Simple organic molecules containing carbon and nitrogen are the essential building blocks of the complex organic chemistry found on Titan, on the early Earth or possibly on early Mars. How did these simple organics come to be on these bodies in the first place? Were they indigenous? Or were they deposited by impacting comets together with large amounts of water?

Comets are remnants from the formation of the Solar System

Scientific opinion is shifting towards the view that comets have played a significant role in seeding the chemical building blocks of life. "An impacting comet has the power to destroy life on Earth, but we now think that comets may have also helped to create life in the first place," says Gerhard Schwehm, ESA's project scientist for the Rosetta mission.

Comets are loose agglomerations of dust and ice that orbit the Sun in the outer reaches of the Solar System. Unlike the planets, they are thought to have undergone little chemical processing since their birth with the rest of the Solar System 4.5 billion years ago. Preserved inside them may be unadulterated samples of the raw material out of which the Solar System formed.

"We know that organic material is present in comets from remote-sensing observations from Earth and spacecraft such as Giotto, which flew by comet Halley in 1986. But we haven't been able to tell how complex the organic molecules are," says Simon Green a Solar System scientist from the Open University, Milton Keynes, United Kingdom and co-investigator on ESA's Rosetta spacecraft. Finding out will be one of Rosetta's tasks when it arrives at Comet Wirtanen in 2012. "The Rosetta orbiter and lander will carry sophisticated payloads that will study the composition of the dust and gas released from the comet's nucleus and help to answer the question: did comets bring water and organics to Earth?", says Schwehm.