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Grain growth in planet-forming disks is the first step toward the formationof planets. The growth of grains and their inward drift leaves a distinctimprint on the dust surface-density distribution and the resultingsurface-brightness profile of the thermal continuum emission. We determine thesurface-brightness profile of the continuum emission using resolvedobservations at millimeter wavelengths of the disk around TW Hya, and infer thesignature of dust evolution on the surface density and dust opacity. ArchivalALMA observations at 820 micron on baselines up to 410 kilolambda are comparedto parametrized disk models to determine the surface-brightness profile. Underthe assumption of a constant dust opacity, a broken radial power law bestdescribes the dust surface density, with a slope of -0.53 +/- 0.01 from the 4.1au 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 belowthe detection limit beyond ~60 au. The shape of the dust surface density isconsistent with theoretical expectations for grain growth, fragmentation, anddrift, but its total dust content and its turn-over radius are too large for TWHya's age of 8-10 Myr even when taking into account a radially varying dustopacity. Higher resolution imaging with ALMA of TW Hya and other disks isrequired to establish if unseen gaps associated with, e.g., embedded planetstrap grains at large radii or if locally enhanced grain growth associated withthe CO snow line explains the extent of the millimeter-continuum surfacebrightness profile. In the latter case, population studies should reveal acorrelation between the location of the CO snow line and the extent of themillimeter continuum. In the former case, and if CO freeze out promotes planetformation, this correlation should extend to the location of gaps as well.

Steepening of the 820 micron continuum surface-brightness profile signals dust evolution in TW Hya's disk