  In situ research on cometary chemistry began when measurements from the
Giotto mission at comet 1P/Halley revealed the presence of complex organics in
the coma. New telescopes and space missions have provided detailed remote and
in situ measurements of the composition of cometary volatiles. Recently, the
Rosetta mission to comet 67P/Churyumov-Gerasimenko more than doubled the number
of parent species and the number of isotopic ratios known in comets. 39 of the
66 parent species have also been detected in pre- and proto-stellar clouds,
making the similarities of cometary ices with pre-stellar material very
intriguing. Most isotopic ratios are non-solar. The variations in D/H in water
between different comets indicate a large range in the protoplanetary disk
where comets formed. All of these results point to a non-homogeneized
protoplanetary disk where comets received their material. This diverse origin
is in contrast to the Sun, who received its material from the bulk of the
collapsing cloud. Detection of N2 and Ar in the coma of 67P and the presence of
very 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 standing
question about the origin of the terrestrial atmospheric xenon. While we can
exclude comets as being the source of the bulk terrestrial water due to their
generally high D/H, the noble gases in the Earth atmosphere are compatible with
a cometary delivery. This means that the amount of organics delivered by comets
may be highly significant.
