Carbon-rich lunar core evidenced by isostructural phase transitions in iron

The successful Chang'e mission has reignited scientific interests in the Moon, with analyses of returned samples advancing our understanding of lunar composition and evolution ^1-4 . Pioneering models proposed that the Moon harbors a liquid iron-sulfur outer core and a pure face centered cubic (fcc) structured iron (Fe) inner core ^5-8 . However, the Apollo-era data described a lunar inner core with seismic velocity of 27% lower than that of fcc-Fe ^{5 , 6} , posting a long-standing challenge to understand the structure and chemistry of the lunar core. Here, we identify new phases of both bcc- and fcc-Fe that could resolve the discrepancy. In-situ high-pressure and high-temperature X-ray diffraction (XRD) and acoustic velocity measurements show that the Fe alloyed with 0.2-1.7 wt.% carbon (C) undergoes an isostructural phase transition around 4-5 GPa accompanied by a notable velocity drop of ~1 km/s. The seismic velocity profile of the lunar inner core can be directly matched by this new phase in Fe-C solid solution (hereafter denoted as fcc-II; conventional fcc-Fe is referred as fcc-I). These findings provide compelling evidence that the lunar inner core is composed of fcc-II Fe-C with a temperature potentially below 1500 K.