Why in the News
Planetary scientists at the Physical Research Laboratory (PRL), Ahmedabad, have confirmed the existence of a hidden lunar impact basin, the Australe Basin, using mineralogical data gathered by Chandrayaan 1. This is the first time a concealed impact basin has been confirmed from mineralogy, and the basin had remained untraced because erosion along its rims defeats modern imaging techniques. The study, published in The Planetary Science Journal, places the basin along the southeastern hemisphere of the Moon and finds it could predate the South Pole Aitken Basin, the largest and oldest basin known. The tension is that the oldest impact record on the Moon is precisely the record surface topography has erased, so the ordering of lunar history now rests on a method that reads composition instead of shape.
What is the Australe Basin?
- Australe Basin: It is a large lunar impact basin located along the southeastern hemisphere of the Moon, formed by a violent space impact such as an asteroid or meteorite strike.
- Why it stayed hidden: Its rims have suffered erosion, which removed the distinct outer rim that imaging techniques rely on to identify a basin.
- Its signature: It carries distinct morphology and gravity signatures together with an unusual mineralogical composition.
- Its volcanic province: It sits in a province characterised by 248 small basalt ponds arranged in a circular pattern, unlike previously known basins classified by their smooth and vast hardened lava surfaces.
How did mineralogy find a basin that imaging could not?
- Moon Mineralogy Mapper: The mineralogy was detected using data from this National Aeronautics and Space Administration (NASA) imaging spectrometer, designed to build a mineralogical map of the lunar surface and operating between 405 and 3000 nanometres.
- The payload context: It was one of 11 scientific payloads on Chandrayaan 1, of which six were contributions from international space agencies including NASA and the European Space Agency (ESA).
- The method: Scientists studied the absorption bands exhibited by key lunar minerals, namely pyroxenes, olivine and plagioclase, which identify composition where topography carries no usable signal.
- What the composition showed: The basalts within the basin are relatively lower in calcium and higher in magnesium than the majority of lunar basalts, which are high in calcium bearing minerals.
Why does the age claim matter, and how much of the Moon is still unmapped?
- The benchmark: The South Pole Aitken Basin is the largest and oldest known basin on the Moon, formed over 4 billion years ago.
- The claim: PRL scientists hold that the Australe Basin could be older than the South Pole Aitken Basin, which would move the earliest dated event in the lunar impact record.
- The detection deficit: Roughly 300 impact basins are believed to exist on the Moon and only 74 have been detected so far, so most of the lunar impact record remains unidentified.
- Why the eroded ones are the old ones: Basins with distinct outer rims are the ones imaging finds, so a detection method keyed to rims systematically misses the most degraded features.
What does the finding mean for future lunar missions?
- The landing site link: The Chandrayaan 3 landing site, now known as Shiv Shakti point and located roughly 350 km away, also carries higher concentrations of magnesium, possibly material originally from the South Pole Aitken Basin transported there.
- Material spread to the south pole: Magnesium bearing lithologies are widespread across the Australe region, and since the region lies close to the lunar south polar region, material excavated by the impact is likely to have been deposited across the south pole.
- Reading a landing site in context: The study provides a framework to interpret data from landing missions in a broader geological context, by studying the regions that could have contributed material to those sites.
- The missions it serves: The mineralogical picture bears on NASA’s proposed Moon Base mission and on Chandrayaan 4, India’s lunar sample return mission, since such sites become targets for sample return.
Challenges to lunar impact basin research
- Remote sensing cannot date a surface: Spectrometry identifies composition but assigns no absolute age, so an ordering claim rests on inference until a sample is dated in a laboratory. Eg. The age of the Australe Basin relative to the South Pole Aitken Basin is stated as the research team’s opinion rather than as a measured date.
The Fix: Target the province for a sample return so radiometric dating can settle the sequence. - Space weathering degrades the spectral signal: Continuous micrometeorite bombardment and solar wind alter the optical properties of the lunar surface, which mutes the absorption bands a spectrometer reads. Eg. The basin’s own rims were eroded past the point where imaging could detect them.
The Fix: Calibrate orbital spectra against returned samples of known composition so the weathering offset is corrected rather than estimated. - Coverage gaps at the poles: The lunar south polar region sits in extreme illumination conditions, so instruments that depend on reflected sunlight return poor data exactly where interest is concentrated. Eg. Permanently shadowed craters near the south pole are the targets of the proposed Moon Base and remain the least characterised terrain.
The Fix: Pair reflectance mapping with active instruments such as radar and neutron spectrometry that do not depend on solar illumination. - Sample return is technically unproven for India: Retrieving lunar material requires ascent from the surface, rendezvous in lunar orbit and a controlled return, none of which India has yet demonstrated together. Eg. Chandrayaan 4 is planned as India’s first lunar sample return mission.
The Fix: Validate the docking and ascent elements separately in Earth orbit before committing them to a lunar sequence. - Surface operations disturb the record they study: Landings and rover activity churn the regolith that later missions are sent to sample, which compromises the evidence itself. Eg. Understanding how the regolith in the south polar regions has evolved over billions of years is stated as a requirement for the missions planned there.
The Fix: Fix exclusion zones around high value sampling terrain before the operating missions arrive rather than after.
Conclusion
A basin no imaging technique could see was found by asking what the surface is made of instead of what it looks like. That reverses the usual order of lunar geology, where shape identifies a feature and composition then explains it, and it puts the most degraded parts of the record back within reach. The finding is published and the age ordering remains an interpretation rather than a measurement. What to watch is whether the same mineralogical method is turned on the basins that remain undetected, and whether this province becomes a named target for the planned sample return.
Back2Basics: Chandrayaan 1
- What it was: It was India’s first lunar mission, launched by the Indian Space Research Organisation in October 2008 and placed in orbit around the Moon.
- Launch vehicle: It was launched on a Polar Satellite Launch Vehicle from the Satish Dhawan Space Centre, Sriharikota.
- Its payloads: It carried 11 scientific instruments, six of them contributed by international space agencies including NASA and ESA.
- Its principal finding: Data from the mission led to the detection of water and hydroxyl molecules on the lunar surface, which reshaped the understanding of lunar resources.
Matching Previous Year Question
“[2017, GS3, 10 marks] India has achieved remarkable successes in unmanned space missions including the Chandrayaan and Mars Orbitter Mission, but has not ventured into manned space mission, both in terms of technology and logistics? Explain critically.”