(2015) David A. Rothery, Oxford University Press, £7.99, pbk, xv + 153pp, ISBN 978-0-198-73527-4
The Oxford Very Short Introductions series began in 1995 and now contains over 400 volumes on a wide range of topics, as they say, "for anyone wanting a stimulating and accessible way into a new subject. They are written by experts, and have been translated into more than 40 different languages" . The books are compact, lightweight, and an easy fit even for quite a small pocket – but the print size is small and the paper, though strong, is thin, so this supposedly short introduction to 'moons' contains a great deal of material. For obvious reasons it is almost all about moons of the Solar System, though at the end it discusses 'exomoons' of planets of other stars, including the possibility that habitable Earthlike worlds could orbit gas giant planets there (as in Return of the Jedi and Avatar), and that any inhabitants might have difficulty believing that the Earth, devoid of a gas giant, is habitable
It is strange now to recall how short the list of known moons was even in the 1970s, and for how long it had remained that way. I grew up knowing that apart from our Moon, Mars had two moons, Jupiter twelve, Saturn nine, Uranus five and Neptune two – and that was it. Most of them had been known since the 19th century, with few discovered since, so the discovery of Pluto's largest moon. Charon. in 1978 was a sensation for that reason, and it was soon to be followed by a cascade of discoveries by spacecraft which continues to this day. By 2015, as Rothery notes, even 184 of the asteroids were known to have moons, and the total moon population in the Solar System runs into hundreds and is still rising.
The book covers a great deal of ground despite its size, so much so that it is easier to note what is not here. There is no discussion of the resources of the moons and their possible uses – not even Phobos, which already features prominently in NASA's plans for manned missions to Mars. It precedes the flyby of Phobos by Mars Express, which not only confirmed the existence of large voids within the moon, as Rothery expects, but unexpectedly revealed that Phobos is composed of Mars rocks, blasted into orbit by impacts and accumulated into the 'rubble pile' which Rothery describes on the assumption that Phobos and Deimos were captured asteroids. That is the only instance I have spotted where the book is already dated. Instead it was full of information that was wholly or partly new to me, and a short list of those may serve to illustrate the wide range of topics covered:-
• Of the impact basins on the Moon, Aitken is the oldest at 4.1 – 4.0 billion years, and Orientale the youngest at 3.7. Their filling by lava was not a single episode but a gradual process beginning 3 billion years ago and lasting 2 billion years.
• The relative abundances of the three main oxygen isotopes are the same in Earth and Moon rocks, indicating thorough mixing in the collision of Gaia and Theia 4.5 billion years ago, and foreshadowing this year's discovery that it was a head-on impact rather than a graze.
• The Apollo landing sites are protected by the US Congress Apollo Lunar Legacy Bill, but the Russian Lunokhod landers are privately owned, having been sold at auction by their makers in 1983.
• Ganymede, largest moon of Jupiter, may have concentric shells of different forms of water ice (thought to exist on Jupiter itself in James Blish's classic story 'Bridge').
• A scientific paper published in March 1979 predicted that Io would have a molten interior, just 6 days before Voyager 1 discovered the volcanic evidence for it.
• The internal heating and water vapour plumes of Enceladus are powered by 2:1 resonance with the moon Dione, but the complex surface of Miranda may be due to a previous 3:1 resonance with Umbriel, rather than shattering and reassembly as first thought.
• The trojan moons of Saturn may be less than 1 billion years old. In addition to the four groups of outer satellites of Jupiter, presumably formed by the breakup of captured asteroids, another 20 at least are not members of any known group.
• Only the brightest of the narrow rings of Uranus is bounded by the orbits of shepherd satellites, though the outermost one surrounds the orbit of Mab. The inner rings may have been formed by collisions within the last 600 million years. The lava flows from Io's volcanoes are 'too hot for sulphur' (i.e. above its vaporisation point?) and so must contain molten rock, on which sulphur condenses as it cools.
• The apparently smooth areas on Enceladus appear so from a distance due to intense fracturing, not to ice deposition as previously thought.
• The older craters on Miranda are draped with mantles of dark material, origin unknown.
• The asteroid Hermes, the first 'Earth-crosser' to be discovered, is actually binary, with two components 400 metres across and 1200 metres apart.
• All Pluto's moons are prograde with respect to the planet, retrograde with respect to the Solar System, so may all be fragments blasted off Pluto by impacts rather than captured from the Kuiper Belt.
•The orange dwarf star J1407's gas giant planet is confirmed to have a large system of rings, implying that it also has multiple moons.
I have listed these discoveries in the order they appear in the book, which demonstrates not just that there is a wide range of topics and a great deal of information, but also that it is organised by topics rather than dealing with each planet and each moon in turn. While the book is a great source of information, which I could see myself taking to lectures or conferences for quick reference, I could therefore wish that the index and the tables at the end were more comprehensive. For an example which has already happened, I wanted to look up those groupings of Jupiter's outer satellites. But the table says only "Fifty-nine outer" and gives the overall range of their orbits, and the index has no entry for Jupiter itself, let alone subheadings. Unless you remember the name of at least one of the moons from which the groups take their names, you have to look through all the entries for 'moons/of Jupiter' till you find them. Just a little more detail there would have been helpful – but that's a minor grump about a very informative and useful book, far more so than the words 'Very Short Introduction' would suggest.
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