Free Astronomy Magazine November-December 2014

15 NOVEMBER-DECEMBER 2014 ASTROBIOLOGY tope of hydrogen that is heavier than nor- mal hydrogen due to a neutron bound to a proton of the atomic nucleus. Deuterium is quite rare and, in fact, for every million atoms of normal hydrogen (H) only 26 are of the heavy type (D), but this ratio increases by 6 times considering its presence in water molecules, which can be explained by a slightly higher speed of the reactions that create heavy water com- pared to those that create normal water. Precisely because small mass differences between isotopes cause differences in their behaviour during chemical reactions, it is possible to understand, staring from the ratio between deuterium and hydrogen in R endering of a permanent- ly shadowed cra- ter near the lunar south pole, in which water ice has been pre- served. [NASA/ GSFC/SVS]