Questions and answers with H. Jay Melosh
DOI: 10.1063/PT.4.2387
By Jermey N. A. Matthews
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H. Jay Melosh, after whom Asteroid #8216 is named, earned his bachelor’s degree in physics from Princeton University in 1969 and his PhD in physics and geology from Caltech in 1973. His primary areas of research have been impact cratering, planetary tectonics, and the physics of earthquakes and landslides. He has contributed to studies of the giant-impact origin of the moon, the prehistoric asteroid impact 65 million years ago that extinguished the dinosaurs, and the origin and transfer of life between the planets. Melosh is a science team member of NASA’s Deep Impact mission, which in 2005 successfully created an artificial crater in a comet. He is also a coinvestigator on NASA’s GRAIL mission, which is studying the Moon’s gravity field.
Melosh is currently a Distinguished Professor of Earth and atmospheric science at Purdue University, Indiana, and professor emeritus at the University of Arizona in Tucson, where he spent 27 years teaching planetary science courses. His first textbook, Planetary Surface Processes (Cambridge University Press, 2011), is a product of those lectures. He says his other single-authored book, Impact Cratering: A Geologic Process (Oxford University Press, 1989), is treated as a textbook, even though it’s not.
Physics Today recently caught up with Melosh to discuss his text.
PT: After four decades of research and more than 180 technical articles, what motivated you to write this textbook?
Melosh: It came about because there is no text on this subject suitable for upper-level undergraduate or graduate courses. I had planned to turn my notes into a text for many years, but other projects always seemed to get in the way. The past few years have been no different, but the need for such a text has simply gotten to the point that I had to find the time to write the book, which owes a great deal to late-night writing sessions.
PT: How has the field of planetary sciences in general changed since you first entered it and how does your book compare to texts from a few decades ago?
Melosh: When I first started teaching this subject, the only other [planetary bodies] we knew much about from a geologic perspective were the Moon and Mars. At that time one could center a course about those two bodies rather than about the processes that act on their surfaces. Older textbooks on planetary science employed a planet-based exposition that I believe is presently obsolete. However, as planetary exploration brought more planets and moons into view, an object-focused organization became unwieldy, and a process orientation now makes much more sense. Besides, because of my background in physics—through a PhD in quark physics—I am much more inclined to think in terms of processes than in terms of history alone. Although the time evolution of the solar system is important, too.
Computer modeling, of course, plays a big role in evaluating the consequences of different hypotheses, which we then compare to observations. While the physics of an individual process may be simple, Nature is messy and computers are one of our principal tools for combining simple processes into the complex fabrics necessary to mimic observations and thus either validate or refute different hypotheses about what we see.
PT: What have we learned from observations and discoveries of other planetary bodies that shed light on the role of impact craters on Earth?
Melosh: Studies of impact craters on Earth and those on other planetary bodies are largely complementary. Most terrestrial craters are rapidly degraded by our hyperactive surface environment. Impact craters elsewhere in the solar system are much less altered, and so we can often observe the pristine morphology that is so quickly erased on Earth. Observations of extraterrestrial craters allow us to evaluate the cratering process in a variety of different environments, with different substrates—rock, ice or a mixture—[in different] gravity fields, and over an enormous range of sizes, from micron-sized zap pits to craters with diameters comparable to that of their target planet. While the physics of the cratering process may be similar across all of these circumstances, the variations offer many hints as to the details and the implications of impact cratering for planetary evolution.
PT: What is your reaction to the early scientific findings by the Mars rover Curiosity, and what do you anticipate could yet be uncovered by it?
Melosh: For me, Curosity’s most exciting discovery so far is the millimeter-sized basaltic spherules that appear to be ubiquitous within Gale crater. My own opinion is that these are likely to be melt droplets, or perhaps vapor condensates, from very large impacts elsewhere on Mars. Layers of impact spherules are known from ancient Earth rocks, from 65 million years ago back to Archean rocks 3.5 billion years old, but they are very rare. However, Mars’s lack of terrestrial erosion and consequent slow surface degradation may have preserved such impact ejecta layers in quantities far larger than what we can imagine on Earth.
PT: Are there differences in the pedagogical approaches of physicists and geologists to planetary surface processes, and have you seen a convergence over the years?
Melosh: In the early days, geologists tended to evaluate planetary surface features by their similarity in appearance to terrestrial features, sometimes without even considering large differences in size scale. Physicists tended to ignore ‘dirty'—that is, complex and often ambiguous—surface processes, focusing instead on ‘clean’ quantifiable aspects like mass and gravitational or magnetic fields. At the present time, I see much more convergence of approach, with geology becoming more quantitative and deductive, while more physicists have become interested in complex processes and even the historical aspect of how solar systems evolve. Geologists still put more emphasis on evolution through time, but I think that is an entirely appropriate emphasis. We are all interested in how, and by what means, planetary bodies and systems arose and presently evolve. And let us not forget chemistry, which cuts across all planetary science, and biology, which makes planets especially fascinating to us humans.
PT: What books are you reading at the moment?
Melosh: I am deeply interested in learning more about the chemical changes caused by impacts and their implications for the origin and evolution of life, so I am immersed in reading Jibamitra Ganguly’s book, Thermodynamics in Earth and Planetary Sciences (Springer, 2008), as well as the multiauthored Brock Biology of Microorganisms (13th edition, Benjamin Cummings, 2010). I already use scattered results from those fields, but I am reading the books to fill in gaps in my understanding. For fun, I read books on the history of science to appreciate how ideas and theories develop; I put a lot of emphasis on this in both of my published books. My current bedtime reading is M. J. Nye, From Chemical Philosophy to Theoretical Chemistry (University of California Press, 1994).
