2020 Nier Prize to Thomas Kruijer

It is my honor and great pleasure to present Thomas Kruijer as the recipient of the 2020 Nier Prize of the Meteoritical Society. The Nier Prize recognizes outstanding research in meteoritics by young scientists. Thomas receives this award for his work on the short-lived hafnium–tungsten isotope system, which he used to make fundamental contributions to understanding the early solar system. I am giving this citation because Thomas was my PhD student and postdoc, first at ETH Zürich and then at the University of Münster. Thomas received his PhD with honors in 2013, and then was a postdoc in Münster until 2017. Following this, he was a research staff member at the Lawrence Livermore National Laboratory, and since 2020, he is full professor at the Museum for Natural History in Berlin. Thomas has made fundamental contributions to a range of topics, including the formation of the Moon and the chronology of Mars, but perhaps the most influential is his work on iron meteorites. To this end, Thomas solved one of the major problems in the hafnium–tungsten chronology of iron meteorites, namely how to correct for the effects of secondary neutron capture induced during exposure to cosmic rays. Thomas showed that platinum isotopes are an excellent neutron dosimeter for iron meteorites, and using this approach, he obtained very precise hafnium–tungsten core formation ages for iron meteorites, which, for the first time, made it possible to resolve sub-million-year age differences. He showed that core formation in iron parent bodies occurred between 1 and 3 million years after the start of the solar system and also showed that this implies very early and rapid accretion of the parent bodies within less than 1 million year of solar system formation. Although this by itself is a very important finding, Thomas noticed something else in the age data, namely that the Hf-W ages seem to fall into two distinct groups. To investigate whether this reflects formation in different regions of the solar nebula, he also measured nucleosynthetic Mo isotope anomalies in the same samples. He indeed found that iron meteorites derived from two genetically distinct reservoirs that are characterized by distinct nucleosynthetic isotope signatures and Hf-W ages. Whereas many irons are linked to enstatite and ordinary chondrites (together termed the non-carbonaceous meteorites), some other irons are linked to carbonaceous chondrites. It had been known for quite some time that carbonaceous chondrites are isotopically distinct from most other meteorites, but what Thomas' work showed is that this isotopic difference extends to differentiated meteorites such as the irons, which, importantly, formed much earlier than the chondrites. It is this observation which demonstrates that the noncarbonaceous and carbonaceous meteorite reservoirs were established early and remained spatially separated for several million years. Thomas then argued that the most plausible way to achieve this separation is the formation of Jupiter, and with this assumption attempted, for the first time, to date the growth history of Jupiter. Another important implication of this work is that carbonaceous-type meteorites, including some irons, derive from bodies that originally formed in the outer solar system. I have been very privileged to work with Thomas for quite some years, and to see him develop to an extremely productive and creative young scientist. He already has had a remarkable impact in meteoritics and planetary science after only 7 years of having obtained his PhD. Perhaps most importantly, Thomas' work helped us recognizing how we can use meteorites to reconstruct the dynamical evolution of the early solar system in time and space, which bridges the gap between meteoritics and planetary science as well as astronomy. When Thomas applied for a PhD position in my group back in 2009, his Master's advisor told me that Thomas always expressed a strong desire to work on fundamental problems. I think this desire became quite true. I am very proud to be able to present to you the recipient of the Nier Prize for 2020, Thomas Kruijer. Open Access funding enabled and organized by Projekt DEAL. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.