NO ONES HOME: THE FATE OF CARBON ON LIFELESS EARTHSALTHOUGH SEVERAL THOUSANDS OF EXOPLANETS ARE NOW KNOWN INCLUDING MANY TERRESTRIAL PLANETS THEIR POSSIBLE GEOLOGY AND CLIMATES REMAIN POORLY UNDERSTOOD AND UNDERSTUDIED. YET UNDERSTANDING HOW ELEMENTS SUCH AS CARBON ARE CYCLED BETWEEN A PLANETS INTERIOR SURFACE AND ATMOSPHERE IS CRUCIAL TO PREDICT HOW LIFELESS PLANETS OPERATE AND BY CONTRAST BE ABLE TO DETECT DEVIATIONS FROM ABIOTICBACKGROUNDS DUE TO BIOLOGY THE HOLY GRAIL OF EXOPLANET SCIENCE. AS A FIRST FEASIBLE STEP TOWARDS THE DIFFICULT LONG-TERM GOAL OF UNDERSTANDING HOW KEY REACTIVE ELEMENTS (H C N O S) ARE CYCLED IN THE ATMOSPHERES SURFACES AND INTERIORS OF TERRESTRIAL EXOPLANETS THROUGH TIME WE PROPOSE TO CARRY OUT A SELF-CONSISTENT THEORETICAL STUDY OF THE FATE OF CARBON IN THE ATMOSPHERES AND AT THE SURFACES OFEARTH-LIKE LIFELESS EXOPLANETS. WE WILL: 1. MODEL THE NEAR-SURFACE GEOCHEMISTRY AND GEOPHYSICS OF THE CARBON CYCLE TO DETERMINE NET CARBON GAS FLUXES AS A FUNCTION OF TERRESTRIAL PLANET SIZE AND REDOX CONDITIONS;2. MODEL THE ATMOSPHERIC FATE OF CARBON SPECIES AS A FUNCTION OF STELLAR INPUT; 3. PERFORM SIMULATIONS THAT SELF-CONSISTENTLY COMBINE GEOLOGICAL AND ATMOSPHERIC PROCESSES;4. CONVERT RESULTING ATMOSPHERIC COMPOSITIONS TO SPECTRA TO BE ARCHIVED AS A PUBLIC DATABASE FOR USE BY OBSERVERS.WE WILL TRACK THE ABIOTIC FATE OF CARBON AND ITS ATMOSPHERIC EXPRESSION ON EARTH-LIKE PLANETS AS A FUNCTION OF THREE KEY PARAMETERS: PLANET SIZE SURFACE AND ATMOSPHERIC REDOX CONDITIONS AND STELLAR IRRADIATION. TO DO SO WE WILL FURTHER DEVELOP AND USE STATE-OF-THE-ARTPLANETARY GEOLOGICAL ("GEO") AND ATMOSPHERIC ("ATMOS") MODELS.WE HAVE PREVIOUSLY DEVELOPED A CODE THAT COUPLES GEOPHYSICAL EVOLUTION AND WATER-ROCK GEOCHEMISTRY (NEVEU ET AL. 2015 GRL 42 10197). USING THIS CODE WE WILL CALCULATE THE SPECIATION OF CARBON SPECIES VERSUS DEPTH IN SUBAERIAL OCEANS THEIR POSSIBLE INCORPORATION INTO THE CRUST BY WATER-ROCK INTERACTION AT THE SEAFLOOR OR BY SUBDUCTION OF SEDIMENTS AND OUTGASSING AS A FUNCTION OF TEMPERATURE PRESSURE AND FLUID/ROCK COMPOSITION. WE WILL EXPAND THIS CODE WITH BENCHMARKED PARAMETERIZATIONS OF LAND AND SEAFLOOR WEATHERING AND OUTGASSING RATES. THIS MODELING WILL RESULT IN DETAILED BOUNDARY CONDITIONS TO BE IMPLEMENTED INTO AN EXISTING ATMOSPHERIC PHOTOCHEMICAL-CLIMATE MODEL (DOMAGAL-GOLDMAN ET AL. 2014 APJ 792 90).THE ATMOSPHERIC MODEL WILL BE USED TO PREDICT SPECIES MIXING RATIOS FROM NET SURFACE FLUXES GIVEN PLANETARY AND STELLAR PARAMETERS.THE MODELS WILL BE BENCHMARKED AGAINST WHAT IS KNOWN OF THE SURFACES AND ATMOSPHERES OF THE EARTH (PRESENT AND PRIOR TOATMOSPHERIC OXYGENATION) AND TITAN.ATMOSPHERIC MODEL OUTPUTS WILL BE FED BACK INTO THE GEOLOGICAL MODEL IN COMBINED SIMULATIONS OF CARBON CYCLING. WE WILL INVESTIGATE IN DETAIL THE MUTUAL FEEDBACKS BETWEEN GEOLOGICAL AND ATMOSPHERIC PROCESSES SO FAR UNDERSTUDIED FOR TERRESTRIAL EXOPLANETS.THE RESULTING ATMOSPHERIC COMPOSITIONS WILL BE CONVERTED TO PREDICTED EXOPLANET SPECTRA USING THE SPECTRAL MAPPING ATMOSPHERIC RADIATIVE TRANSFER MODEL (SMART; MEADOWS&CRISP 1996 JGR 101 4595). THIS GRID OF SPECTRA WILL BE MADE FREELY AVAILABLE TO THE EXOPLANET COMMUNITY.THIS PROPOSAL IS RELEVANT TO THE EXOPLANETS RESEARCH PROGRAM (E.3) OBJECTIVES AS IT SUPPORTS DIRECTLY THE SCIENTIFIC GOALS OF ADVANCING OUR KNOWLEDGE AND UNDERSTANDING OF EXOPLANETARY SYSTEMS. IT INVOLVES THE CHARACTERIZATION OF EXOPLANETS (INCLUDING THEIR SURFACES INTERIORS AND ATMOSPHERES) [...] INCLUDING THE DETERMINATION OF THEIR COMPOSITIONS DYNAMICS ENERGETICS AND CHEMICAL BEHAVIORS.THIS INVESTIGATION WILL ALSO ADVANCE UNDERSTANDING THE CHEMICAL AND PHYSICAL PROCESSES OF EXOPLANETS (INCLUDING THE STATE AND EVOLUTION OF THEIR SURFACES INTERIORS AND ATMOSPHERES). FURTHERMORE THIS PROPOSAL IS NOT AIMED AT INVESTIGATING THE HABITABILITY OF AN EXOPLANET AND THEREFORE NOT RELEVANT TO THE HABITABLE WORLDS PROGRAM ELEMENT (E.4).