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29 august 2020

A stupid glitch in my university library's system, too tedious to explain, cut off my access to new e-books for almost a year. I finally discovered a workaround, and got back into the game with William Sheehan's comforting, informative, and handsome book about the planet Saturn.

Planetary science – which I follow solely in excellent popular writing like Sheehan's – is an intriguing pursuit. The study of exoplanets, on the one hand, is all about generalizing from small data points. Planets that orbit other stars are so far away that they offer nothing more than literal data points. In the past couple of decades, scientists' idea of "planet" as a constituent of our universe has involved study of a rapidly growing dataset.

The science of nearby planets, though, concerns the contingent details of hyperlocal, unique objects. Take the rings of Saturn, which comprise much of Sheehan's text and most of his illustrations. Much study of Saturn's rings has been devoted to specifying what the heck they are, without thought of how this specification could be generalized to the wider heavens. For over 350 years, when Galileo first noticed them as little blobs next to the planet, the rings of Saturn were sui generis, until the inadvertently hilarious 1977 discovery of rings around Uranus. Even after rings had been discovered around Jupiter and Neptune, the value of knowledge about any of the ring systems was fairly low. They do offer an interesting workshop in celestial mechanics, but the rings of Saturn are mainly interesting to humans because they are beautiful and unusual. Study of them is pure research at its most academic and at its best.

Saturn is a bright object, though not as dazzling as Jupiter or Venus and not as red as Mars. Its great appeal, again, lies in the rings, which are instantly recognizable – even though I daresay that most people who know them instantly by sight have never seen them directly. Seeing Saturn's rings through a telescope for the first time is almost like a joke. What appears as a speck to the naked eye and a small disc through binoculars is, in even a beginner telescope, suddenly and iconically Saturn.

Unless you look during one of the phases when the rings are edge-on to Earth, in which case you see just the disc of Saturn. This was the issue that had Galileo scratching his head: sometimes he could see blobs on either side of the planet, and sometimes he couldn't. The old legend of Saturn eating his children seemed weirdly prescient. Later on, better telescopes increasingly provided better and better definition of entire ring systems, as well as the moons that "shepherd" the rings and maintain them in place. Sheehan's book is a history of astronomy via a single planet; among his themes are the contributions of amateur observers and the curious way in which earthbound observations that originally seemed cranky (like the detection of "spokes" within the rings) were later confirmed by spacecraft.

The rings of Saturn are apparently made of ice: good old conventional water ice like the stuff in your freezer. The rings consist of countless tiny particles of ice, far smaller than the cubes you have on hand. Ice reflects light well, but Saturn's rings are far brighter than the similar rings of other planets. Sheehan explains that the rings of Saturn are relatively brand-new. They are only somewhere between ten and 100 million years old. Which seems old, but in astronomical time it is yesterday. At the oldest estimate, the rings have only been around for about two percent of the life-span of Saturn itself. Rings around other planets are older and have accumulated more dust over the aeons, and are thus much darker.

People will never land on Saturn. The planet has no real surface, for one thing, and is full of noxious corrosive gases. Pity, because seeing the rings from Saturn would be a tourist attraction on the grand scale. People might some day land on one of Saturn's moons. Inhospitable as they too might seem, covered in ice or smog or worse, they may well harbor some forms of life. Water is abundant on some moons of Saturn, and beneath forbidding surfaces there might be some biological activity akin to that which takes place far from sunlight in the depths of terrestrial oceans. Till then, we will have to send up more spacecraft. They only cost a few billion a throw. We waste far more on amusements – like reading, and writing for the Internet – that tend to do little but dull our perceptions of our own planet. We might as well chip in to learn about others.

Sheehan, William. Saturn. London: Reaktion, 2019.