  Grain growth in planet-forming disks is the first step toward the formation
of planets. The growth of grains and their inward drift leaves a distinct
imprint on the dust surface-density distribution and the resulting
surface-brightness profile of the thermal continuum emission. We determine the
surface-brightness profile of the continuum emission using resolved
observations at millimeter wavelengths of the disk around TW Hya, and infer the
signature of dust evolution on the surface density and dust opacity. Archival
ALMA observations at 820 micron on baselines up to 410 kilolambda are compared
to parametrized disk models to determine the surface-brightness profile. Under
the assumption of a constant dust opacity, a broken radial power law best
describes the dust surface density, with a slope of -0.53 +/- 0.01 from the 4.1
au radius of the (already known) inner hole to a turn-over radius of 47.1 +/-
0.2 au, steepening to -8.0 +/- 0.1 at larger radii. The emission drops below
the detection limit beyond ~60 au. The shape of the dust surface density is
consistent with theoretical expectations for grain growth, fragmentation, and
drift, but its total dust content and its turn-over radius are too large for TW
Hya's age of 8-10 Myr even when taking into account a radially varying dust
opacity. Higher resolution imaging with ALMA of TW Hya and other disks is
required to establish if unseen gaps associated with, e.g., embedded planets
trap grains at large radii or if locally enhanced grain growth associated with
the CO snow line explains the extent of the millimeter-continuum surface
brightness profile. In the latter case, population studies should reveal a
correlation between the location of the CO snow line and the extent of the
millimeter continuum. In the former case, and if CO freeze out promotes planet
formation, this correlation should extend to the location of gaps as well.
