Stanton J. Peale: Deciphering the motions of planets and moons.

Planetary science lost one of its most highly respected members with the recent passing of Prof. Stanton (Stan) Peale. At last December’s American Geophysical Union meeting, Stan presented a talk on effects of the ellipsoidal shape of Mercury’s solid inner core on the tilt of the planet’s spin axis, but he was not feeling well. He was later diagnosed with a rare form of leukemia that proved resistant to treatment, followed ultimately by a severe leg breakage because of a fall, which hastened his decline. Supported by his wife of 55 years, Priscilla, his sons, Robert and Douglas, and many other family, friends, and colleagues, Stan died on May 14, 2015 at the age of 78 in Santa Barbara, California. His final scientific paper was submitted just days earlier. Stan obtained his doctorate in astronomy from Cornell University in 1965. He accepted a faculty position at the University of California, Los Angeles, but then moved to the physics department at the University of California, Santa Barbara in 1968, where he worked for the remainder of his life. Stan’s research focused on the dynamical evolution of planets and moons, in particular the often complex consideration of how an object’s physical state, including its interior structure and response to tidally driven deformation, was coupled to its orbital and rotational motions. Much of Stan’s early work concerned planetary rotation, motivated by the discoveries that Mercury’s spin rate is 1.5-times its mean motion about the Sun, while Venus rotates in an opposite sense to its orbital motion. Both findings contrasted with the expectation that each planet would be synchronously rotating—with a day equal to its year—because of solar tidal friction. With Peter Goldreich, Stan developed the concept of “spin-orbit coupling,” in which an orbiting body subject to tidal interaction is captured into a resonant state that maintains a nonsynchronous spin. Stan later derived a theoretical framework describing all possible rotational states for a planet or moon that remains the standard for such analyses today. In 1976, Stan developed an ingenious technique to infer the existence and extent of a molten core at Mercury, which seemed necessary to produce the magnetic field detected by Mariner 10 but was unexpected for such a small planet. Stan showed that Mercury’s interior structure could be constrained by measurements of small fluctuations in its spin rate caused by its asymmetric shape, the tilt of its rotational axis, and harmonics of its gravitational field. More than 30 years later, Stan and colleagues used his approach in combination with Earth-based radar observations (led by Jean-Luc Margot) and gravity data acquired by the MESSENGER spacecraft (led by Sean Solomon) to demonstrate that Mercury does indeed have a liquid core, a critical constraint to its composition and thermal history. In 1979, Stan and his coauthors famously predicted that there would be widespread volcanism on Jupiter’s innermost large moon, Io (1). This was later confirmed during the Voyager 1 flyby, in one of that spacecraft’s most dramatic discoveries. Io’s orbit is slightly noncircular, and Stan recognized that variable flexing of the moon’s shape by Jupiter would produce an ongoing source of dissipation that would maintain a molten interior. Stan’s later research included a multitude of papers on the origin and evolution of satellites, and the detection and orbital evolution of exoplanets orbiting other stars. His many awards included the Newcomb Cleveland Prize of the American Association for the Advancement of Science (AAAS; 1979), the NASA medal for exceptional scientific achievement (1980), fellowship in the AAAS (1981), the James Craig Watson award of the National Academy of Sciences (1982), fellowship in the American Geophysical Union (1988), designation of the asteroid Peale 3612 (1988), the Dirk Brouwer Award of the Division of Dynamical Astronomy of the American Astronomical Society (1993), and election to the National Academy of Sciences (2009). As a person, Stan was just as remarkable. He was brilliant, but also kind. He laughed often, and when not in a state of concentration, he was usually smiling. He was candid with a self-effacing sense of humor. Despite the complex nature of Stan’s work and his level of achievement, he was never condescending, treating colleagues with great respect. Acknowledging the accomplishments of others came effortlessly to him, a reflection of his appreciation for all things good and worthy. You could hope for no better fortune than to have Stan peer-review your work, for he would replicate your calculations, and if he found a flaw or weakness in reasoning it would be pointed out with fairness and sensitivity. Stan would likewise respond with care to critiques of his own works, and with genuine and often explicitly expressed gratitude to any who helped him improve them. As a result of all of these things, Stan was one of the most admired (and indeed, beloved) members of our field. Stan found joy in many aspects of life. He delighted in the rare bloom of the titan arum flower at the University of California, Santa Barbara greenhouse, the ballet, the symphony. There was a boyish wonder and Stanton J. Peale. Image courtesy of Henry Throop (Planetary Science Institute).