Comets are some of the solar system's most primitive building blocks, with many dating to soon after its formation. Scientists think that these dirty snowballs probably helped seed Earth with key ingredients for life, such as organic compounds.
The European Space Agency's (ESA) Rosetta spacecraft is helping scientists learn more about the role these icy nomads have played in the evolution of the solar system and life on Earth by analyzing the composition of Comet 67P/Churyumov–Gerasimenko. In August, Rosetta became the first spacecraft to orbit a comet, and in November, its Philae lander became the first probe to make a soft touchdown on a comet's surface. Rosetta is also the first mission to escort a comet as it travels around the sun. [See images from ESA's Rosetta mission]
Now, Rosetta has helped solve a mystery about how Earth became the watery world it is today. Before Rosetta began orbiting Comet 67P/C-G in August, it was using an instrument known as ROSINA (short for Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) to analyze the chemical fingerprint of gases in the comet's fuzzy envelope. Scientists focused on data from the instrument regarding water to help uncover whether asteroids or comets delivered the water in Earth's oceans.