Prof. Dr. Ulrich Christensen, director emeritus at the Max Planck Institute for Solar System Research (MPS) in Germany, has been elected an international member of the U.S. National Academy of Sciences (NAS). The association thus recognizes Christensen's significant contributions to the understanding of dynamical processes inside the Earth and other planets. Membership in the NAS is considered one of the highest honors for scientists. From 2002 until 2020, Christensen led the MPS department “Planets and Comets” as director. Since his retirement, he has continued his research as head of the emeritus group "The Interior of Planets”; he is involved in the current space missions InSight and BepiColombo.
The interior of the Earth is a dynamic place: under the influence of the high pressure at depths of up to 2900 meters, the rock of the Earth's mantle is deformable and over the course of millions of years turns over in gigantic flows; in the Earth’s underlying, liquid core, swirling movements of the liquid iron-nickel mixture generate the magnetic field of our planet in a dynamo process. Both processes are the subject of Christensen's research. In order to understand the internal dynamics of the Earth, the geophysicist relies primarily on numerical simulations.
At the beginning of his scientific career, Christensen's research interest focused on the Earth's mantle. This most massive layer of the Earth's inner structure is divided into several sub-layers that differ in their chemical composition and mineralogical properties. Christensen addressed the question of how the boundaries between the individual sublayers affect the dynamics of the flowing rock. In later years, the MPS researcher turned his attention increasingly to processes in the Earth's core. For example, he was able to identify the flow of energy in the Earth's liquid core as the key physical parameter that determines the strength of the magnetic field.
Considerations of this kind enabled Christensen to apply the dynamo theory of the Earth to other celestial bodies. In addition to Earth, for example, the gas giants Jupiter and Saturn, Mercury, and several moons in the solar system also display magnetic fields. The aim of Christensen’s work is to relate the strength of the respective magnetic field to the processes inside the body.
Christensen's participation in the InSight Mars mission of the U.S. space agency NASA addresses a similar question. Measurements of the mission's own seismometer, which was developed and built with the help of the MPS, are intended to provide information about the internal structure and dynamics of Mars - and thus help to explain why our neighboring planet has developed so completely differently from our own. A magnetic field, for example, existed there at most a long time ago.
In recent years, Christensen's research interest has also shifted even further away from Earth - to the mutual features of the magnetic properties of planets and stars, which, unlike the Sun, rotate particularly quickly.
Prof. Dr. Ulrich Christensen studied physics at the Technical University of Braunschweig, where he also received his doctorate. His habilitation at the University of Mainz was followed by research stays in the USA and the Netherlands, among others. In 1992, Christensen accepted an appointment at the Institute of Geophysics at the University of Göttingen; in 2002, he became director at the Max Planck Institute for Aeronomy, which today bears the name Max Planck Institute for Solar System Research. In the 18 years under his leadership, the MPS department "Planets and Comets" contributed to numerous international space missions, which visited among other planets Mercury, Venus, Mars, and Saturn, as well as the comet Churyumov-Gerasimenko, the asteroid Vesta, and the dwarf planet Ceres.
Christensen's many awards and honors include the Gottfried-Wilhelm Leibniz Prize of the German Research Foundation, the August Love Medal of the European Geosciences Union, and the Inge Lehmann Medal of the American Geophysical Union. The NAS membership, which he has now been awarded, recognizes Christensen's outstanding research achievements; it is considered one of the highest honors for scientists worldwide. Of the approximately 3000 members of the NSA, only about one-sixth are international members.