Are comets at the origin of life on earth ?

For over 60 years now, the University of Liege Astrophysics and Geophysics Institute (IAGL) has been concerned with the study of comets. Again today, four of its researchers have just made a major discovery for the understanding of these celestial objects : a lot of extra heavy nitrogen detected in two comets suggests the presence of great quantities of complex organic molecules in them. The results obtained by the Liege team will be published in the next number of the international magazine, Science (issue 12 sept.). They will help to revive the debate on whether comets present a link to the origin of life on earth.

A comet is composed of an ice and dust nucleus some kilometres in diameter. It spends most of its time at the outer limits of the solar system, but when it happens to venture nearer the sun, it warms up and develops a tail made of gas and dust which can stretch to millions of kilometres. During their early existence, the planets were bombarded by comets. As has been suggested for several decades, the latter may have provided a large part of the water and organic molecules necessary for the appearance of life on earth. Comets which escaped these violent collisions conserve in their ice the imprints of the physico-chemical conditions prevailing in the solar system at the time of its formation, 4.6 billion years ago. Thus, the study of these « hairy » objects is indispensable for the understanding of the history of the solar system and of our planet in particular.

During its passage in 1997, the Hale-Bopp comet was exceptionally brilliant. This was an opportunity, seized by Professor Claude Arpigny and his team, to point in that direction the NOT (Nordic Optical Telescope) sited in the Canaries, and study the comet’s chemical composition. These observations gave rise to the first optical detection of the heavy nitrogen isotope in the comet. Their analysis showed that the comet contained too large a quantity to come only from simple molecules, whose abundance is known. It thus suggests the presence of a great quantity of complex organic molecules in these comets.

Before announcing their results, the Liege astronomers preferred to wait for confirmation on the basis of more precise measurements. But heavy nitrogen is a difficult element to detect on a less brilliant comet than the exceptional Hale-Bopp. To meet their difficulty, the Liege team had to wait for the UVES high-resolution spectrograph to be set in train on the site of one of the four giant telescopes of the VLT (Very Large Telescope) installed in Chile. The first favourable opportunity for their study was provided by the C/2000 WM1 (LINEAR) comet, visible in the southern hemisphere in autumn 2002. The very high quality observations made with the VLT confirmed the astronomers in their conclusion, which is today published in the prestigious magazine, Science : comets prove to be large reservoirs of complex organic molecules.

The Liege research results open up new horizons in the exploration of comets and at the same time provide precious clues to the early existence of the solar system. This does not, however, imply that comets contain living organisms : the physico-chemical circumstances under which organic molecules can evolve into living organisms probably do not exist in comets. On the other hand, these new results are concordant with the hypothesis that complex organic molecules necessary to the appearance of life may have been deposited on the earth by comets some four billion years ago. So we see that, whereas their presence in the sky used to be a cause for panic, comets may well be, in fact, messengers of life.