William B. McKinnon
Professor
Ph.D., California Institute of Technology, 1981
Planetary Geophysics and Dynamics
Professor McKinnon's research focuses on the icy satellites
of the outer solar system and the physics of impact cratering. 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. Professor McKinnon and his students and colleagues
are dedicated to exploring this frontier, concentrating on the origin,
structure, evolution, and bombardment history of outer planet satellites
and 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 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.
Barr, A.C., and W.B. McKinnon (2007). Convection
in ice I shells and mantles with self-consistent grain size. J. Geophys. Res. 112,
E02012, doi:10.1029/2006JE002781.
McKinnon, W.B. (2006). On convective instability in the ice I shells
of outer solar system bodies, with detailed application to Callisto. Icarus 183,
435450.
McKinnon, W.B. (2006). Origin and early evolution of Io. In Io After Galileo,
R.M.C. Lopes and J.R. Spencer, eds. (Springer Praxis Books), 6188, 2006.
Dombard, A.J., and W.B. McKinnon (2006).
Elastoviscoplastic relaxation of impact crater topography with application
to Ganymede and Callisto. J. Geophys. Res. 111,
E01001, doi:10.1029/2005JE002445. Dombard,
A.J., and W.B. McKinnon (2006). Folding of Europa’s icy lithosphere:
An analysis of viscous-plastic buckling and subsequent topographic
relaxation. J. Struct.
Geol. 28, 22592269.
Durham, W.B., W.B. McKinnon,
and L.A. Stern (2005).
Cold compaction of water ice. Geophys. Res. Lett. 32, L18202, doi:10.1029/2005GL023484.
Bagenal, F., T.E. Dowling, and W.B.
McKinnon, eds. (2004). Jupiter - The Planet, Satellites and
Magnetosphere,
Cambridge Univ. Press.
McKinnon, W.B., and M.E. Zolensky
(2003). Sulfate content of Europa’s ocean and shell: Evolutionary
considerations and geological and astrobiological implications. Astrobiology 3,
879-897.
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314-935-5604 
mckinnon@wustl.edu
314-935-7361 
