|Astronomy & Geophysics 44 (2) 2.35|
|Dust and the diffuse universeAstrophysics of the Diffuse Universe, Michael A Dopita and Ralph S Sutherland, Springer Verlag, 2003, hbk $69.95, ISBN 3 540 43362 7.The Physics of Interstellar Dust, Endrik Krügel, Institute of Physics Publishing, 2003, hbk £90, ISBN 0 7503 0861 3.Dust in the Galactic Environment, 2nd edition, D C B Whittet, Institute of Physics Publishing, 2003, pbk £34.99, ISBN 0 7503 0624 6.The Physics of the Interstellar Medium 2nd edition, J E Dyson and D A Williams, Institute of Physics Publishing, 1997, pbk £26.99, ISBN 075030460x.|
The insubstantial haze of gas, plasma and dust between the stars and within galaxies is currently a productive research area; the development of new observational techniques and powerful modelling methods and new theoretical approaches means it will continue to be a lively field in the future. A feature of this research field is, however, its extreme breadth, demanding combinations of mineralogy, chemistry, physics and such expertises as magnetohydrodynamics for a full understanding of the subject. Faced with such diversity, and wanting to introduce the field to a wider audience, authors have some difficult choices to make. Simplification with the attendant risk of over-simplification is one option, while covering a restricted part of the field another. In my attempt to cover this field in print, I chose a third way: a draft of a comprehensive overview of this field, consisting mostly of typed pages and of substantially greater thickness than these three books stacked on top of one another, sits on a shelf in my office. My co-author and I have already devoted multiple man-years to it, but there it rests as an embarrassing reminder of an uncompleted job.
Among the authors to be commended for completing published texts, both E Krügel and D C B Whittet chose to limit their objectives by focusing on dust, a single, albeit important, component of diffuse matter. I am somewhat surprised that IOP Publishing has released these books simultaneously. A librarian with a limited budget is likely to choose one of the books, whereas he or she might have been more inclined to buy both if they had appeared at different times or on the lists of different firms.
However, Krügel's and Whittet's books are distinct treatments of the subject and a good library should have both.
Having given essentially no astrophysical motivation for the study of this field, Krügel begins with topics that are taught here at Leeds in a third-year undergraduate electromagnetism course. He then examines the scattering theory of radiation before reviewing statistical mechanics and thermodynamics including black-body radiation. Some simple quantum mechanics and solid-state physics follows. All of the material is presented clearly and concisely, but many researchers will already be able to find it on their bookshelves. Roughly the first 200 pages out of a total of around 600 will be of more interest to a student than a practising professional. Astronomical applications begin in earnest on page 214, but they are interspersed with further treatments of relevant general physics and chemistry, including an introduction to radiative transfer. The sections on PAHs and nucleation are perhaps those in the remaining several hundred pages covering material most likely to be unfamiliar to some professionals.
Whittet presents the field much more from the perspective common to most astrophysicists. Rather than reviewing all of the electromagnetic theory and scattering theory with which Krügel begins, Whittet states results from Mie theory, but also directs readers to appropriate references. He deals far more with the observed ices, depletion, and desorption. For instance, he addresses hot cores in some detail, giving insight into the way that dust affects gas-phase chemistry.
I find Whittet's book contains more of what interests me, whereas Krügel's might be suitable for a course introducing students to physics that professionals have already encountered. Neither book mentions the important dynamical role that dust can play in media that are sufficiently weakly ionized for dust-neutral collisions to control the degree to which the magnetic field is coupled to the neutral particles. This topic is extremely important for ambipolar diffusion in star formation and the structures of shocks in star-forming regions.
