Free Astronomy Magazine November-December 2014

ASTROBIOLOGY lution of a protoplanetary disk in which heavy water had already been eliminated by the chemical processes induced by our star. The period considered was the first million years in which the Sun started to shine, of course while taking into account the lower incidence of galactic cosmic rays (caused by the solar magnetosphere), the simultaneous production of less energetic solar cosmic rays, the X-radiation coming from the same Sun and from radionuclides decay. Even considering possible variables, the results of the study clearly indicate that the amount of deuterium present in the ices of the solar system cannot have been all processed within the protoplanetary disk, since those processes seem too slow in the period considered and substantially non-influential in later periods, when the ices had already been incorporated in the planetesimals. This means that a significant portion of water −both normal and heavy− present today in the solar system is primor- dial and derives directly from ice present in the protosolar cloud. And this also applies to the Earth's water, since it derives in large part from falling comets and asteroids. Researchers estimate that between 30% and 50% of the water that bathes our planet is older than the solar system −figure that in the case of comets goes up to 60-100%. Besides telling us that we swim in a liquid that in part can have an age much closer to that of the universe than that of the Earth, the conclusions drawn by Cleeves's team (published in Science in late September) tell us above all that the water in our solar sys- tem cannot be much more abundant than that available to other planetary systems, and this applies also to the quantities of pre- biotic organic matter contained in the water. The conditions that can lead to the appear- ance of life are therefore not specific to a particular planetary system, but, on the con- trary, they are more linked to the properties of the interstellar medium and to the large- scale uniformity of the latter. Obviously stars which are very different from the Sun will have an equally different influence on the ices inherited from the interstellar medium, but the majority of the stars are solar-type dwarfs or a little smaller, and it is to these that we look with more optimism in the search for extraterrestrial life. n A rtistic repre- sentation of the icy surface of Europa, a satel- lite of Jupiter, that with its huge subsurface ocean is considered, after the Earth, the environment most suitable for life in our solar system. [Anony- mous]