“A meaningful understanding of the Earth and planets requires explaining their differences. This explanation of planetary processes is difficult partly because it entails a wide range of scales–from microscale, operating at the atomic level, to macroscale, determined by boundaries thousands of kilometers apart. David Stevenson’s graduate study was mainly in theoretical condensed-matter physics, but he is remarkable in his grasp of large-scale planetary processes such as mantle convection and the dynamos. He is also remarkable in his instinct to attack the jugular,’ that is to go for the most important problems and for the versatility of his approaches thereto. Sometimes he achieves a new insight by a back-of-the-envelope’ calculation, which make one think within 30 seconds of hearing about it, Why didn’t I think of that?’ Other times insight may entail elaborate computer modeling. In these cases, he often adapts techniques evolved by others and usually enlists a graduate student, postdoc, or other bright collaborator. This reflects his exceptional ability to assimilate the work of others and to use it to achieve new depths of understanding.
“Here are some examples of David Stevenson’s contributions to our understanding of how the Earth and planets came to be as they are.
“How did the Earth, with its large moon, originate?’ On this problem, Stevenson’s contributions demonstrate his eclectic and synthesizing abilities. While Cameron and Hartmann were first to point out the need for a great impact to account for the moon’s iron deficiency, it was Stevenson who led the charge at the 1984 Kona conference to persuade almost everyone else it had to be so and, in subsequent reviews, to explore the dynamical difficulties. Similarly, he picked up Abe’s demonstration of the inevitability of a magma ocean from the outgassing of a steam atmosphere and pursued its implications.
“How do cores differentiate?’ This is both a microscale (separation of iron from silicate) and macroscale (accumulation of iron blobs big enough to sink in a turbulent environment) problem. The latter is more difficult, and Stevenson’s 1990 paper on the fluid dynamics remains the established word.
“Why does the Earth’s mantle appear to be so close to homogeneous in composition?’ A much more difficult problem, emphasized by Ringwood as the main objection to a magma ocean. In a series of papers with Solomatov in 1993, Stevenson showed that it was feasible as a trade-off of crystal growth and settling and the decline of convection from turbulence. He also has explored the maintenance of this condition in papers on mantle convection, subduction, and the underlying chemistry and physics, as in his 1994 paper for Philosophical Transactions of the Royal Society of London.
“How do crusts differentiate?’ This is, of course, a preoccupation of many geoscientists. Stevenson, in a series of papers from 1986 to 1991, most of them with Scott, developed physical models of magma migration, a combination of the microscale (melting, segregation, and surface tension) with the macroscale (conduit and porous flow within a solid state convecting medium) to explain evidences of sporadic behavior and differentiation at depth. This physical modeling is rather complementary to the more geochemical work of McKenzie and collaborators. Stevenson has also collaborated in the investigation of seismological constraints on magmatism.
“What are the energy sources of terrestrial planet dynamos?’ Since proposed by Braginsky and Verhoogen in the early 1960s, it has been concurred to be the solidification of the inner core, from both gravitational settling and freezing. However, it was Stevenson who first pointed out (in 1983) that this hypothesis was supported by a different outcome in another experiment: Venus, which, if it is like the Earth in all respects except size, must have a completely fluid core, and hence no energy source for magnetic field. This conjecture of a fluid core has since been confirmed by Konopliv and Yoder, who infer a high Love number for Venus from solar tides. Stevenson has also explored the possibilities of dynamo action in Mercury, showing that it is not too small.
“What is the pattern of core dynamics?’ Stevenson investigated the problem of geodynamo generation in several papers from 1974 to 1983, with particular emphasis on the dipole and its near-alignment with the rotation axis. While Braginsky and Busse have done more on the fundamentals and Glatzmaier and Roberts more on the modeling, Stevenson’s particular contribution has been to explain the differences in magnetic fields among the planets, a fine example of comparative planetology requiring knowledge of the differences in physics between the Earth and other planets, both on the microscale questions of fluidity and electrical conductivity and the macroscale questions of the convecting regime and the influence of rotation.
“What is the pattern of mantle dynamics?’ More recently, in collaboration with Tackley, Glatzmaier, and others, Stevenson has explored the nature of mantle convection, with emphasis on the effect of the 670-km endothermic phase transition. This work is still in progress, but indicates that the effect of the phase transition and other nonlinear interactions of the flow system with material properties is to cause temporal variations in the convection, some of them rather episodic avalanches.’
“What are the effects of the high pressures in major planet interiors on the physical properties of hydrogen and helium?’ In a series of papers from 1974 to 1980, Stevenson, partly in collaboration with Salpeter, developed the condensed matter theory to infer the phase diagrams of hydrogen, helium, and hydrogen-helium mixtures; most importantly, that the transition from molecular to metallic hydrogen occurs around 300 GPa. He also developed estimates of thermal and electrical conductivity, the miscibility of hydrogen and helium, and the solubilities of ices important to the thermal and compositional evolution of the major planets.
“What are the implications of the masses, shapes, thermal radiations, magnetic fields, and atmospheric compositions of the planets and large satellites in the outer solar system for their structure and evolution? Since the mid-1970s, Stevenson has continually developed and refined models of planetary interiors to reconcile condensed matter theory with the data gained by spacecraft and telescopes. The major planets are thus inferred to be quite varied in character: Saturn’s heat requires ongoing settling-out of helium, Uranus’ and Neptune’s do not, while in Jupiter it is unsure; and the near axisymmetry of Saturn’s magnetic field indicates a stable stratified conducting layer overlying its dynamo region. Very recently, Stevenson has interpreted the magnetic and gravitational results from the Galileo mission to infer that Ganymede must have an internal source for its magnetic field, but that the magnetic signatures of Europa and Io may be induced by Jupiter’s field.
“What were the circumstances of origin of major planets?’ On the most important problem of the formation of Jupiter, after which the other planets follow, Stevenson and Lunine [1988] showed that there would be a high concentration of ice, hence rapid core formation, by diffusive redistribution of water vapor just inside the radius of condensation. Stevenson has also modeled chemical heterogeneity and imperfect mixing in the nebula and has used the differences in the structure of Uranus and Neptune to infer differences in late-stage impact history.
“What are the physical constraints on bodies intermediate between major planets and stars in size?’ In a few papers between 1978 and 1991, Stevenson extended his condensed matter theory to infer the likely thermal properties, transport properties, and atmospheres of brown dwarfs.
“Dave Stevenson is the Jimmy Levine of geophysics: the omniscient endomorph who incorporates the work of others with his unique insights to produce ongoing models of the planets, satellites, and dwarf stars. The breadth of his view leads to frequent invitations to give summarizing lectures at conferences and to write review papers (for example, four in the Annual Reviews series). He is wiser than us, but beloved because he recognizes and uses what we have to offer: his response is always to point out That’s a good suggestion!’, while tactfully ignoring what is wrong. Thus, in contrast to some other bright people, his overall contribution to our science is greater through the work of those he inspires, as well as through his own.”
—WILLIAM M. KAULA, University of California, Los Angeles
