Breakthrough to our origins

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Science  19 Dec 2014:
Vol. 346, Issue 6216, pp. 1433
DOI: 10.1126/science.aaa4569


Long before I dreamed of becoming a scientist, I wondered why Earth was teeming with life, while Mars was a barren, rocky outpost and Venus was shrouded in a dense atmosphere. Somewhere, I read that comets were the reason. Strike a rocky planet with one volatile- and organic-rich comet and an Earth-like environment results. Two comets produce a Venus. No comets: Mars. Of course, this theory was far too simple to explain the differences in the evolution of the inner planets, but no one had ever explored comets up close to know how they might alter a planet's composition and history—until now. Last month, after many years of planning and a decade in transit, the European Space Agency's spacecraft Rosetta reached Comet 67P/Churyumov-Gerasimenko, surveyed landing sites, and deployed its probe, Philae, to execute the first-ever soft landing on a comet's surface (see the News story, p. 1442).

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