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Existing on the extreme edges of the exoplanet population comma ultra short period (USP) rocky planets offer new opportunities to study the physicochemical properties of planetary interiors comma surfaces comma and atmospheres. The immense irradiation and tidal forces experienced by these planets open observational avenues that remain closed for their long-period counterpartsdoublePoint their primordial atmospheres photoevaporate comma opening their surfaces to observation their tidally locked daysides melt and vaporize comma enabling observation of surface material in new phase states and unless heat is redistributed efficiently comma their rock vapor atmospheres may condense on their cold nightsides. We propose for 11 orbits of WFC3 data to measure the phase curve of the lava world TOI-2431b comma a rocky planet with a 5.4-hour orbital period comma the shortest among planets with a known density. It is the best rocky planet for thermal emission studies using HST-WFC3 comma and with the majority of the sky now surveyed by TESS comma it is likely to remain so. The planet is expected to have lost its primordial atmosphere comma but it may well possess a rock vapor atmosphere sustained by its surface magma ocean. These observations will allow us todoublePoint 1) determine the Bond albedo of the planet comma which can constrain the properties of the surface magma ocean 2) to detect a rock vapor atmosphere or a thicker volatile atmosphere from its spectrum or the presence of a hotspot offset indicative of heat redistribution and 3) constrain the interior structure by refining the planetary radius and iron mass fraction of the planet comma which will help inform USP planet formation models and the design of future observations.