Book Review of Protoplanetary Dust: Astrophysical and Cosmochemical Perspectives, Daniel Apai and Dante S. Lauretta, editors (2010) Cambridge University Press, New York, 377 pp., $120.
About 1% of the mass of interstellar matter in the Universe is dust. Most of the refractory elements are locked up in these tiny motes, leading to a depletion of those elements in the gas phase. This dust is a critically important component in the formation of stars and of planets. Dust cocoons around fledgling stars can be studied telescopically by their absorption and scattering of light. Laboratory simulations can provide constraints on the astrophysical processes that produce and alter dust in space. Dust that was incorporated into our own solar nebula and survived intact can be extracted from meteorites or collected by spacecraft and analyzed by sensitive instruments. The approaches provided by astronomical observations, laboratory experiments, and cosmochemical analyses are complementary. However, the practitioners of these disciplines seldom communicate with each other - and never more effectively than now.
Protoplanetary Dust comprises ten chapters by pairs or threesomes of authors from the different disciplines, each focused on the same question. These arranged marriages are intended to reveal new insights into the origin and evolution of dust in the Universe, and they succeed wonderfully. I found this novel approach riveting.
An instructive example of the complementary nature of the different approaches is provided by a chapter on dust particle size evolution. Astronomy is limited by the high optical depths in the inner parts of accretion disks, which preclude direct observation of the planet-forming regions. Conversely, meteorites only sample materials from the inner parts of our solar disk. Observations from astronomy and cosmochemistry can be combined with experimental data on dust coagulation to enhance understanding of how and where dust particles accrete to form planetesimals and ultimately planets.
Perhaps the most fascinating demonstration of the success of this book is illustrated by a figure that correlates the exquisite chronology of the early solar system afforded by short-lived radioisotopes in meteorites with the timescales of protostars, disks, and extrasolar planets estimated from astronomical observations. Anchoring these two very different evolutionary reference frames is a huge step forward.
The chapters cover a wide range of topics dealing with the origin, composition, physical state, and evolution of protoplanetary dust. Of particular interest to geochemists are Chapter 4 (Chemical and isotopic evolution of the solar nebula and protoplanetary disks), Chapter 5 (Laboratory studies of simple dust analogs in astrophysical environments), and Chapter 6 (Dust composition in protoplanetary disks).
The book is a marvel of coherency, and the editors have done a commendable job of ensuring that the same style and level of writing are maintained throughout. The intended audience is very broad, and although a few examples of astronomical or cosmochemical jargon creep in, such terms are defined in an excellent glossary. A few chapters end by identifying directions for future research - it would have been helpful to students if all of the authors had shared their own key questions. Helpful appendices provide background on mineralogy, mass spectrometry (why only this analytical technique?), and light absorption and scattering.
My only critique of this wonderful book is in its uneven treatment of condensation. This process (or sometimes the reverse process of evaporation) is commonly invoked to explain fractionations of refractory and volatile elements in solar system solids. Many chapters accept without question that condensation is the explanation for the isotopic homogeneity of nebular materials, but one states that 'To date, there is no incontrovertible evidence for direct condensation of rocky meteoritic material in the solar nebula.' It is not surprising that the authors of this book do not reach consensus on this on-going controversy. However, condensation is of such overriding importance in understanding protoplanetary dust that I wish the subject had been fully developed in its own chapter.
Protoplanetary Dust is a terrific edition (No. 12) to the Cambridge Planetary Science Series. The authors are authorities in their respective fields, and the interdisciplinary perspective crafted by the editors is a delight. This book should be required reading for all cosmochemists (and astronomers), and it would serve as an excellent text for an interesting graduate course on the origin of solar systems.