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doi:10.1038/nindia.2015.87 Published online 3 July 2015

Moon is not dead, geologically speaking

Biplab Das

Amid debates on whether the Moon is still geologically active, an Indian research team has discovered fresh signs of tectonic activity in the south pole of the Moon1

Analysing images captured by sophisticated instruments on board India's Moon mission Chadrayaan-1, Saumitra Mukherjee and his colleague Priyadarshini Singh from the School of Environmental Sciences at Delhi's Jawaharlal Nehru University detected a 20-km long cliff-like structure known as ‘lobate scarp' between two craters. They also found distinct signs of debris avalanches on the inner walls of impact craters. The team says such geological features are the handiwork of tectonic activities such as movements of the lunar crust that are still shaping the the Moon's surface.

Narrow angle camera image of a partially crosscut crater in the scarp region.
© Mukherjee, S. et al.
“The lobate scarps mostly form in the highlands of lunar south polar regions. They are small and young,” Mukherjee told Nature India.  He said that by identifying lobate scarps on the lunar surfaces, it is possible to locate flat surfaces between two lobate scarps where future spacecraft could land.   

For a long time, scientists had believed that volcanism ended within a billion years after the birth of the Moon. Seismometers placed on the Moon by the Apollo missions recorded moonquakes which had been mostly attributed to meteorite strikes or the Earth's gravitational pull on the Moon.

But evidence for lunar tectonic activities had long eluded scientists. In 2010, geologist Thomas Watters and his colleagues from the US-based Smithsonian Institution identified lobate scarps at many sites across the lunar surfaces. They claimed that some had formed less than a billion years ago, others could be as young as a hundred million years.

The Indian researchers identified similar scarps by analysing microwave sensor (Mini-SAR) data and images captured by the Lunar Reconnaissance Orbiter Camera on Chandrayaan-1. Besides scarps, they found faults as lineaments – surface manifestations of displacements of rocks beneath the ground – in two craters – Cabeus B and Wiechert J near the lunar south pole.

“Both craters have piles of crushed rocks that could be debris from avalanches, possibly triggered by subsurface movements of the lunar crust,” explains Mukherjee. “The formation of such geological features including lobate scarps may be associated with the release of compressive stress in lunar crust leading to decrease in the global lunar radius.”

This corroborates the previous discovery of lobate scarps by Watters and his team mates who suggested that such geological upheavals might have shrunk the distance between the Moon's centre and the surface by about 300 feet.

Detailed analyses of the data and images ruled out the possibility of any impact event by meteorites that could have formed the geological features like scarps and debris avalanches. In addition, a higher surface roughness in the lunar regions of the deformation validates significantly recent formation of the feature since it has not been inundated or degraded by meteorite impacts, Mukherjee claims.

“Lobate scarps must be young; otherwise everyday small meteor bombardment would have obliterated them,” he adds. “These are the evidences based on which it can be said that the Moon is still geologically active.


References

1. Mukherjee, S. et al. Identification of tectonic deformations on the south polar surface of the moon. Planet. Space. Sci. 112, 46-52 (2015)

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