Book Review: Planetary Surface Processes, by H. Jay Melosh

Planetary Surface Processes
H. Jay Melosh, Cambridge University Press, 2011

I could tell from the first page that this book was going to become a primary resource for this blog. Many of my posts are explanations of pretty pictures, and the geologic processes that created the shapes of planetary surfaces. Planetary Surface Processes provides a rigorous overview of every process that shapes the appearance of planetary surfaces. And it's extremely well-organized. Rather than taking a world-by-world approach, Melosh considers the physical processes that shape all worlds, from the largest-scale (with chapters on global shapes and the balance between strength gravity) through regional-scale processes (chapters on tectonics, volcanism, impact cratering, regolith/weathering) to processes that locally modify underlying landforms (mass movement, wind, water, and ice). His examples are drawn from across the solar system, helping to emphasize his points about how variations in initial conditions of composition, surface gravity, size, and temperature affect the outcomes of the same physical forces affecting different places.

The book was developed for a planetary science course that Melosh taught for many years at the University of Arizona (he is now at Purdue). I was amused to see that the book's Amazon reviews are from professors who teach their own courses and are relieved to have it available. As a text it would support a class for advanced undergraduates and/or new grad students who have come in with physics or astronomy or other non-geology majors.

While it is full of equations, most of them are of simplified and schematic form, designed to help the reader understand the relationships among physical properties that control a process. Almost none of the equations seem to require more than the most basic understanding of calculus -- most are just algebra -- and where Melosh does mention higher-level concepts (like tensors, which are really necessary for a proper understanding of the mathematics of stress and strain), he does so in a parenthetical way, benefiting people who have the necessary background without derailing the comprehension of readers who don't. End-of-chapter problems provide opportunities for the reader to work through these relationships, though no answer key is provided. (In his Amazon review, Erik Asphaug remarks, "I do not regard this as a serious drawback, since sometimes it is good to leave exercises to the instructor to figure out, so that we don't get too lazy about things....")

And the text is also just enjoyable, whether Melosh is acknowledging the human characters of historical figures important in planetary geology, or simply guiding the reader. I will certainly be referring to it -- to remind me of the difference between Pratt and Airy isostasy; of the fates of materials at different distances from an asteroid impact; of the significance of a moon's moment of inertia; and even of the most basic equations used to explain the shapes of planets. It's already been a helpful refresher on concepts I last worked through more than a decade ago. As the years between the present and my geologic education grow in number, I expect to be finding such a reference text more and more useful!

Ultimately, this is a physics text, so it does not go into much detail on the chemistry of the solar system except where it most directly affects physical processes. I would love to see another book that takes the same organizational approach as Planetary Surface Processes, beginning with the composition of the nebula that formed the solar system, and walking through the chemical processes, major to minor, that differentiated this primordial cloud into the diverse compositions of planets, moons, cores, mantles, crusts, atmospheres, and dust that we see today. Who's going to write that one?