Gaku Nishiyama

and 4 more

Linear gravity anomalies (LGAs) on the Moon have been interpreted as ancient magmatic intrusions formed during the lunar expansion. The composition of such ancient subsurface intrusions may offer a hint for the lunar thermodynamic state in the initial stage of lunar history. To pose a first compositional constraint on magmatism related to lunar expansion, this study analyzed the spectrum and gravity around craters on LGAs, such as Rowland, Roche, and Edison craters. Using spectral datasets around the craters, we first surveyed non-mare basaltic exposures that we hypothesize originate from subsurface intrusions. This hypothesis is then investigated in comparison between the GRAIL data and post-cratering gravity simulated with the iSALE shock physics code. Our spectral analysis reveals no basaltic exposure around Rowland crater. Further, the observed termination of LGA at the crater rim contradicts the gravity simulation which assumes that LGA predates Rowland crater. These results suggest that LGA formation postdates the Rowland formation and that lunar expansion lasted even after the Nectarian age. On the other hand, we found that both Roche and Edison craters possess basaltic exposures in their peripheries. Because the gravity reduced inside Roche crater can be reproduced in our simulation, the discovered basaltic exposures are possibly LGA materials ejected from these craters. The composition of those exposures suggests that the LGA intrusions are composed of low-titanium magma. This indicates that ancient magma during the expansion did not contain ilmenite-rich melt provided by a plume ascending from the ilmenite-bearing layer above the core.