  Planets form in the disks around young stars. Their formation efficiency and
composition are intimately linked to the protoplanetary disk locations of "snow
lines" of abundant volatiles. We present chemical imaging of the CO snow line
in the disk around TW Hya, an analog of the solar nebula, using high spatial
and spectral resolution Atacama Large Millimeter/Submillimeter Array (ALMA)
observations of N2H+, a reactive ion present in large abundance only where CO
is frozen out. The N2H+ emission is distributed in a large ring, with an inner
radius that matches CO snow line model predictions. The extracted CO snow line
radius of ~ 30 AU helps to assess models of the formation dynamics of the Solar
System, when combined with measurements of the bulk composition of planets and
comets.
