WE PROPOSE TO COMBINE MODELS OF PLANET INTERIORS CLIMATE PHASE EQUILIBRIA AND PHOTOCHEMISTRY TO DELINEATE THE HABITABLE ZONE BOUNDARIES AND ATMOSPHERIC COMPOSITIONS OF WATER WORLD EXOPLANETS (WITH AT LEAST 1% WATER BY MASS). KUCHNER (2003) AND LEGER ET AL. (2004) FIRST PROPOSED THE POSSIBILITY OF WATER WORLDS - WATER-RICH SUPER-GANYMEDE EXOPLANETS THAT FORMED FROM VOLATILE ICE-RICH MATERIAL BEYOND THE SNOW LINE BUT THAT NEVER ATTAINED MASSES SUFFICIENT TO ACCRETE OR RETAIN LARGE AMOUNTS OF H/HE NEBULAR GAS. THIS PATHWAY FOR PRODUCING LOW-MASS WATER-RICH PLANETS HAS PLAYED OUT AS A ROBUST PREDICTION OF PLANET FORMATION SIMULATIONS. IF LOCATED AT AN APPROPRIATE ORBITAL SEPARATION FROM THEIR HOST STAR WATER WORLDS MAY HOST A GLOBAL SURFACE WATER OCEAN. HABITABLE WATER WORLDS ARE ESPECIALLY TIMELY AND RELEVANT BECAUSE 1) WATER WORLDS FORMED FROM REMNANT CORES OF EVAPORATED MINI-NEPTUNES COULD BE ONE OF THE DOMINANT FORMATION MECHANISMS FOR VOLATILE-RICH HABITABLE ZONE PLANETS AROUND M DWARF STARS (LUGER ET AL. 2015) AND 2) THEIR LARGER SIZES RELATIVE TO TERRESTRIAL PLANETS MAKE THEM MORE AMENABLE TO OBSERVATIONS WITH CURRENT AND UPCOMING TELESCOPES SUCH AS HST AND JWST. THE CLASSICAL HABITABLE ZONE DOES NOT APPLY TO WATER WORLDS WITH GLOBAL OCEANS. WATER WORLDS LACK EXPOSED LANDMASS FOR CONTINENTAL SILICATE WEATHERING FEEDBACK AND MAY HOST A LAYER OF HIGH PRESSURE ICE (ICE PHASES VI AND VII) THAT SEPARATES THE LIQUID OCEAN AND ATMOSPHERE FROM THE PLANET'S CRUST AND INTERIOR. IN THE ABSENCE OF A CARBONATE-SILICATE CYCLE THE FORMATION OF CLATHRATES AND THE SOLUBILITY OF CO2 IN THE OCEAN DETERMINES ITS CONCENTRATION IN THE WATER WORLD'S ATMOSPHERE. TO DATE STUDIES OF THE SOLUBILITY-MEDIATED HABITABLE ZONES OF WATER WORLDS HAVE ONLY CONSIDERED LIQUID-VAPOR EQUILIBRIA IN THE H2O-CO2 SYSTEM (KITZMANN ET AL. 2015 LEVI ET AL. 2017) WHEREAS WATER WORLDS ARE EXPECTED TO FORM FROM A COMET-LIKE MIXTURE OF ASTROPHYSICAL ICES (INCLUDING H2O CO2 CH4 H2S N2 NH3 H2S AND CH3OH). DUE TO CHEMICAL INTERACTIONS IN THE LIQUID PHASE THE PRESENCE OF OTHER CHEMICAL SPECIES THAT DISSOCIATE IN WATER COULD HAVE AN ORDER UNITY EFFECT ON THE SOLUBILITY OF CO2 (AND CONSEQUENTLY THE ATMOSPHERIC CONCENTRATION OF CO2 AND THE PLANETARY SURFACE TEMPERATURE). A MORE THOROUGH EXPLORATION OF THE SOLUBILITY-MEDIATED HABITABLE ZONE OF WATER WORLDS IS NEEDED. WE PROPOSE TO IMPROVE UPON EXISTING MODELS FOR WATER-WORLD EXOPLANETS 1) BY DEVELOPING THE FIRST SELF-CONSISTENTLY COUPLED MODELS OF EXOPLANET INTERIOR STRUCTURE ATMOSPHERIC RADIATIVE TRANSFER LIQUID-VAPOR THERMOCHEMICAL EQUILIBRIA AND PHOTOCHEMISTRY AND 2) BY SUBSTANTIALLY EXPANDING THE RANGE OF COMPOSITIONAL PARAMETER SPACE CONSIDERED. WE WILL APPLY THESE MODELS TO ASSESS HOW PARTITIONING OF SPECIES BETWEEN THE LIQUID HYDRATE AND VAPOR PHASES AFFECTS THE ATMOSPHERIC COMPOSITION TRANSMISSION SPECTRUM AND THERMAL EMISSION SPECTRUM OF OCEAN-COVERED PLANETS. WE WILL ALSO DELINEATE THE SOLUBILITY-MEDIATED HABITABLE ZONE FOR WATER WORLDS AND HOW IT DEPENDS ON THE COMPOSITION OF VOLATILES ACCRETED BY THE PLANET. ONE OF THE PRIMARY RESULTS OF THIS PROJECT WILL BE PREDICTIONS FOR ATMOSPHERIC ABUNDANCE PATTERNS THAT ARE SIGNATURES OF GLOBAL WATER OCEANS. THIS MODELING EFFORT IS NECESSARY IF WE ARE TO IDENTIFY POTENTIALLY HABITABLE PLANETS WITH GLOBAL OCEANS USING JWST OBSERVATIONS. THIS PROPOSAL IS DIRECTLY RELEVANT TO THE NASA HABITABLE WORLDS PROGRAM BECAUSE IT ADDRESSES THE HABITABILITY OF WATER-RICH EXOPLANETS. THIS WORK INVOLVES MODELING WATER BODY PHYSICS AND CHEMISTRY AS THEY PERTAIN TO HABITABILITY AND HABITABILITY OVER TIME. BY DEVELOPING OBSERVATIONAL DIAGNOSTICS OF THE PRESENCE OF GLOBAL OCEANS ON WATER WORLD EXOPLANETS THIS PROJECT WILL INFORM TARGET AND OPERATIONAL CHOICES FOR CURRENT NASA MISSIONS (I.E. HST SPITZER JWST TESS) AND WILL BE ESSENTIAL FOR THE PLANNING OF A FUTURE SPACE-BASED EXOPLANET DIRECT IMAGING MISSION (E.G. LUVOIR HABEX).