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Summary
When Europe's Rosetta spacecraft started studying comet 67P/Churyumov-Gerasimenko late in 2014, some scientists feared the comet might turn out to be a boring lump of ice and dust. They needn't have worried. Papers in this issue of Science show that 67P is pocked with pits, incised by cracks and cliffs, and decorated with ripples and flows of dust—all signs of an extraordinarily active place. Many of the intriguing landforms testify to the power of the sun, which heats up 67P during part of every orbit, igniting jets of gas and dust that resculpt the surface of the comet. Other discoveries could be primordial, dating from the comet's formation more than 4.5 billion years ago. Mission scientists say the complexity of the comet suggests that the comet-forming regions of the early solar system were more turbulent and chemically diverse than theorists have thought.
