The context   

Comets are small icy planetesimals formed in the outer solar system. The nucleus, typically a few kilometers in diameter,  is essentially constituted of water ice mixed with carbon oxides, methane, ammonia, and of dust particles. When the comet approaches the Sun, the ices sublimate, forming a gaseous and dusty coma. Radiation and solar wind blow this material to form the spectacular cometary tails.

Because comets are thought to be remnants of the early stages of the solar system, they are considered as the most pristine solar system bodies. Understanding their origin, evolution ans composition is a clue to the history of our solar system. Moreover comets contain complex organic molecules, and may play a key role in the transfer of water and organics from the interstellar medium to the early Earth, then contributing to the origin of life.

Recent work and projects

We have started a systematic study of comets using high-resolution spectroscopy with the Ultraviolet Visible Echelle Spectrograph (UVES) of the ESO Very Large Telescope (VLT) located in the Paranal Observatory . Thanks to the high throughput of the instrument, faint comets can now be investigated, namely to identify the spectral signature of new species, to determine chemical compositions and isotope ratios, as well as spin temperatures via ortho-to-para molecular ratios. Part of this research is done in collaboration with the McDonald Observatory Planetary Systems group, the Besançon Observatory, the European Space Agency and the Leiden Observatory.

First focusing on the measurement of the carbon 12C /13C and nitrogen 14N /15N isotope ratios in various comets at small and high heliocentric distances, we found surprisingly low 14N /15N isotope ratios from the CN radical, a possibly key ingredient to prebiotic chemistry. Our observations suggest the existence of several sources of cometary CN with very different isotope ratios. A few pictures illustrate the method.

These studies should be extended during the Deep Impact Space Mission. On July 4, 2005  Comet Tempel 1 will be impacted by a 370 kg mass which should eject ice and dust debris from below the comet surface, revealing for the first time fresh unaltered material. The impact  will produce a brightening of the comet, making it observable from the ground.  Using the VLT UVES spectrograph, we will obtain high-resolution spectra before and after the event to measure isotopic ratios and determine spectral changes possibly revealing new species.

   Preprints  (astro-ph)
   All publications since 2000  (ADS)
   Publications in refereed journals since 1995

Press releases
14.09.2003 - Space Flight Now - Europe opens new window to our origins
  12.09.2003 - ULG - Les comètes sont-elles à l'origine de la vie sur Terre?
  12.09.2003 - ESO-PR - Optical  Detection of Anomalous  Nitrogen in Comets 

  21.05.2001 - APOD - Another Comet LINEAR breaks up 
  18.05.2001 - ESO-PR - Comet LINEAR splits further

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