Inner Core Composition of the Moon and Ganymede Constrained by Thermal Equation of State of Fe0.99C0.01

Core composition is essential for understanding the state, structure and evolution of terrestrial planets and satellites. Here we present synchrotron energy-dispersive X-ray diffraction measurements on Fe0.99C0.01 alloy, a possible candidate inner-core component, in a Paris-Edinburgh press up to 8.0 GPa and 1773 K. The thermal equation of state of face-centered cubic (fcc or gamma) Fe0.99C0.01 was established at 0.8-8.0 GPa and 973-1773 K (V0 = 48.3(3) & Aring;3, K0 = 143(35) GPa, K0 ' ${K}_{0}<^>{\mathit{\prime }}$ = 4, (partial derivative K/partial derivative T)P = -0.02(9) GPa K-1, and thermal expansion alpha T = a + bT with a = 7(2) x 10-5 K-1 and b = -0(7) x 10-8 K-2). In particular, adding 1 at.% carbon decreases the density of fcc Fe by similar to 0.09 g/cm3 at 8.0 GPa and 1773 K. Comparison with geophysical observations suggests that the density of Fe-C alloy (0.2-6.7 wt.% C) could not account for the inner core density of the Moon and Ganymede. The gamma-Fe0.99C0.01 also displays a higher velocity by similar to 1.45-1.59 km/s than the lunar inner core. This result implies that carbon should be not the sole light element in the inner core of the Moon and Ganymede.