  An unsolved problem in step-wise core-accretion planet formation is that
rapid radial drift in gas-rich protoplanetary disks should drive
mm-/meter-sized particles inward to the central star before large bodies can
form. One promising solution is to confine solids within small scale
structures. Here we investigate dust structures in the (sub)mm continuum
emission of four disks (TW Hya, HL Tau, HD 163296 and DM Tau), a sample of
disks with the highest spatial resolution ALMA observations to date. We
retrieve the surface brightness distributions using synthesized images and
fitting visibilities with analytical functions. We find that the continuum
emission of the four disks is ~axi-symmetric but rich in 10-30AU-sized radial
structures, possibly due to physical gaps, surface density enhancements or
localized dust opacity variations within the disks. These results suggest that
small scale axi-symmetric dust structures are likely to be common, as a result
of ubiquitous processes in disk evolution and planet formation. Compared with
recent spatially resolved observations of CO snowlines in these same disks, all
four systems show enhanced continuum emission from regions just beyond the CO
condensation fronts, potentially suggesting a causal relationship between dust
growth/trapping and snowlines.
