  In a protoplanetary disk, the inner edge of the region where the temperature
falls below the condensation temperature of water is referred to as the 'snow
line'. Outside the snow line, water ice increases the surface density of solids
by a factor of 4. The mass of the fastest growing planetesimal (the 'isolation
mass') scales as the surface density to the 3/2 power. It is thought that
ice-enhanced surface densities are required to make the cores of the gas giants
(Jupiter and Saturn) before the disk gas dissipates. Observations of the Solar
System's asteroid belt suggest that the snow line occurred near 2.7 AU. In this
paper we revisit the theoretical determination of the snow line. In a
minimum-mass disk characterized by conventional opacities and a mass accretion
rate of 10^-8 solar masses per year, the snow line lies at 1.6-1.8 AU, just
past the orbit of Mars. The minimum-mass disk, with a mass of 0.02 solar, has a
life time of 2 million years with the assumed accretion rate. Moving the snow
line past 2.7 AU requires that we increase the disk opacity, accretion rate,
and/or disk mass by factors ranging up to an order of magnitude above our
assumed baseline values.
