During the collisions, to very small relative speeds, these grains were united, acrecionaron an a others to form the nuclei of comets, we see as them nowadays. The recent observations in situ of the nucleus of the comet the Halleys, conducted by the space probes FERTILE VALLEY and GIOTTO, and also of grains and of gas (mainly H20) that is come off him give much credit to this hypothesis. In any case, the cometary nuclei, constituted half of ice and half of the dust, were the elementary bricks from which they simultaneously formed the solid nucleus of the giant planets (in which the rest of the gas was grouped by accretion) and a good number of the satellites surround that them. The majority of them still takes the track of an intense bombing of such bricks of hie it (for example Calisto, around Jupiter), whereas others as Europe (another Jupiter satellite) shows a smooth surface that it indicates that there was in them an intense smelting of the ice and the disappearance of impact craters. The origin of the terrestrial water But, how arrived the water at our planet? In the internal zone of the Solar System, the aqueous vapour could not be condensed due to the too high temperature.
In order to include/understand the origin of the terrestrial water, it is possible to be imagined that, formed from a material essentially dry solid, the terrestrial planets were bombed next by a multitude of comet nuclei. These, been born in the outside of the Solar System, would have been projected soon towards the interior by the gravitational disturbances of giant planets, especially of Jupiter and Saturn. Simple calculations of trajectories show then that Venus, the Earth and Mars would have received approximately equal amounts of cometary nuclei. Nevertheless, instead of this purely cometary origin of the water, the scientists prefer another explanation at the moment.