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Colliding Worlds - Simone Marchi ****

The title of this book recalls Immanuel Velikovsky’s Worlds in Collision, one of the most notorious works of pseudoscience from the 1950s, which argued that human history since Biblical times has been shaped by collisions between planets and other bodies in the Solar System. As outrageous as Velikovsky’s theories were, they contained a tiny grain of truth. On a much longer timescale – billions of years rather than millennia – the Solar System really has been shaped by collisions. And that’s what this wide-ranging new book by Simone Marchi is all about.

The planets were built by collisions – first through the aggregation of dust and pebble-sized fragments, then by the merger of larger protoplanets. Chance played a part here – computer simulations show very different outcomes depending on whether it’s a grazing collision or head-on. This could explain why Venus and Earth ended up so different, despite being similar in size and distance from the Sun. And it was another collision – with a hypothetical planet named Theia – that gave us the most obvious thing Venus lacks, namely the Moon.

Collisions were much more frequent – because there was more interplanetary debris around – four billion years ago when life first appeared on Earth. These collisions may have caused problems for newly arisen life forms – or alternatively they may have been crucial to their development. Either way, it’s well-established that subsequent asteroid impacts helped shape evolution by triggering mass extinctions, which freed up niches for new species to emerge. The best-known example is the Chicxulub event 66 million years ago, which killed off the dinosaurs and cleared the way for the eventual emergence of our own species.

In our own time, much smaller asteroid ‘collisions’ can be of great value to scientists when they result in meteorites, which are the easiest way to acquire samples of extraterrestrial material and assess their composition. They also help to establish the age of the Solar System and, in the case of material that originated on Mars, tell us something about that planet too.

Although collisions form the main thread running through the book, it’s actually wider in scope than that, covering both the history of the Solar System, and the history of our understanding of it. That includes our understanding of our own planet, and one of the eye-opening facts is how long it took the academic world to recognise the role played by collisions in geological history. Basically, the topic got lost for decades in the gap between the geology and astronomy departments.

As far as I can tell, this is Marchi’s first book, and it’s his specialist subject – he’s a space scientist at the Southwest Research Institute in Boulder, Colorado. But don’t let that put you off – the book is entirely non-technical, and written very much with the general reader in mind. Marchi’s first-hand accounts of work he was personally involved in – such as NASA’s Dawn mission to the asteroid belt – make especially fascinating reading. For anyone wanting an up-to-date account of the Solar System and the processes that shape it, this is the perfect place to start.

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Review by Andrew May
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