  The water snowline in protoplanetary disks is one of the most pivotal
locations for planet formation: it sets a critical boundary of chemical
composition in the disk and likely serves as a favorable site of planetesimal
growth and planet formation. However, the water snowline at the mid-plane
cannot be directly traced by water line observations as the dust emission
around the water snowline is highly optically thick at these line frequencies.
Alternatively, the water snowline maybe traced through the sharp changes in
physical properties (surface density and dust size distribution) at this
transition or through the snowlines of other molecules that sublimate
co-spatially with water.
  In this chapter, we discuss the ngVLA's capability of tracing the mid-plane
water snowline through mapping sharp transitions in the continuum spectral
index caused by changes of dust size distributions and by using the NH3
snowline as a proxy. We found that with the current design of the ngVLA
ability, a sharp transition in spectral index across the water snowline can be
readily traced in relatively bright disks. A detection of the NH3 snowline is
challenging under the current design, but may still be possible if the
NH3-to-water abundance in the disk is close to that of the ISM ices. In
summary, the proposed ngVLA is the only facility can provide sufficient
sensitivity and spatial resolution to trace the evolution of water snowline at
the disk mid-plane and to reveal its role in accelerating the planetesimal
formation and in building the architecture of planetary systems.

Earth and Planetary Astrophysics