Lunar Low-Titanium Magmatism during Ancient Expansion inferred from
Ejecta originating from Linear Gravity Anomalies
Abstract
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.