From the masses of planets orbiting our Sun, and relative elemental abundances, it is estimated that at birth our Solar System required a minimum disk mass of ∼0.01 M within ∼100 AU of the star 1-4 . The main constituent, gaseous molecular hydrogen, does not emit from the disk mass reservoir 5 , so the most common measure of the disk mass is dust thermal emission and lines of gaseous carbon monoxide 6 . Carbon monoxide emission generally probes the disk surface, while the conversion from dust emission to gas mass requires knowl-1

Water and simple organic molecular ices dominate the mass of solid materials available for planetesimal and planet formation beyond the water snow line. Here we analyze ALMA long baseline 2.9, 1.3 and 0.87 mm continuum images of the young star HL Tau, and suggest that the emission dips observed are due to rapid pebble growth around the condensation fronts of abundant volatile species. Specifically, we show that the prominent innermost dip at 13 AU is spatially resolved in the 0.87 mm image, and its center radius is coincident with the expected mid-plane condensation front of water ice. In addition, two other prominent dips, at distances of 32 and 63 AU, cover the mid-plane condensation fronts of pure ammonia or ammonia hydrates and clathrate hydrates (especially with CO and N 2 ) formed from amorphous water ice. The spectral index map of HL Tau between 1.3 and 0.87 mm shows that the flux ratios inside the dips are statistically larger than those of nearby regions in the disk. This variation can be explained by a model with two dust populations, where most of solid mass resides in a component that has grown into decimeter size scales inside the dips. Such growth is in accord with recent numerical simulations of volatile condensation, dust coagulation and settling.

We report observations of resolved C 2 H emission rings within the gas-rich protoplanetary disks of TWHya and DMTau using the Atacama Large Millimeter Array. In each case the emission ring is found to arise at the edge of the observable disk of millimeter-sized grains (pebbles) traced by submillimeter-wave continuum emission. In addition, we detect a C 3 H 2 emission ring with an identical spatial distribution to C 2 H in the TWHya disk. This suggests that these are hydrocarbon rings (i.e., not limited to C 2 H). Using a detailed thermo-chemical model we show that reproducing the emission from C 2 H requires a strong UV field and C/O>1 in the upper disk atmosphere and outer disk, beyond the edge of the pebble disk. This naturally arises in a disk where the ice-coated dust mass is spatially stratified due to the combined effects of coagulation, gravitational settling and drift. This stratification causes the disk surface and outer disk to have a greater permeability to UV photons. Furthermore the concentration of ices that transport key volatile carriers of oxygen and carbon in the midplane, along with photochemical erosion of CO, leads to an elemental C/O ratio that exceeds unity in the UV-dominated disk. Thus the motions of the grains, and not the gas, lead to a rich hydrocarbon chemistry in disk surface layers and in the outer disk midplane.

CO is widely used as a tracer of molecular gas. However, there is now mounting evidence that gas phase carbon is depleted in the disk around TW Hya. Previous efforts to quantify this depletion have been hampered by uncertainties regarding the radial thermal structure in the disk. Here we present resolved ALMA observations of 13 CO 3-2, C 18 O 3-2, 13 CO 6-5, and C 18 O 6-5 emission in TW Hya, which allow us to derive radial gas temperature and gas surface density profiles, as well as map the CO abundance as a function of radius. These observations provide a measurement of the surface CO snowline at ∼30 AU and show evidence for an outer ring of CO emission centered at 53 AU, a feature previously seen only in less abundant species. Further, the derived CO gas temperature profile constrains the freeze out temperature of CO in the warm molecular layer to <21 K. Combined with the previous detection of HD 1-0, these data constrain the surface density of the warm H 2 gas in the inner ∼30 AU such thatg cm 10 auwarm gas 2.9 3.0 2 1 2 . We find that CO is depleted by two orders of magnitude from -= R 10 60 AU, with the small amount of CO returning to the gas phase inside the surface CO snowline insufficient to explain the overall depletion. Finally, this new data is used in conjunction with previous modeling of the TW Hya disk to constrain the midplane CO snowline to 17-23 AU.

The initial mass distribution in the solar nebula is a critical input to planet formation models that seek to reproduce today's Solar System 1 . Traditionally, constraints on the gas mass distribution are derived from observations of the dust emission from disks 2, 3 , but this approach suffers from large uncertainties in grain growth and gas-to-dust ratio 2 . On the other hand, previous observations of gas tracers only probe surface layers above the bulk mass reservoir 4 .Here we present the first partially spatially resolved observations of the 13 C 18 O J=3-2 line emission in the closest protoplanetary disk, TW Hya, a gas tracer that probes the bulk mass distribution. Combining it with the C 18 O J=3-2 emission and the previously detected HD J=1-0 flux, we directly constrain the mid-plane temperature and optical depths of gas and dust emission. We report a gas mass distribution of 13 +8 −5 ×(R/20.5AU) −0.9 +0.4 −0.3 g cm −2 in the expected formation zone of gas and ice giants (5-21 AU). We find the total gas/millimetersized dust mass ratio is 140 in this region, suggesting that at least 2.4 M ⊕ of dust aggregates have grown to >centimeter sizes (and perhaps much larger). The radial distribution of gas mass is consistent with a self-similar viscous disk profile but much flatter than the posterior extrapolation of mass distribution in our own and extrasolar planetary systems.The primary theory for the formation of giant planets is the so-called core accretion scenario, where a rock+ice core forms through the coagulation of planetesimals until it becomes sufficiently massive to accrete a gaseous envelope 1 . In this theory, the spatial distribution of gas in the primitive nebula is not only critical to the later accretion of the atmosphere of giant planets, but also plays an 1