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In situ research on cometary chemistry began when measurements from theGiotto mission at comet 1P/Halley revealed the presence of complex organics inthe coma. New telescopes and space missions have provided detailed remote andin situ measurements of the composition of cometary volatiles. Recently, theRosetta mission to comet 67P/Churyumov-Gerasimenko more than doubled the numberof parent species and the number of isotopic ratios known in comets. 39 of the66 parent species have also been detected in pre- and proto-stellar clouds,making the similarities of cometary ices with pre-stellar material veryintriguing. Most isotopic ratios are non-solar. The variations in D/H in waterbetween different comets indicate a large range in the protoplanetary diskwhere comets formed. All of these results point to a non-homogeneizedprotoplanetary disk where comets received their material. This diverse originis in contrast to the Sun, who received its material from the bulk of thecollapsing cloud. Detection of N2 and Ar in the coma of 67P and the presence ofvery volatile S2 point to low formation temperature of 20-30 K of the comet.The xenon isotopic ratios measured in 67P can explain the long standingquestion about the origin of the terrestrial atmospheric xenon. While we canexclude comets as being the source of the bulk terrestrial water due to theirgenerally high D/H, the noble gases in the Earth atmosphere are compatible witha cometary delivery. This means that the amount of organics delivered by cometsmay be highly significant.

Cometary Chemistry and the Origin of Icy Solar System Bodies, the view after Rosetta