In 1997 George Wetherill received the highest scientific award in the nation — the National Medal of Science. He was elected to the National Academy of Sciences in 1974. He received the 1984 G. K. Gilbert Award of the Geological Society of America, the 1986 G. P. Kuiper Prize of the American Astronomical Society, and the 1991 Harry H. Hess Medal of the American Geophysical Union. In 2003 Wetherill was awarded the Henry Norris Russell Lectureship, the highest honor bestowed by the American Astronomical Society.

The asteroid region between Mars and Jupiter contains only small bodies. It is not known if only small bodies formed there, or if planets as large as Mars were formed and later removed. Knowing this answer will help clarify the evolution of other regions of the solar system. The illustrated “standard” model assumes that few planet- forming small solid bodies were in the region near 2.5 AU (the distance between the Earth and the Sun). Under these circumstances, after only a few million years bodies the size of today’s largest asteroids (yellow circles) are formed, along with intermediate (red) and small (blue) ones. Unlike the observed asteroids, these modeled orbits are fairly circular.

Early in his career, George Wetherill and his Carnegie coworkers used an improved technique for dating Earth rocks using the fact that uranium ores provide two isotopic systems, which permit testing for unwanted geological disturbances the minerals may have experienced when they were deep inside the Earth. Along with coworkers at the department and the Geophysical Laboratory, Wetherill determined the poorly known decay constant of the half-life of very long-lived radioactive rubidium by comparing it with well-dated uranium and thorium ages that came from the same rocks. This knowledge made it possible to use rubidium to date the formation of ordinary rocks, such as granite, instead of having to use only rare uranium ores. Later, Wetherill became interested in the origin of meteorites, and this led him to explore the dynamics of the asteroid belt and the formation of the solar system.

Inspired by the research of Russian scientist Victor Safronov, who showed that groups of tiny planetesimals could grow into large bodies such as the terrestrial planets, Wetherill was one of the first to develop calculations of the orbital evolution and dynamics of planetesimal accretion and growth. These calculations result in the formation of a large group of probabilistic outcomes from a very similar initial condition. Along with the work of others, this result enabled him to make predictions of the size and orbits of the inner planets, as well as how collisions between bodies in the asteroid belt could result in asteroid impacts on Earth, such as the one that led to the extinction of the dinosaurs.

Wetherill’s computations have revealed how important Jupiter may be in protecting the Earth and other inner planets from bombardment. The gravitational field from Jupiter’s enormous mass effectively provides a shield from asteroids and comets. The discoveries of planets orbiting other stars by DTM’s Paul Butler and others are providing further challenges. Wetherill is considering if giant planets may form under different conditions, such as those proposed by DTM’s Alan Boss, rather than those of the standard model. Collaborative work with the former postdoctoral fellows Satoshi Inaba and M. Ikoma is testing whether the standard model can explain the formation of the outer planets.

Wetherill came to the department as a Staff Member in 1953. Between 1960 and 1975 he was a professor and department chairman at the University of California, Los Angeles. He came back to Carnegie in 1975 as director of the department, a position he held until 1991.

SELECTED PUBLICATIONS

• Chambers, J. E., and G. W. Wetherill. 2001. Planets in the asteroid belt, Meteoritics Planet. Sci. 36, 381-399.
  
  
• Inaba, S., H. Tanaka, K. Nakazawa, G. W. Wetherill, and E. Kokubo. 2001. High-accuracy statistical simulation of planetary accretion II: comparison with N-body simulation, Icarus 149, 235-250.
  
  
• Kortenkamp, S. J., G. W. Wetherill, and S. Inaba. 2001. Runaway growth of planetary embryos facilitated by massive bodies in a protoplanetary disk, Science 293, 1127-1129.
  
  
• Kortenkamp, S. J., and G. W. Wetherill. 2000. Terrestrial planet and asteroid formation in the presence of giant planets I: relative velocities of planetesimals subject to Jupiter and Saturn perturbations, Icarus 143, 60-73.
  
  
• Wetherill, G. W., and S. Inaba. 2000. Planetary accumulation with a continuous supply of planetesimals, Space Sci. Rev. 92, 311-320.

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