We have developed an online radiative-transfer suite (https://psg.gsfc.nasa.gov) applicable to a broad range of planetary objects (e.g., planets, moons, comets, asteroids, TNOs, KBOs, exoplanets). The Planetary Spectrum Generator (PSG) can synthesize planetary spectra (atmospheres and surfaces) for a broad range of wavelengths (UV/Vis/near-IR/IR/far-IR/THz/submm/Radio) from any observatory (e.g., JWST, ALMA, Keck, SOFIA), any orbiter (e.g., ExoMars, Juno), or any lander (e.g., MSL). This is achieved by combining several state-of-the-art radiative transfer models, spectroscopic databases and planetary databases (i.e., climatological and orbital).PSG has a 3D (three-dimensional) orbital calculator for most bodies in the solar system, and all confirmed exoplanets, while the radiative-transfer models can ingest billions of spectral signatures for hundreds of species from several spectroscopic repositories. It integrates the latest radiativetransfer and scattering methods in order to compute high resolution spectra via line-by-line calculations, and utilizes the efficient correlated-k method at moderate resolutions, while for computing cometary spectra, PSG handles non-LTE and LTE excitation processes. PSG includes a realistic noise calculator that integrates several telescope / instrument configurations (e.g., interferometry, coronagraphs) and detector technologies (e.g., CCD, heterodyne detectors, bolometers). Such an integration of advanced spectroscopic methods into an online tool can greatly serve the planetary community, ultimately enabling the retrieval of planetary parameters from remote sensing data, efficient mission planning strategies, interpretation of current and future planetary data, calibration of spectroscopic data, and development of new instrument/spacecraft concepts. Number of species: 52 Number of isotopologues: 118 JPL Molecular Spectroscopy [16] Wavelength range: 2.65 µm to radio Number of lines: 888,113 Number of species: 383 CDMS Cologne Database for Molecular Spectroscopy [17] Wavelength range: 1.81 µm to radio Number of lines: 1,612,154 Number of species: 792 Exo-Transmit Opacities database [33] Wavelength range: 0.1 to 170 µm Number of spectral points: 7454 Number of temperatures: 30 (100 to 3000 K) Number of pressures: 13 (1E-9 to 1000 bar) Number of species: 30 The MPI-Mainz UV/VIS Spectral Atlas [34] UV cross-sections for O3 [35] UV cross-sections for CO2 [36] Wavelength range: 0.01 to 1 um Number of cross-sections: 70 Number of species: 22 (e.g., H2O, CH4, CO2, N2O, O3)

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The evolution and history of water on Mars plays a key role in the assessment of the habitability of the planet across time. There is abundant geomorphological evidence suggesting that Mars had a wetter past (Bibring et al., 2006;Carr & Head, 2003), yet the duration and extent of this more humid past remains a topic of substantial debate. For instance, the large deltas, basins, and valleys on Mars are suggestive of large bodies of water that were stable over relatively long periods of time. Some estimates suggest past volumes of water in excess of a 500 m deep Global Equivalent Layer (GEL; Carr & Head, 2003), which is many times larger than the current estimates of labile water on Mars (∼30 m, Lasue et al., 2013).The large enrichments of D/H measured in atmospheric water suggest that a large fraction, beyond 80%, of this water was lost over time (Jakosky, 2021;Villanueva et al., 2015), and Ar and O isotopic ratios measured with MAVEN (Jakosky et al., 2017) and TGO (Alday, Wilson, et al., 2021) indicate that Mars has lost a large fraction of its atmosphere. Because Mars is less massive than Earth, the neutral escape of volatiles is easier on Mars, considering the similar equilibrium temperatures of the two planets, although Mars is obviously colder. Recent results indicate that most of this escape occurred via neutral and nonionized processes (Brain et al., 2015), in which temperature and its variability across geological times were key factors defining the state of the Martian atmosphere. Recent results from dust storms suggest that dust storms can greatly heat the atmosphere, leading to the upward transport and more readily escape of water (