Evolution of C/O in gas (left) and ice (right) from Eistrup et al. (2018). We note the location of the water (solid), CO₂ (dashed), O₂ (dotted), and CO (dot-dashed) ice lines. A step-function in C/O is seen early on in the disk’s life, which becomes complicated by chemistry as the disk ages. After ~ Myr CO is converted into frozen CO₂ outward of the CO₂ ice line, which lowers the gas C/O while raising the ice C/O, eventually becoming more carbon rich than the gas.