William McKINNON

Professor
Department of Earth, Environmental, and
Planetary Sciences, and McDonnell Center
for the Space Sciences
Washington University in St. Louis

Professor William McKinnon teaches in the Department of Earth, Environmental, and Planetary Sciences at Washington University in St. Louis, where he is also a member of the McDonnell Center for the Space Sciences. He received his S.B. in Earth and Planetary Sciences from MIT in 1976 and his Ph.D in Planetary Science and Geophysics from Caltech in 1981. Starting with the Voyager mission to the outer planets, he and his students and collaborators have devoted themselves to the study of icy satellites and Kuiper belt worlds such as Pluto. This includes understanding the relative importance of large impacts, orbital dynamics, and internal processes for tectonics and other surface modifications, the origin and evolution of impactor populations, and cratering mechanics in ice and other targets. He is particularly interested in extending geological and geophysical perspectives to worlds where ices are the major constituents. He is an Emeritus Co-investigator on NASA’s New Horizons mission to Pluto and the Kuiper belt, and currently a Co-investigator for ESA’s JUICE (Jupiter Icy Moons Explorer) and for multiple instrument teams on NASA’s Europa Clipper. He is a member of the U.S. National Academy of Sciences, and recently served on the Steering Committee of the U.S. Planetary Science and Astrobiology Decadal Survey. In 2023 he received the Kuiper Prize from the Division for Planetary Sciences of the American Astronomical Society.

Exploration of Kuiper Belt in the 21^st Century, What Have We Learned and What is to Come?

Our understandings of the formation and evolution of the deep outer Solar System have been revolutionized by the New Horizons spacecraft encounters with the Pluto-Charon system in 2015 and especially by the subsequent rendezvous in 2019 with the contact binary Arrokoth. These detailed spacecraft explorations took place against continuing and ever deepening telescopic surveys of trans-Neptunian space, and now exquisite compositional information from JWST. The orbital structure of the Kuiper belt is the primary evidence supporting a more compact Solar System during the protosolar nebula phase, followed by migration and orbital instability of the giant planets, including the possible ejection of a third ice giant. Pluto-Charon is emblematic of the dwarf planets that originally formed beyond Neptune, but that were emplaced in the Kuiper belt (in Pluto’s case, safely in the 3:2 mean-motion resonance with Neptune). Pluto is an active rock-ice world with a complex geological history; ongoing processes include tectonism, cryovolcanism, solid-state convection, glacial flow, atmospheric circulation, surface-atmosphere volatile exchange, aeolian processes, and atmospheric photochemistry, microphysics, and haze formation. Pluto likely inherited a large organic mass fraction during accretion, which may be responsible in part for its surface volatile ices and atmosphere. It is a differentiated world, and likely possesses a subsurface ocean overlying a rocky core, and during its thermal evolution may have gone through a high heat flow phase. Charon, Pluto’s major moon, is no longer active, but like the Earth’s moon displays evidence of extensive early tectonism and cryovolcanism.

Looking further out, into the cold classical region of the Kuiper belt, the two gently merged, lenticular lobes of Arrokoth are dynamically aligned, which is a testament to their formation in a low-velocity, gravity-driven local particle cloud collapse within the protosolar nebula: i.e., strong evidence for formation via the streaming instability (or something similar), as opposed to high-velocity, hierarchical planetesimal accretion. The ultimate merger of such a binary requires a dissipative mechanism, of which gas drag is a leading candidate (Kozai-Lidov cycling being too violent unless the binary is already tight); this may imply a very low bulk density for Arrokoth (for which there is independent evidence) or a rethink of protosolar nebular gas densities in the cold classical region. The giant impact that resulted in Charon was itself relatively low-velocity (though of a different order), but between partially differentiated (at least) precursor bodies (necessary to explain the iciness of Pluto-Charon’s small satellites); this impact likely kick-started a hot beginning for both Pluto and Charon. But nothing compares to the tidal heating that Triton (a former KBO) likely underwent subsequent to its capture by Neptune during or prior to the instability epoch. Strong hydrothermal processing at elevated temperatures may explain the ubiquitous CO2 ice detected on its surface by JWST. Triton remains a prime target for future spacecraft exploration. And there were likely once ~1000-4000 Pluto-mass bodies in the trans-Neptunian planetesimal disk. If so, dozens of them are still out there, in the Kuiper belt's scattered disk and its extended/detached component. Silently they orbit the Sun, waiting.