Tim Van Hoolst

The 2019 Runcorn-Florensky Medal is awarded to Tim Van Hoolst for seminal contributions to the geodesy and geophysics of the terrestrial planets and satellites and for leadership in planetary geodesy.

Tim van Hoolst is a Senior Research Scientist at the Royal Observatory Belgium, a project leader in planetary science there, and a professor of astronomy at the Catholic University of Leuven, where he teaches planetary science and asteroseismology. His field of research is the rotation of planets and satellites, their figure and gravity fields, and how these can be used to explore the planet’s interior structure. He has participated as a co-investigator in missions such as BepiColombo to Mercury, JUICE (JUpiter ICy moons Explorer) to the Jovian satellites, and ExoMars. He is a member of ESA’s Solar System and Exploration Working Group. He has published over 100 peer reviewed papers with about 3000 citations. His recently published book, Linear Isentropic Oscillations of Stars – Theoretical
Foundations, with Paul Smeyers on the foundations of asteroseismology, has gained worldwide recognition. He has been appointed chair of the JUICE working group on Internal Structure, Subsurface and Geophysics of Giant Icy Moons.

One of van Hoolst’s most important contributions is to our understanding of the rotational dynamics of icy satellites whose sub-surface oceans decouple their interior from the solid but deformable ice shells. This permits the subsurface oceans to be detected by observing the rotational state of the satellite. He has spearheaded the development of the methodology required. In applying the theory, he and his team showed that Titan’s obliquity could be explained by a subsurface ocean and that the observed librations and gravity field of Enceladus allow its ice shell and subsurface ocean to be characterised. The great advantage of this technique is that it permits remote observations (and potentially even Earth-based ones) to probe satellite interiors, and it is largely thanks to van Hoolst that this is now a reality.

Many of the same techniques can be applied to Mercury, where rotation state and gravity measurements are also available. Over the last decade, van Hoolst and his team have worked on various aspects of this problem, showing in particular that Mercury’s librations can in principle be used to determine the size of its inner core. This is a critical parameter that is currently completely unknown; what their work shows is that it is potentially measurable, for instance with the forthcoming ESA Bepi-Colombo mission.

Van Hoolst and his team have also worked on determining the interior structure of Mars. In particular, they have shown that a Martian inner core could potentially be detectable, if it was of a size which happened to produce a resonant response in either polar motion or nutation.

Van Hoolst received several prestigious prizes, among them the Descartes Prize of the European Union and the Vanderlinden Prize of the Royal Academy of Sciences of Belgium.

Considering his pioneering work during the past decades, his remarkable scientific achievements, his creativity, his outstanding leadership and services to the community, his enormous influence in mentoring many junior colleagues, Tim van Hoolst is highly deserving of the 2019 EGU Runcorn-Florensky Medal.