been little studied, in particular solid CO. As a follow-up we plan to measure the porosity of ice grown under reduced gravity in parabolic flights.