  Theory surrounding the origin of the dust-laden winds from evolved stars
remains mired in controversy. Characterizing the formation loci and the dust
distribution within approximately the first stellar radius above the surface is
crucial for understanding the physics that underlie the mass-loss phenomenon.
By exploiting interferometric polarimetry, we derive the fundamental parameters
that govern the dust structure at the wind base of a red supergiant. We present
near-infrared aperture-masking observations of Betelgeuse in polarimetric mode
obtained with the NACO/SAMPol instrument. We used both parametric models and
radiative transfer simulations to predict polarimetric differential visibility
data and compared them to SPHERE/ZIMPOL measurements. Using a thin dust shell
model, we report the discovery of a dust halo that is located at only 0.5
R$_{\star}$ above the photosphere (i.e. an inner radius of the dust halo of 1.5
R$_{\star}$). By fitting the data under the assumption of Mie scattering, we
estimate the grain size and density for various dust species. By extrapolating
to the visible wavelengths using radiative transfer simulations, we compare our
model with SPHERE/ZIMPOL data and find that models based on dust mixtures that
are dominated by forsterite are most favored. Such a close dusty atmosphere has
profound implications for the dust formation mechanisms around red supergiants.
