C. Ceccarelli (Universite J.Fourier de Grenoble, IPAG, Grenoble, France),
P. Caselli (University of Leeds, School of Physics and Astronomy, United Kingdom),
D. Bockelee-Morvan (Observatoire de Paris, LESIA, France),
O. Mousis (University of Franche-Comte, Institut UTINAM, France),
S. Pizzarello (Arizona State University, Dpt. of Chemistry & Biochemistry, United States),
F. Robert (Museum National d'Histoire Naturelle, LMCM, France),
D. Semenov (MPIA, Germany)

Once upon a time, there was a small cold condensation of gas and dust in an interstellar cloud broken into several clumps and filaments of different masses and dimensions. Then, about 4.5 billions years ago, the condensation became the Solar System. What happened to that primordial condensation? When, why and how did it happen? Answering these questions involves putting together all of the information that we have on the present day solar system bodies and micro particles. But often, this is not enough: combining it with our understanding of the formation process of solar type stars in our Galaxy largely enhances our capacity to reconstruct what happened to ours. In this respect, the chemical composition of matter during the formation process is an extremely useful and, sometimes, almost unique tool. One specific example is offered by the molecular deuteration, namely the ratio between the abundances of the H- and D-bearing isotopologues of the same species. The few examples that we will give in the following show the huge diagnostic power contained in the measured molecular deuteration, a power that we plan here to fully review and exploit, bringing together our knowledge on interstellar astrochemistry, solar type star and planet formation, comets and solar system small bodies, and meteoritic material. Our first example is about the Earth water. Terrestrial water that possesses an enhancement of HDO with respect to H2O by about one order of magnitude with respect to the D/H elemental abundance (1.6x10^(-5)). On the on other hand, bulk mineralogical composition of the Earth’ crust is remarkably similar to that of most primitive chondritic meteorites, which have formed at "dry", water-free conditions. Since comets and some of carbonaceous chondrites show water deuteration of the same order of magnitude, it was suggested that the Earth water was mostly delivered by comets and/or icy bodies raining on the early Earth. The thesis is debatable and debated, and we will review all the discussion on the subject. Nonetheless, the example means to show how the water deuteration in terrestrial water brings us back to the primitive Earth history and likely even earlier than that. What more, the recent discovery by Herschel of the HDO/H2O abundance ratio in comet 103P/Hartley2, twice lower than the value measured in comets believed to originate farther from the Sun, has triggered a debate on the very origin of the comets in the Solar System and on the thermal structure of the Solar Nebula, requiring as well a new class of models. Again, we will review the on going debate. Our second, notable example is provided by the discoveries that meteoric material is highly enriched in deuterium. This is true for soluble and insoluble material, as well as for clay minerals in carbonaceous meteorites. How this high degree of deuteration was acquired? Through inheritance from previous epochs or at the latest phases of the Solar Nebula formation? If the latter is true, where and how? The lessons that we have learned from the studies of molecular deuteration occurring during the pre-stellar to proto-planetary phases will enormously help to understand the origin of the organic material and clay minerals in meteorites and, consequently, how the Solar System formed. In this Chapter we will review the present understanding of the deuteration process across the various phases of a solar type star and planet formation, and compare it with the observations and models of various objects of the Solar System. We will use the deuteration as the Ariadnes thread. The legend says that he thread was given by Ariadne to Theseus to get out from a difficult situation, the Minotaur's labyrinth. Theseus followed the thread that he had previously unrolled to get into the labyrinth to get out of it. Similarly, deuteration can help us to find the way out of the labyrinth of what happened to our Solar System.

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