1 Revision 2 1 2 Carbonic Acid Monohydrate 3 4

12 In the water-carbon dioxide system, above a pressure of 4.4 GPa, a crystalline phase consisting 13 of an adduct of the two substances can be observed to exist in equilibrium with the aqueous fluid. 14 The phase had been found to be triclinic, and its unit cell parameters determined, but the full 15 crystalline, and even molecular, structure remained undetermined. Here, we report new diamond16 anvil cell, x-ray diffraction data of a quality sufficient to allow us to propose a full structure. The 17 crystal exists in the P1 space group. Unit cell parameters (at 6.5 GPa and 140°C) are 18 a = 5.8508(14) Å, b = 6.557(5) Å, c = 6.9513(6) Å,  = 88.59(2),  = 79.597(13), and 19  = 67.69(4). Direct solution for the heavy atoms (carbon and oxygen) revealed CO3 units, with 20 co-planar, but isolated, O units. Construction of a hydrogen network, in accordance with the 21 requirements of hydrogen bonding and with minimum allowed distances between non-bonded 22 2 atoms, indicates that the phase consists of a monohydrate of carbonic acid (H2CO3H2O) with the 23 carbonic acid molecule in the cis-trans configuration. This is the first experimental determination 24 of the crystalline structure of a H2CO3 compound. The structure serves as a guide for ab initio 25 calculations that have until now explored only anhydrous H2CO3 solids, while validating 26 calculations that indicated that high pressures should stabilize H2CO3 in the solid state. If 4.4 27 GPa is the lowest pressure at which the phase is thermodynamically stable, this probably 28 precludes its existence in our solar system, although it may exist on larger, volatile-rich 29 exoplanets. If, however, its range of stability extends to lower pressures at lower temperatures 30 (which possibility has not yet been adequately explored) then it might have been be a stable form 31 of CO2 within the water-rich moons and dwarf planets prior to differentiation, and might still 32 exist on an undifferentiated Callisto. 33 34