Where life began

How life began on Earth is one of the most fascinating problems in modern science. Scientists are piecing together the evidence, and are showing increasing interest in the composition of comets.

These icy leftovers from the formation of the planets may hold the key to unlocking the mysteries of life’s beginning. ESA’s comet chaser Rosetta is leading the quest to find out.

It seems highly unlikely that life in the form of biological cells began in comets. However, there is now very good evidence that some of the so-called chemical ‘building blocks’ of life – organic molecules – can be found in comets.

These chemicals can be seen using techniques such spectroscopy, in which light from the comet is split into a rainbow of colours and analysed for the dark lines that the chemicals create. More importantly, it will be possible to carry out another technique called mass spectrometry with instruments on both Rosetta orbiter and lander. This will give even more accurate details of the comet’s chemical composition.

The nucleus of Comet HalleyAccess the image

Early spectroscopic studies of comet tails, by British astronomer Sir William Huggins, showed that these organic molecules included the highly noxious gas, cyanide. This knowledge caused a panic when, in 1910, the Earth was expected to pass through the tail of Comet Halley. Newspapers ran stories of mass asphyxiation from space.

Life could arise in a wide variety of environments, for example on the satellite of a giant planetAccess the image

In the event, Earth missed the tail and even if it had passed through, the gases in a comet’s tail are so rarefied that no ill effects would have been felt. The Earth had, in fact, passed through a comet’s tail in 1861 without disaster.

Things are very different now. Some scientists believe that, far from inhibiting life, comets may have played a leading role in creating it.

Comets colliding with the early Earth may have seeded our world with the chemicals necessary for life to begin. The icy nature of the comets almost certainly contributed to the quota of water that now exists in Earth’s oceans.

Comet 67P/Churyumov-GerasimenkoAccess the image

By studying Comet 67P/Churyumov-Gerasimenko in situ, Rosetta will produce a full inventory of organic chemicals in the comet, thereby investigating the role of comets in the origin of life on Earth.

In particular, it will search for a collection of molecules known as ‘left-handed’ amino acids. These are the 'bricks' with which all proteins on Earth are built.

During tests of some of Rosetta’s instruments, it was demonstrated that they will be capable of detecting these amino acids. Rosetta will try to determine the ratio of right- and left-handed amino acids in the most primitive bodies in the Solar System to see if, as some astrobiologists believe, left-handed amino acids did originally come from space.

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