NRIXS compressional sound velocity as a function of density. Red squares (Mao et al., 2001); green crossed open squares (Lin et al., 2005); magenta horizontal-lined squares (Murphy et al., 2013); blue vertical-lined squares (Gleason et al., 2013). Solid lines are linear fits to the experimental data sets by Murphy et al. (2013) and Gleason et al. (2013). For comparison, the results by Murphy et al. (2013) with densities rescaled to those for iron of natural isotopic abundance (55.85 vs. 56.95 g/mol) are plotted as a dotted line. The star is the ultrasonic determination reported in Mao et al. (1998). PREM is shown as crosses.