Abstract The impact of a body of unknown origin with Jupiter in July 2009 produced an intense perturbation of the planet’s atmosphere at the visible cloud levels. The vertical cloud structure was deeply affected by the presence of a strongly absorbing dense aerosol layer that was expanded steadily by advection in the local winds. We observed this phenomenon at high spatial resolution with the Hubble Space Telescope in July, August, September and November 2009 using the Wide Field Camera 3. In this work, we present radiative transfer modeling of the observed reflectivity in the wavelength range from the near UV (200 nm) to near IR (950 nm) range. The geometric and spectral variations of reflectivity give information on the main particle properties (optical thickness, size, and imaginary refractive index). The observations can be fitted by introducing small particles into the stratosphere with an optical thickness, at a wavelength of 400 nm, ranging from 0.5 ± 0.2 (center of the Impact Cloud) to 0.17 ± 0.03 (impact periphery). Similar effects are detected in the troposphere; the disturbance increases the particle density at all detectable atmospheric levels, with a total aerosol column density of 5 ± 2 × 109 cm−2. The imaginary refractive indices of the aerosol were also substantially altered, with values of mi ∼ 0.015 at UV wavelengths, resembling the absorption spectrum of absorber candidates previously proposed for SL9. We find a typical e-folding temporal scale of 10 ± 3 days in the most rapidly evolving region of the Impact Cloud.