Keller, H. U. Department Planets and Comets, Max Planck Institute for Solar System Research, Max Planck Society; MPI for Aeronomy, Max Planck Institute for Solar System Research, Max Planck Society;
Ball, A. J., Keller, H. U., & Schulz, R. (1999). Critical questions and future measurements - collated views of the workshop participants. Space Science Reviews, 90, 363-369.