The IOP list also contains the second edition of Dyson and Williams' The Physics of the Interstellar Medium. At a large meeting in John Dyson's honour (kindly sponsored by the Mexican astronomical community and whose chief organizer was Jane Arthur) Mike Dopita compared his book (Astrophysics of the Diffuse Universe co-authored with Ralph Sutherland) favourably with that of Dyson and Williams. When making comparisons with this book (written by two very close friends of mine), one must bear in mind what Dyson and Williams were aiming to achieve. They wished to provide an introduction of appropriate level and length for an in-depth third-year undergraduate course. They succeeded. In addition, they produced a book to which a wide variety of postgraduate students and practising astronomers can turn. Such a reader can quickly get a background in an area that may touch on but may not be at the heart of their research. Dopita and Sutherland have not produced a book of such ready accessibility. Nor have they written a book that comprehensively covers the field of diffuse matter, as they may believe that they have.
Dopita and Sutherland's book is, however, a useful one. It is strongest on the collisional ionization, photoionization, and spectroscopy of gas at temperatures around and in excess of 10 000 K, a worthy subject for a book in itself. If the title indicated those topics as the major focus of the book, I would be able to write very positively about it indeed. However, the title indicates a much broader objective, which is also implied in the preface. As a result, I feel obliged, if not compelled, to address omissions.
Dopita and Sutherland have appreciated that molecules are important in the diffuse universe. They split their treatment of molecular spectroscopy into two parts. They don't mention inversion transitions, such as those in which NH3 emission is observed, but, otherwise, they include the main points. They include a chapter on interstellar chemistry, but they do not discuss one of the most important topics, the chemistry controlling the ionization balance in dark regions. This regulates the rate of low-mass star formation through ambipolar diffusion, a topic neglected by the authors. They also put H2CO+ in a number of reactions where HCO+ is the relevant species.
Indeed, the book contains nothing on the modern theory of star formation in magnetized molecular media. It suffers in comparison with Spitzer's 1978 book in this regard, even though that work was written in the very early days of the field. I mentioned the role of dust in shocks and star formation above. Though Dopita and Sutherland discuss C-type shocks, they do not write of the electrodynamics of dust in them. In short, other than in the treatment of molecular spectra, the authors did not address chemistry or the multifluid magnetohydrodynamics of the molecular phase adequately.
Even in the sections on warmer and hotter phases, important topics have been neglected. Cosmic-ray pressure probably has major effects on supernova remnant propagation and the dynamics of coronal gas in the galactic halo. Cosmic-ray transport and acceleration really must be introduced in a book with the title Astrophysics of the Diffuse Universe. After all, synchrotron emission is observed in many studies of supernova remnants and jets. Given the level of the book and its title, the effect of cloud ablation and evaporation on the expansion of supernova remnants and wind-blown bubbles really should have been addressed in a reasonably rigorous fashion.
All three books under review can be recommended for various features. They are similar in the sense that their authors really took astrophysical spectroscopy as their main interests, which is absolutely fine. Two books are primarily about the spectroscopy of dust and the implications for its composition. The other is mainly about the spectroscopy of ionized diffuse gaseous matter. They are very good on these topics. I think that I will find having Whittet's book, in particular, a plus. However, even the astrophysics of dust includes the magnetohydrodynamics of multifluid media where dust is a major carrier of charge.
The science of diffuse matter is much richer in hydrodynamic, hydromagnetic, and plasma processes than the authors of any of these books have suggested. If we are to explain key phenomena, including the self-regulation of high-mass-star formation, in diffuse astrophysical plasmas, we will have to develop just as deep understanding of these processes as we have of the collisional and radiative mechanisms described so well in these volumes.
|The Beagle Voyages: From Earth to Mars|
Exhibition at the National Maritime Museum, Greenwich, London, from December 2002 to September 2003.
The Beagle Voyages exhibition is at the National Maritime Museum until September.
In 1831 Charles Darwin set out on a journey that came to revolutionize our understanding about the origin and evolution of life. In an echo of his voyage of discovery, a British-led space probe which will embark for Mars in May 2003 to search for traces of life has been named Beagle 2 after Darwin's ship. The National Maritime Museum at Greenwich has constructed a small exhibition that compares some aspects of both missions.
