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Measurements of photometric variability at different wavelengths provideinsights into the vertical cloud structure of brown dwarfs and planetary-massobjects. In seven Hubble Space Telescope consecutive orbits, spanning $\sim$10h of observing time}, we obtained time-resolved spectroscopy of theplanetary-mass T8-dwarf Ross 458C using the near-infrared Wide Field Camera 3.We found spectrophotometric variability with a peak-to-peak signal of2.62$\pm$0.02 % (in the 1.10-1.60~$\mu$m white light curve). Using threedifferent methods, we estimated a rotational period of 6.75$\pm$1.58~h for thewhite light curve, and similar periods for narrow $J$- and $H$- band lightcurves. Sine wave fits to the narrow $J$- and $H$-band light curves suggest atentative phase shift between the light curves with wavelength when we allowdifferent periods between both light curves. If confirmed, this phase shift maybe similar to the phase shift detected earlier for the T6.5 spectral type 2MASSJ22282889-310262. We find that, in contrast with 2M2228, the variability ofRoss~458C shows evidence for a {color trend} within the narrow $J$-band, butgray variations in the narrow $H$-band. The spectral time-resolved variabilityof Ross 458C might be potentially due to heterogeneous sulfide clouds in theatmosphere of the object. Our discovery extends the study of spectralmodulations of condensate clouds to the coolest T dwarfs, planetary-masscompanions.

Cloud Atlas: Rotational Spectral Modulations and potential Sulfide Clouds in the Planetary-mass, Late T-type Companion Ross 458C