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Comets are small celestial bodies consisting mostly of dust and gases (dirty snowbals) that move in elongated elliptical or nearly parabolic orbits around the sun. Many end up colliding with the Sun.

Comets show substantial amounts of organic material as nicely demonstrated by the European cometary mission Giotto in 1986. A future in situ study of organic compounds in cometary material will also be done during the Rosetta mission and its landing platform. On average, dust particles ejected from the Comet Halley nucleus contain 14% organic carbon by mass. About 30% of cometary grains are dominated by the light elements C, H, O, and N, and 35% are close in composition to carbonaceous chondrites.

Among the molecules identified in comets are hydrogen cyanide and formaldehyde. The presence of purines, pyrimidines, and formaldehyde polymers has also been inferred from the fragments analyzed by Giotto and Vega mass spectrometers. There is no direct identification of the complex organic molecules present in the dust grains and in the cometary nucleus.

Many chemical species of interest for exobiology have been detected in Comet Hyakutake in 1996, including ammonia, methane, acetylene, acetonitrile and hydrogen isocyanide. In addition to hydrogen cyanide and formaldehyde seen in several earlier comets, Comet Hale-Bopp was also shown to contain methane, acetylene, formic acid, acetonitrile, hydrogen isocyanide, isocyanic acid, cyanoacetylene, and thioformaldehyde.

Cometary grains might, therefore, have been an important source of organic molecules delivered to the primitive Earth. The study of micrometeorites collected in the Greenland and Antarctic ice sheets shows that the Earth captures interplanetary dust as micrometeorites at a rate of about 50-100 tonnes per day. About 99% of this mass is carried by micrometeorites in the 50-500 ┬Ám size range. This value is much higher than the most reliable estimate of the meteorite flux, i.e. about 0.03 tonnes per day.

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Fig. 1: Comet Ikeya-Zhang animation by Ian Hodgkins.

Fig. 2: Comet Halley by W. Liller.