  Context. Convection is a candidate to explain the trigger of red supergiant
star mass loss. Owing to the small size of the convective cells on the
photosphere, few of their characteristics are known. Aims. Using near infrared
interferometry, we intend to resolve the photosphere of red supergiant stars
and to bring new constraints on their modeling. Methods. We observed the nearby
red supergiant Antares using the 4-telescope instrument VLTI/PIONIER. We
collected data on the three available configurations of the 1.8m telescopes in
the H band. Results. We obtained unprecedented angular resolution on the disk
of a star (6% of the star angular diameter) that limits the mean size of
convective cells, brings new constraints on numerical simulations. Using an
analytical model with a distribution of bright spots we determine their effect
on the visibility signal. Conclusion. We determine that the interferometric
signal on Antares is compatible with convective cells with various sizes from
45% to 5% of the angular diameter. We also conclude that convective cells can
strongly affect the angular diameter and limb-darkening measurements. In
particular, the apparent angular diameter becomes dependent on the sampled
position angles.
