Stanton J. Peale: Deciphering the Motions of Planets and Moons Robin M

RETROSPECTIVE Stanton J. Peale: Deciphering the motions of planets and moons Robin M. Canup1 Planetary Sciences Directorate, Southwest Research Institute, Boulder, CO 80302 Planetary science lost one of its most highly molten core at Mercury, which seemed nec- respected members with the recent passing of essary to produce the magnetic field detected Prof. Stanton (Stan) Peale. At last December’s by Mariner 10 but was unexpected for such a American Geophysical Union meeting, Stan small planet. Stan showed that Mercury’sin- presented a talk on effects of the ellipsoidal terior structure could be constrained by mea- shape of Mercury’ssolidinnercoreonthetilt surements of small fluctuations in its spin rate of the planet’sspinaxis,buthewasnotfeel- caused by its asymmetric shape, the tilt of ing well. He was later diagnosed with a rare its rotational axis, and harmonics of its form of leukemia that proved resistant to gravitational field. More than 30 years later, treatment, followed ultimately by a severe Stan and colleagues used his approach in leg breakage because of a fall, which hastened combination with Earth-based radar obser- his decline. Supported by his wife of 55 years, vations (led by Jean-Luc Margot) and grav- Priscilla, his sons, Robert and Douglas, and ity data acquired by the MESSENGER many other family, friends, and colleagues, spacecraft (led by Sean Solomon) to dem- Stan died on May 14, 2015 at the age of 78 onstrate that Mercury does indeed have a in Santa Barbara, California. His final scien- liquid core, a critical constraint to its com- tific paper was submitted just days earlier. position and thermal history. Stan obtained his doctorate in astronomy In 1979, Stan and his coauthors famously Stanton J. Peale. Image courtesy of Henry from Cornell University in 1965. He accepted a predicted that there would be widespread Throop (Planetary Science Institute). faculty position at the University of California, volcanism on Jupiter’s innermost large Los Angeles, but then moved to the physics moon, Io (1). This was later confirmed dur- department at the University of California, ing the Voyager 1 flyby, in one of that he was usually smiling. He was candid with Santa Barbara in 1968, where he worked for spacecraft’s most dramatic discoveries. Io’s a self-effacing sense of humor. Despite the the remainder of his life. Stan’s research fo- orbit is slightly noncircular, and Stan recog- complex nature of Stan’s work and his level cused on the dynamical evolution of planets nized that variable flexing of the moon’s of achievement, he was never condescend- and moons, in particular the often complex shape by Jupiter would produce an ongoing ing, treating colleagues with great respect. consideration of how an object’s physical state, source of dissipation that would maintain Acknowledging the accomplishments of including its interior structure and response to a molten interior. others came effortlessly to him, a reflection tidally driven deformation, was coupled to its Stan’s later research included a multitude of his appreciation for all things good and orbital and rotational motions. of papers on the origin and evolution of sat- worthy. You could hope for no better for- Much of Stan’s early work concerned plan- ellites, and the detection and orbital evolution tune than to have Stan peer-review your etary rotation, motivated by the discoveries of exoplanets orbiting other stars. His many work, for he would replicate your calcula- that Mercury’sspinrateis1.5-timesitsmean awards included the Newcomb Cleveland tions, and if he found a flaw or weakness in motion about the Sun, while Venus rotates in Prize of the American Association for the reasoning it would be pointed out with fair- an opposite sense to its orbital motion. Both Advancement of Science (AAAS; 1979), ness and sensitivity. Stan would likewise findings contrasted with the expectation the NASA medal for exceptional scientific respond with care to critiques of his own that each planet would be synchronously achievement (1980), fellowship in the AAAS works, and with genuine and often explic- rotating—with a day equal to its year—because (1981), the James Craig Watson award of the itly expressed gratitude to any who helped of solar tidal friction. With Peter Goldreich, National Academy of Sciences (1982), fellow- him improve them. As a result of all of these Stan developed the concept of “spin-orbit ship in the American Geophysical Union things, Stan was one of the most admired coupling,” in which an orbiting body subject (1988), designation of the asteroid Peale (and indeed, beloved) members of our field. to tidal interaction is captured into a resonant 3612 (1988), the Dirk Brouwer Award of Stan found joy in many aspects of life. He state that maintains a nonsynchronous spin. the Division of Dynamical Astronomy of delighted in the rare bloom of the titan arum Stan later derived a theoretical framework the American Astronomical Society (1993), flower at the University of California, Santa describing all possible rotational states for a and election to the National Academy of Barbara greenhouse, the ballet, the sym- planet or moon that remains the standard Sciences (2009). phony. There was a boyish wonder and for such analyses today. As a person, Stan was just as remarkable. He In 1976, Stan developed an ingenious tech- was brilliant, but also kind. He laughed often, Author contributions: R.M.C. wrote the paper. nique to infer the existence and extent of a and when not in a state of concentration, 1Email: [email protected]. 10076–10077 | PNAS | August 18, 2015 | vol. 112 | no. 33 www.pnas.org/cgi/doi/10.1073/pnas.1512536112 Downloaded by guest on October 1, 2021 curiosity in him that made Stan seem much who he was: a relentlessly exacting scientist publishing more than 20 scientific papers RETROSPECTIVE younger than his actual age. He was also who viewed it as his ethical obligation to since turning 70. Before beginning chemo- self-critical and reflective. Stan confessed the community to do things correctly and therapy, Stan told his doctor that he would that he wished he could speak more dra- to explain his findings thoroughly. By his need to delay the treatment schedule by matically, to be “someone that makes example, Stan inspired those of us who several days to finish his contributions to a others hang on their every word.” But Stan knew him to be better scientists, and indeed paper on Mercury. Stan would not let his was not a salesman. He openly acknowl- better people. colleagues down, even then. edged each uncertainty and potential weak- Stan Peale lived a long and full life. ness in his research. The field trusted his Nonetheless, his death feels premature. He work, not just because of his raw ability, was extraordinarily fit, both physically and 1 Peale SJ, Cassen P, Reynolds RT (1979) Melting of Io by tidal which was exceptional, but because of mentally, often riding his bike to work and dissipation. Science 203(4383):892–894. Canup PNAS | August 18, 2015 | vol. 112 | no. 33 | 10077 Downloaded by guest on October 1, 2021.

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