Figure: The pristine comets

Comparison of mid-IR spectra of various astronomical objects with those of the inner and outer disc regions of three young stars

This picture provides a comparison of the mid-IR spectra of interstellar dust grains (upper left), three solar system comets (left) and the outer and inner disc regions of the three young stars observed with MIDI and the VLTI. The similarities of the spectra in the same row are striking. The relative amount of crystalline content increases from the top towards the bottom.

The present observations also have implications for the study of comets. Some - perhaps all - comets in the solar system do contain both pristine (amorphous) and processed (crystalline) dust. Comets were definitely formed at large distances from the Sun, in the outer regions of the solar system where it has always been very cold. It is therefore not clear how processed dust grains may end up in comets.

In one theory, processed dust is transported outwards from the young Sun by turbulence in the rather dense circumsolar disc. Other theories claim that the processed dust in comets was produced locally in the cold regions over a much longer time, perhaps by shock waves or lightning bolts in the disc, or by frequent collisions between bigger fragments.

The present team of astronomers now conclude that the first theory is the most likely explanation for the presence of processed dust in comets. This also implies that the long-period comets that sometimes visit us from the outer reaches of our solar system are truly pristine bodies, dating back to an era when the Earth and the other planets had not yet been formed.

Studies of such comets, especially when performed in-situ, will therefore provide direct access to the original material from which the solar system was formed.

Credit: European Southern Observatory (ESO)

http://www.eso.org/outreach/press-rel/pr-2004/pr-27-04.html