India’s Chandrayaan-1 mission has uncovered mineralogical clues supporting the existence of an ancient impact basin on the Moon that is no longer visible in topographic or gravity data. A study published in The Planetary Science Journal in August 2026 examined the Mare Australe region using data from the Moon Mineralogy Mapper (M3) aboard Chandrayaan-1.

Researchers from the Physical Research Laboratory (PRL), Ahmedabad, found a broad circular distribution of orthopyroxene around Mare Australe. The pattern supports the presence of the previously proposed Australe Basin.

Mare Australe is a distinctive volcanic region containing 248 small basalt ponds arranged in a circular pattern. Earlier studies had linked the region to an impact basin. However, scientists could not confirm it due to the lack of clear ring-shaped features and a central positive Bouguer anomaly.

The latest evidence comes from orthopyroxene, a mineral often linked to material excavated from the Moon’s lower crust or mantle during major impacts. Researchers found that orthopyroxene-bearing spectra largely follow the circular boundary of the Mare Australe basalts. The study said the distribution of orthopyroxene “delineates the boundary” of the obliterated Australe Basin, providing mineralogical evidence of its existence.

Scientists also detected orthopyroxene exposures along the proposed rings of the Australe North Basin. Earlier GRAIL data and geological studies identified this basin as an approximately 880-km-wide impact structure. The findings suggest a dual-impact scenario involving the completely obliterated Australe Basin and the insufficiently preserved Australe North Basin.

The mineral study also found that Mare Australe’s basalts differ from typical lunar basalts. They are dominated by low-to-intermediate-calcium pyroxenes, while lunar basalts elsewhere are generally dominated by high-calcium pyroxenes. This mineralogical pattern remained consistent across basalt deposits formed at different times, ranging from about 3.9 billion years ago to nearly 1 billion years ago. Researchers said this could indicate that the region’s magma source saw little major mineralogical change during its volcanic history.

The study further examined the absence of purest anorthosite (PAN) along the proposed rim of the Australe Basin. PAN is linked to the Moon’s early crust, which formed as the Lunar Magma Ocean solidified. Researchers said its absence could indicate that the Australe Basin formed very early, while the Lunar Magma Ocean was still solidifying. This could place the basin’s formation around the same period as, or possibly earlier than, the South Pole–Aitken (SPA) Basin, estimated to have formed about 4.25 billion years ago.

However, the researchers cautioned that PAN may have escaped detection. Even a small amount of pyroxenes can mask plagioclase signatures. They also noted that the proposed boundary may represent only the inner ring of a larger basin.

The study shows how mineral signatures can help identify ancient lunar structures erased by volcanic resurfacing, impact ejecta and billions of years of geological change. The researchers said the findings could help reconstruct the Moon’s early impact history. They added that detailed geochemical and geodynamic modelling will be needed to understand the unusual geological evolution of the Australe region.