IXS compressional sound velocity as a function of density. Red open circles (Antonangeli et al., 2004b; 2012); magenta crossed circles (Mao et al., 2012); green lined circles (Ohtani et al., 2013). The curve and the lines are fit to the distinct experimental data sets. The star is the ultrasonic determination reported in Mao et al. (1998). PREM is shown as crosses.