Solid state greenhouse effect in planetary ice layers

The principal aim of this project is to reach a better understanding of the so-called "solid-state greenhouse effect" in planetary ices. In contrast to rocky surfaces, where the solar radiation is absorbed within a few micrometers distance, icy surfaces are partially transparent to the incoming solar flux, which is absorbed either gradually over a larger distance or suddenly by a dark subsurface layer. However - similar to the classical greenhouse effect in an atmosphere - the re-radiated infrared flux cannot escape directly, but rather deposits its energy locally below the surface. The importance of this effect for the behaviour of planetary ices was first recognised by Brown and Matson (1987). Their theoretical models showed that it may be relevant for the energy balance of icy surfaces on the satellites of Jupiter and other bodies of the outer solar system. In particular it could be a plausible driver for active phenomena like cryo-volcanic eruptions on icy bodies, including cometary nuclei (Kömle et al., 1990). In recent years a wealth of new data concerning Jupiter`s icy satellites were collected by the Galileo spacecraft, which provides us today a much broader data base. Moreover, recent data from the Mars polar regions mapped by Mars Global Surveyor revealed the existence of large areas probably covered by transparent slabs of CO2-ice. Again, in these regions the solid state greenhouse effect should play an important role for the energy balance and may cause violent evaporation and melting activity. In this project we will investigate some consequences of the sub-surface absorption of solar radiation in various ices and ice/dust conglomerates known to exist on solar system bodies, and compare them with analogous phenomena in terrestrial glaciers. Useful analogues are glaciers with incorporated dark ash layers, as they exist in the volcanic activity regions in Iceland. Three mutually supplementing methods will be applied: (i) Laboratory experiments, using partly existing equipment at the Space Research Institute, Graz, and partly additional instrumentation funded from the project; (ii) field research, mainly performed in co-operation with glaciologists from the University of Reykjavik; and (iii) theoretical work to ensure a proper understanding of the collected experimental data and to allow extrapolation of the results to real planetary environments.

The main topic of the project was an experimental investigation of the so-called "solid state greenhouse effect" in ice and its role for the energy balance in planetary surface layers, in particular on bodies with a low surface pressure, like Mars, most of the icy satellites in the outer part of the solar system, and comets. While several theoretical papers on this effect can be found in previously published literature, very little experimental work was done before to confirm the theory and to check the conditions under which it is valid in practice. In the frame of the present project it was confirmed for the first time by laboratory experiments that the characteristic subsurface temperature maximum predicted by numerical models can indeed be measured in laboratory experiments under space conditions. This lead to a significant improvement of our understanding of the physical processes governing the energy balance in planetary icy bodies. First, experiments with transparent glass beads (with and without absorbing interior layers) under vacuum were performed, which clearly showed that with our experimental setup the effect was measurable. The measurements in ice turned out to be technically more complicated than originally expected, but after several tests which gave inconclusive results, finally the effect could be clearly detected in the prepared compact water ice samples irradiated under vacuum conditions by a solar simulator. However, as compared to the glass bead experiments, the measured subsurface temperature maxima in the semi-transparent ice samples were smaller and less distinct. The measured temperature profiles could be reproduced with a numerical model using a reasonable set of input parameters, However, from the results we conclude that under natural conditions (more light scattering and inclusion of dust particles in the ice) the effect probably is less important than one might have expected from the theoretical work published previousl A second aspect of the project which developed rather fruitful due to the possibility of co-operations with several international partners was the development of innovative tools for the exploration of icy subsurface layers on planetary bodies, with emphasis on thermal properties. In particular the space experiment MUPUS on board the ESA comet mission ROSETTA, which is presently under way to a comet, profited from the tests performed with the support of this project. Other experiments explored an alternative way to penetrate icy subsurface layers under space conditions, namely by heating a probe. The work performed will be useful in the future both for terrestrial applications and for tentative participations in planetary lander missions.