Comets are believed to be responsible for delivering between 4.5 and 4 billion years ago huge amounts of water and organics necessary for the development of life on our planet.
They were formed from the gravitational collapse of the presolar nebula and contain inside the nucleus the primordial chemical composition of interstellar matter. In the last 50 years radioastronomers discovered more than 130 complex organic molecules in interstellar clouds and most of them could be found also in cometary comas (Cosmovici and Ortolani, 1984, Nature, 310,122) especially during the fly-by of the GIOTTO spacecraft up to 600 Km from the nucleus of comet Halley in 1986.
As about 80% of the mass of comets and human beings is constituted by H, O, C, N, which are the most biogenic abundant atoms in the Universe, one could easily imagine that comets, like cosmic messengers, might have spread the same biochemical information on all planets of the Galaxy. This vital information contributed, where the chemo-physical conditions were favorable, as in the case of the Earth, to facilitate Life evolution based on the same universal biochemistry.
Thus cometary astronomers focused their efforts during the last 30 years in the search for water and organic molecules in bright comets from ground and from space by applying modern spectroscopic techniques.
In 1994 the world witnessed the much heralded collision of Comet Shoemaker-Levy 9 with the planet Jupiter giving astronomers a unique opportunity to study the consequences of a catastrophic impact in a planetary atmosphere and the subsequent changes in the chemistry and in the excitation conditions of atomic and molecular species.
One of the most important results from the ground based observations was the detection of water at 22 GHz (Cosmovici C.B., et al.,1996, Planet.Space Sci.44, 735) by using a new fast multichannel spectrometer coupled with the 32 m dish of the Medicina (Bologna) radiotelescope.
The very narrow line-width (40 kHz) and the high brightness temperature (20.000 K) of the detected water emission could be explained only by a MASER effect, the first one observed in the Solar System. No water spectral lines from Jupiter's atmosphere were detected before the cometary impacts and the water cloud in the Jupiter ionosphere could be observed for about two months.
These observations were the first evidence that comets are able to deliver huge amounts of water (about 50 billion tons per comet) and also organics in planetary atmospheres, and that also polyatomic molecules may survive the impacts thus rising the fascinating possibility of life development in an appropriate environment.
It was then decided to use this discovery as a powerful diagnostic tool for planetary search outside the solar system where cometary bombardments are occurring today as they took place on our planet billions of years ago. Moreover, calculations have shown that this line can be observed also in water-rich atmospheres where the necessary MASER pumping can be delivered by photo-deposited energy which can affect the molecular level populations.
The observations of Comet Hyakutake C/1996 B2 when it was at only 0.23 A.U. from the Sun permitted the first detection of the 22 GHz water emission line in a Comet (Cosmovici C.B., et al., Planet.Space Sci., 46, 467, 1998). The detection was confirmed in Comet C/2002 V1-NEAT at 0.11 A.U. from the Sun and in other 5 sun-grazing comets, (Cosmovici et al.,Planet.Space Sci, 2014, 96, 22-28).