William McKinnon

William B. McKinnon

​Professor of Earth and Planetary Sciences
PhD, California Institute of Technology
research interests:
  • Outer Solar System Geology
  • Geophysics
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    • Washington University
    • CB 1169
    • One Brookings Drive
    • St. Louis, MO 63130-4899
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    ​Professor McKinnon’s research focuses on the icy satellites of the outer solar system and the physics of impact cratering. He and his students and colleagues concentrate on the origin, structure, evolution, and bombardment history of outer planet satellites and Pluto.

    The last twenty odd years of planetary exploration can be characterized by both the unveiling of the outer solar system - initially by the Voyager missions, but now by the Galileo mission to Jupiter as well as ground- and space-based telescopes - and the growing realization of the importance of impacts in solar system evolution. Dedicated to exploring this frontier, Professor McKinnon believes 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 the cratering mechanics in ice and other targets.

    Professor McKinnon is interested in extending our geological and geophysical perspectives to worlds where water ice is a major, if not dominant, constituent. These worlds include the satellite systems of Jupiter, Saturn, and Uranus, which resemble miniature solar systems in part. Galileo image and other data received over the last several years has transformed our view of the Jupiter's major moons - Io, Europa, Ganymede and Callisto - in particular. Ganymede and Callisto are especially interesting, as they are very similar in bulk properties, yet startlingly different in appearance. Work has focused on their internal structures, convection in their icy mantles, viscous relaxation of impact crater topography, and on Ganymede, topographic and morphologic evidence for water-rich volcanism. Other work has concerned the links between the extreme volcanism and towering mountains on Io, the solar system's most active solid body.

    Of all of the Galilean satellites, though, Europa, with its complexly tectonically and volcanically deformed icy surface and probable subsurface water ocean, has clearly emerged as the star. Active research involves linking the tectonic patterns seen on the surface to sources of stress, the conditions necessary for subsolidus convection within both the surface ice shell and the interior silicate mantle, and the geophysical and geochemical consequences thereof. The latter are of deep interest because of the theoretical possibility of a subsurface biosphere, hosted in hydrothermal systems on Europa's ocean floor.

    Understanding the ice-rock bodies of the deep outer solar system is especially challenging. Pluto and its moon Charon are important because they are survivors representative of the planetesimals that accreted to form Uranus and Neptune. Triton, Neptune's major satellite, may have been captured from solar orbit, and thus be similar to Pluto. Work in this area concerns the origin and interrelationships of these bodies and the ice-rock bodies of the Kuiper Belt, within which Pluto is found and from which Triton probably escaped. Recent work has redated the volcanic terrains on Triton, taking into account the impacts of smaller Kuiper Belt objects, and finds Triton's terrains to be much younger than previously thought.

    The reconnaissance of the solar system is now complete, except for the significant exception of Pluto and the Kuiper Belt beyond. The cratering records of the solid planets and satellites contain the most direct evidence of the accretion of the solar system, a process that is not absolutely complete as the 1994 Shoemaker-Levy 9 impacts with Jupiter attest. Much remains to be learned, especially as concerns the impact process itself and effects on target bodies. The Earth has not been spared from such collisions, and while infrequent today, the effects can be catastrophic. Magellan radar images of Venus and Mars Global Surveyor high-resolution images and laser altimetry open up other vistas. Past work has dealt with atmospheric effects on cratering, and the formation of multiringed craters, on Venus.

    Planetary science suffers from no shortage of data to study and interpret, and the next twenty years should see new infusions from the Hubble Space Telescope and the Cassini mission to Saturn, new missions to Mars and Mercury, and if all goes well, missions to Europa and Pluto as well. Professor McKinnon is actively involved in NASA's effort to plan and launch these new missions to the frontiers of the solar system, particularly a return to orbit Europa and the first reconnaissance mission to Pluto and the Kuiper Belt. Planetary science remains vital and exciting, a field of central importance for our species and - as we are, like it or not, in charge around here - for all other species as well.