Chandrayaan-1 finds clues to a Moon basin lost in time

Data from India’s Chandrayaan-1 mission has provided mineralogical evidence supporting the presence of an ancient impact basin on the Moon that is no longer visible in topographic or gravity data, according to a study published in The Planetary Science Journal, dated August 2026.

Researchers from the Physical Research Laboratory (PRL), Ahmedabad, studied the Mare Australe region using data from the Moon Mineralogy Mapper (M3) aboard Chandrayaan-1. The study found a widespread, circular distribution of orthopyroxene around Mare Australe, supporting the presence of the previously suggested Australe Basin.

Mare Australe is a distinctive volcanic region where 248 small basalt ponds are arranged in a circular pattern. Earlier studies had suggested an associated impact basin, but the absence of identifiable ring morphology and a central positive Bouguer anomaly had left its existence uncertain, the researchers said.


The new mineralogical evidence comes from the distribution of orthopyroxene, a mineral associated with material excavated from the lower crust or mantle during large impacts. The researchers found that orthopyroxene-bearing spectra predominantly trace 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 for its presence.

The researchers also found orthopyroxene exposures along the proposed rings of the Australe North Basin, an approximately 880-km impact basin identified through earlier Gravity Recovery and Interior Laboratory (GRAIL) data and geological investigations.

Together, the findings point to a dual-impact scenario involving the completely obliterated Australe Basin and the insufficiently preserved Australe North Basin, according to the study.

The mineralogical analysis also revealed that the basalts of Mare Australe are unusual compared with typical lunar basalts. They are dominated by low-to-intermediate-calcium pyroxenes, whereas lunar basalts elsewhere are generally dominated by high-calcium pyroxenes.

The researchers found that this mineralogical signature persisted across Mare Australe despite its basalts being emplaced at different geological times, from about 3.9 billion years ago to as recently as 1 billion years ago. This suggests that the magma source in the region may not have undergone major mineralogical changes during its volcanic history, the study said.

The study also examined the absence of purest anorthosite (PAN) along the proposed rim of the Australe Basin. PAN is associated with the Moon’s early crust formed during the solidification of the Lunar Magma Ocean.

According to the researchers, the absence of PAN could possibly indicate that the Australe Basin formed very early, when the Lunar Magma Ocean was still solidifying. This would make the basin either contemporaneous with or potentially older than the South Pole–Aitken (SPA) Basin, which is estimated to have formed around 4.25 billion years ago.

The researchers, however, noted that PAN may not have been detected because even a minor addition of pyroxenes can mask plagioclase signatures. They also said the proposed boundary could represent only the inner ring of a potentially larger basin.

The study highlights how mineralogical signatures can help reconstruct ancient lunar structures that have been erased by volcanic resurfacing, impact ejecta and the Moon’s long geological evolution.

The researchers said the findings have implications for reconstructing the Moon’s early impact history, while detailed geochemical and geodynamic modelling will be needed to understand the processes behind the unusual geological evolution of the Australe region.