Constraining Formation Hypotheses for Irregular Mare Patches on the Moon With Orbital Reflectance Spectra

Irregular mare patches (IMPs) are enigmatic volcanic features on the Moon's surface, whose lack of cratering and crisp appearance imply they formed <100 Ma ago, similar to 1 Ga after the expected turnoff of lunar volcanism. Multiple contrasting formation hypotheses have been put forth to explain their young appearance, including recent emplacement via eruptions of juvenile volcanic material or outgassing, versus ancient volcanic deposits that were emplaced billions of years ago but only appeared young due to highly porous material. If IMPs formed recently, this would require a reinterpretation of lunar thermal evolution. To help constrain formation hypotheses, we provide a comprehensive mineralogical analysis of IMPs using visible to near infrared hyperspectral data from the Moon Mineralogy Mapper. IMPs appear spectrally dominated by high-calcium pyroxene and are spectrally similar to their host mare and fresh craters. IMPs do not show clear indications of significant glass, implying that pyroclastic eruption was not significant in IMP formation. Based on spectral comparisons to terrestrial magma foam, we find that this also contradicts the glassiness expected for a magma foam exposed at the surface. Thus, we find it is unlikely that IMPs are composed of recently erupted material and may instead be the result of recent or ongoing surface modification of materials similar in composition and likely contemporaneously emplaced with the mare. We favor previous hypotheses that collapse processes or drainage into subsurface voids or porous materials may have been the major drivers of IMP surface rejuvenation, supported by their proximity to collapse features. Plain Language Summary Irregular mare patches (IMPs) are volcanic features on the surface of the Moon that appear to have formed in the past 100 million years, long after the expected end of lunar volcanism. Multiple contrasting explanations have been put forward to explain how IMPs formed, including recent eruption of volcanic material or gas explosions, versus an ancient material that only appears to have formed recently due to unique physical properties. If IMPs are the product of a recent eruption, this would require a reevaluation of the evolution of the state of the Moon's interior and the history of volcanism on its surface. We use reflectance spectra from the Moon Mineralogy Mapper spacecraft to determine the dominant minerals present and place constraints on the proposed formation hypotheses. We find that IMPs appear spectrally similar to their surrounding mare and are unlikely to have formed through the recent eruption of new material. We support previous hypotheses that collapse processes or drainage into subsurface voids or porous materials may have been the major drivers of IMP surface rejuvenation. These results suggest that an ongoing geologic process is contributing to IMPs' youthful appearance.