Chandrayaan-1 spectra point to a lunar basin older than South Pole-Aitken

Researchers at the Physical Research Laboratory in Ahmedabad say Chandrayaan-1 recorded the mineral signature of an impact basin that topography no longer shows. The paper, in The Planetary Science Journal, places the feature, the Australe Basin, on the Moon's southeastern hemisphere around Mare Australe. Lead author Neha Panwar and her colleagues argue it may predate the South Pole-Aitken Basin, which formed more than four billion years ago and is the largest and oldest impact structure still counted as confirmed.

The detection is compositional, not a rim on a photograph. The team mapped a broad circular distribution of orthopyroxene around Mare Australe in Chandrayaan-1 spectrometer data. They read that ring as ejecta from a basin whose outer walls have been worn down by later impacts, volcanism and the ordinary grinding of the lunar surface. Gravity data and a distinct morphology sit underneath the mineral map. The basalts inside the structure are low in calcium and high in magnesium, the opposite of most lunar mare basalts. The floor holds 248 small basalt ponds rather than one smooth lava sheet.

Two absences matter. The team did not find the wide blankets of purest anorthosite that younger large basins throw out. That gap is why they date the event early, possibly while the crust was still changing. They also note a later, roughly 880-kilometre Australe North Basin that would have broken and buried parts of the older structure. Volcanic fill then hid more of it.

About 300 impact basins are thought to exist on the Moon. Only 74 have been identified. Imaging finds rims. The oldest basins are the ones that no longer have rims. A method that reads minerals can in principle recover those. If Australe is real and older than South Pole-Aitken, the early bombardment timeline moves. Sample-return planners, including teams working toward Chandrayaan-4, gain a target whose chemistry is already described from orbit.

Chandrayaan-1 left Sriharikota on 22 October 2008 on PSLV-C11 with NASA's Moon Mineralogy Mapper and Mini-SAR aboard. The spacecraft died in 2009. The spectrometer archive has been public for years. The new claim is not a new instrument. It is a new reading of an old file, which is how a large share of planetary science now proceeds.

Independent imaging teams will look for any remaining topographic hint. Gravity models will be re-fit. The paper does not hand over a crater wall you can point to on a tourist map. It hands over a mineral circle and a suggested order in time. That is enough to put Australe into the next round of lunar basin catalogues, and enough to keep the oldest label provisional until another group runs the same spectra.

Panwar told The Indian Express that the volcanism inside Australe is mineralogically odd. High-magnesium, low-calcium basalts are not the default fill for a lunar basin. That oddity is part of the age argument and part of the reason a lander or a sample cache in the region would be worth the trajectory. Catalogues that still treat South Pole-Aitken as the beginning of the large-basin clock now have a competing first page.

For ISRO the practical point is simpler. An orbiter that stopped transmitting seventeen years ago is still producing results that change how the far-side southeast is described. The instrument time already paid for keeps paying.