  Context: Mass-loss occurring in red supergiants (RSGs) is a major contributor
to the enrichment of the interstellar medium in dust and molecules. The
physical mechanism of this mass loss is however relatively poorly known.
Betelgeuse is the nearest RSG, and as such a prime object for high angular
resolution observations of its surface (by interferometry) and close
circumstellar environment. Aims: The goal of our program is to understand how
the material expelled from Betelgeuse is transported from its surface to the
interstellar medium, and how it evolves chemically in this process. Methods: We
obtained diffraction-limited images of Betelgeuse and a PSF calibrator
(Aldebaran) in six filters in the N band (7.76 to 12.81 mic) and two filters in
the Q band (17.65 and 19.50 mic), using the VLT/VISIR instrument. Results: Our
images show a bright, extended and complex circumstellar envelope at all
wavelengths. It is particularly prominent longwards of 9-10 mic, pointing at
the presence of O-rich dust, such as silicates or alumina. A partial circular
shell is observed between 0.5 and 1.0" from the star, and could correspond to
the inner radius of the dust envelope. Several knots and filamentary structures
are identified in the nebula, one of the knots, located at a distance of 0.9"
west of the star, being particularly bright and compact. Conclusions: The
circumstellar envelope around Betelgeuse extends at least up to several tens of
stellar radii. Its relatively high degree of clumpiness indicates an
inhomogeneous spatial distribution of the material lost by the star. Its
extension corresponds to an important intermediate scale, where most of the
dust is probably formed, between the hot and compact gaseous envelope observed
previously in the near infrared and the interstellar medium.
