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William (Bill) Roger Ward was a preeminent theoretician who studied the formation and evolution of planetary systems. He was born on January 11, 1944, and passed away at the age of 74 on September 20, 2018, at his home in Prescott, Arizona, after a battle with brain cancer. Ward’s many research contributions were widely recognized within the scientific community, as evidenced by the numerous awards he received over his distinguished career. These included the NASA Exceptional Scientific Achievement Medal (1981), the 2004 Brouwer Award of the Division of Dynamical Astronomy of the American Astronomical Society (AAS), the 2011 Kuiper Prize of the Division of Planetary Sciences of the AAS, Fellowship in the American Geophysical Union (2005) and the American Association for the Advancement of Science (2006), and his election to the American Academy of Arts and Sciences in 2012, and to the National Academy of Sciences in 2015. He is survived by his beloved wife of 51 years, Sandra, their children Brad, Stephanie, and Scott, his brother Jeff, and his sister Patty.
Ward received undergraduate degrees in physics and mathematics in 1968 from the University of Missouri at Kansas City, where he was valedictorian of his graduating class. He went on to receive his PhD in planetary sciences from Caltech in 1972, advised by Peter Goldreich. Ward worked for a time at Harvard’s Center for Astrophysics before moving to the Jet Propulsion Laboratory in 1977. He joined Southwest Research Institute in Boulder, Co. in 1998, and retired from SwRI as an Institute Scientist in 2014. His research career spanned nearly five decades, with his last paper completed just a few months prior to his death.
Ward contributed a wealth of new and fundamental concepts to our understanding of planet and moon formation, and dynamical and orbital evolution. He was extraordinarily creative, with superb physical intuition and analytic skills. Most of his papers were single author, or with only one co-author, reflecting his primary role in all his published research. He was a visionary whose thinking tended to be ahead of that in the field at the time, and as a result, his research was often underappreciated at the time of its initial publication. However, as later numerical methods confirmed the fundamental validity of his elegant analytical treatments, or as new data became available, many of his works went on to become classics in the field.
With Goldreich in 1973, Ward proposed that kilometer-sized planetesimals formed through the collapse of gravitationally unstable regions of the protostellar disk. This idea is central to current leading theories for how pebble-sized particles grow into objects large enough to accumulate into planets through two-body collisions. It was Ward that first discovered, also in 1973, that the tilt of Mars’s spin axis (its obliquity) undergoes large variations of many tens of degrees. These spin pole swings have profound implications for past variations of the Martian climate, and Ward developed a formalism for describing the direct influence of changes in planetary obliquity and orbital eccentricity on planetary climate.
With Alastair Cameron in 1976, Ward proposed that the impact of a Mars-sized object with the early Earth led to the origin of the Moon, anticipating many elements of the current leading giant impact theory. In 1981, Ward recognized that the secular dynamical structure of our solar system – i.e ., that associated with distant interactions among the planets that depend on the apsidal and nodal precession rates of their orbits – would have been strongly influenced by the dispersal of the early Sun’s massive, hydrogen-rich gas disk, thereby resulting in a “sweeping” of the system frequencies capable of generating highly eccentric and inclined orbits observed in regions such as the asteroid belt.
Perhaps Ward’s most influential contributions were contained in an extensive series of papers that explored the behavior of a planet as it forms within a gaseous circumstellar disk. Earlier works by Goldreich and Scott Tremaine had developed the foundations for describing gravitational interactions between a planet and the density waves its gravity induces in a companion disk. Ward focused on sources of asymmetry in this problem, and in a landmark 1986 paper he concluded that the gravitational torques on the planet from the disk orbiting exterior to the planet would dominate over those from the interior disk, causing the planet’s orbit to spiral inward towards its central star. We now know of countless extrasolar systems with planets in very close stellar orbits that may have undergone such large-scale inward migration. Ward’s ideas on this topic came long before such discoveries, however, and were initially met with skepticism. This frustrated him greatly, and inspired him to continue to write papers on the topic, each addressing a different aspect of what he was certain was a process of fundamental importance. Our field ultimately benefited enormously from Ward’s perseverance, for over the next decade he generated a wealth of papers that provided much of the foundation of modern work in this important area. This work included a widely referenced paper from 1997, in which he clarified the distinction between Type I vs. Type II migration (the latter occurring once the planet grows large enough to open a radial gap in the disk), and created a unified model describing the transition between these two regimes.
Ward had other influential papers on the evolution of the lunar orbit, satellite formation, and planet obliquity evolution, including models for the origin of gas giant satellites (with myself in 2002-2010), the origin of Saturn’s obliquity (with Doug Hamilton in 2004), and the complex evolution of a pre-lunar disk produced by a giant impact (Ward 2012, 2014, and 2017).
Bill took his science seriously, but himself less so. His office walls were covered with blackboards, replete with beautifully scripted diagrams and derivations. While his publications were typically mathematically dense, his verbal descriptions of his work revealed an understanding based on physical intuition, as well as an uncanny sense of which processes mattered, and which could be ignored. When he spoke of his work, the complex topics he conveyed became clear. He was also a fount of wisdom on how to do science. Despite his ease in developing and navigating within complex models, he emphasized simplicity. When tackling a new problem, he would start with initially idealized approaches, from which he would learn how each parameter or process affected the overall outcome. He would then methodically and incrementally increase the complexity of his approach until he was confident in the overall result. He had a youthful joy in conceiving new ideas, a mischievous sense of humor, and a love of sharing fine food and drink with friends and colleagues. From his office at night while he was writing grant proposals could often be heard jazz or Frank Sinatra at considerable volume. On doing research, he wisely noted and advised that “uninteresting problems are often as or more difficult than interesting problems, so just work on the interesting ones.” We are grateful for the many interesting problems he pursued, as well as for the inspiration he provided us, both through his science and his person.
Contributed by Robin M. Canup, February, 2019
BAAS Citation: BAAS, 2019, 51, 2