Chandrayaan-1 data confirms hidden Australe Basin, possibly older than South Pole-Aitken

Planetary scientists have confirmed the existence of a hidden impact basin on the Moon using mineralogical data from Chandrayaan-1, and the structure may be older than the oldest known impact craters on the lunar surface.

A team from the Physical Research Laboratory (PRL), Ahmedabad, identified the Australe Basin, which had remained undetected because erosion along its rims made it extremely difficult to spot even with modern imaging techniques. The findings were published in The Planetary Science Journal.

The basin lies along the Moon's southeastern hemisphere and shows distinct morphology, gravity signatures and unusual mineralogical compositions. Researchers suggest it could be older than the South Pole-Aitken Basin, the largest and oldest known impact basin, which formed more than four billion years ago.

Scientists estimate that roughly 300 impact basins exist on the Moon, of which only 74 have been detected so far. These large craters, often several hundred kilometres across, form from violent space activity such as asteroid or meteorite strikes and frequently carry remnants of volcanic activity.

Neha Panwar, planetary scientist and lead author of the study, said the volcanism inside the Australe Basin has a very unique mineralogy. Unlike most lunar basalts, which are high in calcium, these basalts are relatively lower in calcium and higher in magnesium. The basin was also found in a province marked by 248 small basalt ponds arranged in a circular pattern, unlike other known impact basins classified for their smooth, vast hardened lava surfaces.

Tracing the basin

Chandrayaan-1 carried 11 scientific payloads, six of them contributions from international space agencies including NASA and the European Space Agency. The mineralogy of the Australe Basin was detected using data from NASA's Moon Mineralogy Mapper, an imaging spectrometer designed to map the lunar surface mineralogically, operating between 405 and 3000 nanometre wavelengths. Scientists studied absorption bands exhibited by key lunar minerals such as pyroxenes, olivine and plagioclase.

Relevance for future missions

The study could offer detailed mineralogical insights into the region, particularly amid renewed interest from several countries, NASA's proposed Moon Base mission and India's Chandrayaan-4 lunar sample return mission. Panwar said the work provides a framework to interpret data from landing missions in a broader geological context by studying regions that could have contributed material to landing sites, adding that such unique sites can be important targets for future sample return missions.

PRL scientists noted that the Chandrayaan-3 landing site, now known as the Shiv Shakti point, also possessed higher concentrations of magnesium, which could be linked to material originally from the South Pole-Aitken Basin having been transported to the site located about 350 km away. Panwar said the Australe region also has widespread magnesium-bearing lithologies, and since it lies close to the Moon's South Polar region, material excavated by the impact would likely have been deposited across the South Pole. With future missions planned in the South Polar Region, understanding how the regolith there evolved over billions of years is important, she said.