  Comets contain abundant amounts of organic and inorganic species. Many of the
volatile molecules in comets have also been observed in the interstellar medium
and some of them even with similar relative abundances, indicating formation
under similar conditions or even sharing a common chemical pathway. There is a
growing amount of evidence that suggests comets inherit and preserve
substantial fractions of materials inherited from previous evolutionary phases,
potentially indicating that commonplace processes occurred throughout
comet-forming regions. Through impacts, part of this material has also been
transported to the inner planetary system, including the terrestrial planets.
While comets have been ruled out as a major contributor to terrestrial ocean
water, substantial delivery of volatile species to the Earth's atmosphere, and
as a consequence also organic molecules to its biomass, appears more likely.
Comets contain many species of pre-biotic relevance and molecules that are
related to biological processes on Earth, and have hence been proposed as
potential indicators for the presence of biological processes in the search of
extraterrestrial life. While the delivery of cometary material to Earth may
have played a crucial role in the emergence of life, the presence of such
alleged biosignature molecules in the abiotical environment of comets
complicates the detection of life elsewhere in the universe.

Earth and Planetary Astrophysics