Free Astronomy Magazine November-December 2014

ASTROBIOLOGY reasons we do not have direct knowl- edge of the efficien- cy with which the protosolar cloud pro- duced heavy water, but we can deduct it from the studies con- ducted on the oldest and best-preserved material at our dis- posal, namely certain types of meteorites and long-term com- ets, real and actual relics dating back to the time of the condensation of the cloud from which the solar system formed, very little processed and thus faithful cus- todians of the com- position of the cloud itself. In knowing with sufficient preci- sion the typical D/H ratios of the water in different regions of the solar system, Cle- vees and colleagues decided to verify if those values could have been reached starting from scratch, through the radiative action of the emerg- ing Sun. The team has therefore created some models capable of simulating the evo- water molecules, under which conditions those molecules formed. The necessary in- gredients for the formation of water with high D/H ratio are: low temperatures, oxy- gen, molecular hydrogen (H 2 ) and an ioni- zing source. The environ- ments that best satisfy these requirements are the denser regions of the interstellar medium. The temperature inside them is typically only ten kel- vin (approximately minus 263 degrees C), oxygen is present since dispersed in the universe by gen- erations of supernovae, hydrogen is omnipresent and the ionizing flux is ensured by the galactic cosmic rays. For obvious T he volcanoes of ice water found on Encela- dus, moon of Sat- urn, are one of the many exam- ples of how the precious liquid is widespread in the solar system. [Michael Carroll] Below, the radar map of the north polar region of Mercury, super- imposed on an image in white light produced by the MESSEN- GER probe. The coloured areas are the least lit up by the Sun. The yellow re- presents deposits of water ice pre- sent at the bot- tom of perma- nently shad- owed craters. [NASA/Johns Hopkins Univer- sity/Carnegie In- stitution of Washington/Na- tional Astronomy and Ionosphere Center, Arecibo Observatory]