The first part of the exhibition concentrates on Darwin's voyage and its difficult beginning. HMS Beagle was converted from a naval 10-gun coffin brig, a design with a reputation at the time for sinking. In some letters we also learn that Darwin's father tried to talk his 22-year-old son out of becoming the expedition's naturalist: A wild scheme, no good can come of it. Two other scientists had already turned down the job, but Darwin was determined to go and the exhibition shows how he was motivated by the new science of geology and in particular was stimulated by reading Charles Lyell's Principles of Geology. The geological aspects of Darwin's discoveries are emphasized, including his demonstration of the formation of coral atolls through subsidence of underlying submarine volcanoes. Any visitor who has done fieldwork will want to try and read his notebooks, which are on display, and to glimpse a few pages of Darwin's original observations.
The exhibition offers some nice juxtapositions of 1831 and 2003 science and technology, for instance Darwin's chemical blowpipe for analysing rock specimens lies beside Beagle 2's mass spectrometer for analysing martian rocks, soil and atmosphere. Some of the rock samples that caught Darwin's eye in the Andes are exhibited and so is what we now regard as a piece of the martian crust the 1911 Nakhla meteorite.
Studying martian meteorites and over 24 are now recognized has given scientists a better understanding about martian geology. The publication in 1996 of the controversial claim that one of them contains traces of fossilized martian bacteria has also stimulated much research and helped motivate the planned missions to Mars, including Beagle 2.
The second part of The Beagle Voyages deals with Beagle 2. Centred on a replica of the 60 kg lander and its solar arrays, there are descriptions of the instrument package and a video loop that shows the parachute and landing-bag inflation tests. Beagle 2 will hitch a lift on Mars Express and will be ejected shortly before the spacecraft goes into orbit. The journey and timescale of Beagle 2's voyage is shown, illustrating how the approaching conjunction of Mars and Earth makes a relatively rapid transit (seven months) possible. In contrast, HMS Beagle took five-and-a-half years to complete its journey. Darwin sometimes had to wait a year to exchange letters with home; for most of its 180 sol mission on Mars, Beagle 2's signals will take nine minutes to reach us via Mars Express and Mars Odyssey.
There is not enough space in this exhibition to give a comprehensive record of either voyage. However, sufficient material is given to show visitors some fascinating topics and comparisons between the 19th- and 21st-century missions. As the exhibition runs until September 2003 it will overlap with Beagle 2's voyage from Earth to Mars and anyone wanting to learn about the little Beagle will enjoy it.
John Bridges, Department of Mineralogy, Natural History Museum.
|Early EarthThe Early Earth: Physical, Chemical and Biological development, eds C M R Fowler et al., Geological Society, London, Special Publication 199, hbk £85, ISBN 1 86239 109 2.|
Archaean rocks are the oldest on Earth, the pale, striped granite gneisses at the cores of the continents. Little of their original nature is apparent at first glance, but now and again you will find a layer of startlingly familiar rocks, conglomerates or pillow lavas perhaps, as a sign that the world of over 2500 million years ago had something in common with ours. Just how much remains under debate: was there plate tectonics? How did the continents grow and the atmosphere evolve? What part did life play?
The questions have remained unanswered because the tectonic, geophysical and geochemical signals from the first half of Earth history have been so distorted by the events of the second.
The Early Earth is a compilation of papers that reflects the variety of powerful tools now available to reach back 2000 million years. It arose from a discussion meeting sponsored by the RAS and the Geological Society and reflects this origin in the breadth and depth of the contributions. The editors have collected into one comprehensive volume many diverse aspects of the study of Archaean rocks, from the stable isotope signature of an early biosphere to ancient lithosphere structure. Geology, petrology, seismology, geochemistry and even a bit of astronomy combine to produce a well-structured and informative volume. The Early Earth is a valuable addition to the literature, both of the Archaean eon and of planetary processes in general. And it reflects the state of progress in earth sciences today, in which ever-more sophisticated tools unravel the history of the planet.
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