Extended Data Figure 5. Onset of MgO exsolution and associated exsolution power for two typical models.

(a) The MgO equilibrium concentration in the core (same figure as Extended Data Fig. 4b), corresponding to our nominal CMB temperature evolution. The onset of MgO exsolution from the core occurs when the MgO equilibrium concentration drops below the MgO content in the core, which is reported here in two cases: 2.9 wt.% for the Mars-sized impactor, and 2.1 wt.% for the “fast-spinning” impactor. For the thermal evolution model in Extended Data Fig. 4a, this onset is at 1.1 Ga and 2.3 Ga, respectively. (b) and (c) Exsolution power for these two cases which is proportional to the MgO exsolution rate plotted in Extended Data Fig. 4d. It is noteworthy that power at a given time is independent of initial MgO content (as long as MgO is being exsolved). The latter only affects the onset of exsolution and therefore the duration of energy release. We also note that the power produced is in excess of 3 TW, and therefore sufficient to drive a dynamo by compositional buoyancy. Finally, the power drops dramatically with the onset of inner core growth, because of the associated drop in core cooling rate and MgO exsolution rate. The core at the present day is still exsolving MgO, and should produce ~1 TW of power, significantly less than the power produced by inner core growth.