Interstellar physics on the rocks; Monte-Carlo simulations of ice mantles

Formation and destruction of ice mantles play an important role in many astrophysical environments including regions where new stars and planets are born and on icy bodies in our own solar system such as comets. Although we have a general picture about how the mantles form and desorb again, little is known about the basic physical processes. Even the formation of the most abundant ice - H2O - is not understood. Several physics and chemistry groups, both experimentalists and theoreticians, started focusing on the underlying physics. These studies mainly provide fundamental information about separate processes occurring in the ice mantle and do not directly show how processes interact under the extreme conditions typical for the interstellar medium. The goal of the present application is to overcome this problem. We propose to use the new concept of the continuous-time, random-walk Monte Carlo method applied to interstellar grains. This concept has been co-developed by the applicant and is unique in its potential to simulate interstellar ices under interstellar conditions for long periods of time. We will start with the situation in which only H and O are involved. Subsequently different species are systematically added - complicating the chemistry in the grains - under more and more complex conditions including physical processes as photodesorption and photodissociation. The interpretation of the results is a prerequisite to understanding those processes that are the starting point in star and planet formation. The Herschel Space Observatory, to be launched in early 2008, provides important new information on gaseous water in interstellar space and will measure quantitatively the water abundance as a function of temperature, UV field and other parameters; understanding the processes by which water ice forms and desorbs will be essential for the interpretation of these data.