Simone Marchi - Colliding Worlds : How Cosmic Encounters Shaped Planets and Life

Colliding Worlds reveals the untold story of how violent cosmic collisions shaped the formation and evolution of rocky planets, including our own. It is now well established that our Moon was born in a colossal collision, but more subtleand often counterintuitiveconsequences of ancient collisions are emerging, thanks to sophisticated models of Solar System formation, the analysis of ancient rocks both from Earth and extraterrestrial, and most exciting of all, the results of space missions to Mars and to several asteroids.

The signs of cosmic collisions are widespread: anywhere we lay our sight on a solid planetary surface, from the innermost planet Mercury to asteroids such as Vesta and Ceres, we find countless craters that have resulted from the impact of rocks from space. And rocks from the Moon, Mars, and asteroids that land on Earth as meteorites bear signs of ancient catastrophes. The Earth must surely have had its fair share of big impacts. But nature plays tricks on us, and our geologically active Earth has erased traces of these violent events.

The painstaking work of geologists, geochemists, and planetary scientists shows that the Earth was not immune to these catastrophic events; quite the contrary. Collisionsand lots of thempunctuated the evolution of our planet. We humans owe our existence to a random asteroid that played havoc some 66 million years ago, famously causing the extinction of most dinosaurs, among 75 percent of all living species. But there is more to the story than meets the eye. This book presents the latest research, drawing on the data obtained by Nasas Dawn mission to the asteroids Vesta and Ceres, by the Mars rovers, and missions to Mercury and Venus, as well as the exciting findings from ground and space telescopes concerning systems of exoplanets. With further new space missions planned for the 2020s, the future will certainly bring new discoveries, but the overall picture of the violent early days of the Solar System is now coming into focus, and that is the story I will recount here. There is much fascinating detail I have had to leave out, but keen readers are directed to the endnotes, which provide some additional information and references.

The story told in these pages follows closely my own scientific research, taking the present form thanks to researchers from different disciplines who share a common interest on these fascinating topics. I could not have written this book without their genuine interest. Aspects of the book have been shaped by discussions with, among others, Jeff Andrews-Hanna, Jim Bell, Ben Black, Bill Bottke, Robin Canup, Clark Chapman, Robert Citron, Rogerio Deienno, Cristina De Sanctis, Nadja Drabon, Lindy Elkins-Tanton, Luigi Folco, Laurence Garvie, Vicky Hamilton, David Kring, Hal Levison, Don Lowe, Tim Lyons, Tyler Lyson, Hap McSween, Alessandro Morbidelli, David Nesvorny, Cathy Olkin, Ryan Park, Silvia Protopapa, Carol Raymond, Karyn Rogers, Raluca Rufu, Everett Shock, John Spencer, Kleomenis Tsiganis, and Rich Walker. Thank you all for your advice on specific aspects. Needless to say, any errors remaining in the final text can be laid at my door. A special thank you to the late Jay Melosh, a pioneer in collisional processes, who provided a supportive review in the early stages of this project.

A big thank you goes to my editor, Latha Menon, for having believed in this project, and for outstanding professional guidance throughout the writing of this book. Thanks to Jenny Nuge for helping sort out the graphical aspects of the book. I am indebted to Tyler Lansford for kindly reading the first draft of the book and taking on the daunting task of making sense of my English.

I kept the writing of this book secret, even from close friends. They now ask, how did you find the time? I wrote it thanks to an assiduous, almost maniacal, approach. To cope with the vast scope of the book amid busy life, I set aside all the Sunday mornings over the span of a year, the alarm clock set at 6 a.m., regardless of whichever part of the world I laid down the previous night. For this, I am grateful to my wife, Cristina, who lovingly and unconditionally supported my endeavor by reading early drafts, discussing broader implications, and putting out family fires when needed, allowing me to concentrate on my book. The bulk of the writing took place at Trident Booksellers & Caf in Boulder, and was fueled by an untold amount of coffee. On many mornings, I could be found writing these pages tucked away at a small table at the back, amidst shelves of secondhand books. I delivered the manuscript just as the whole world was starting to shut down for the COVID-19 pandemic.

Enough said.

Simone Marchi

Boulder, Colorado

March 2021