Editorial: Shrinking Moon

Published in The Hindu on August 28, 2010

After last year’s discovery of traces of water across the surface of the Moon, particularly at higher latitudes, challenged the long-held notion that Earth’s only natural satellite is bone dry, scientists have now come up with another interesting find. Images of the Moon captured by high-resolution cameras on board the Lunar Reconnaissance Orbiter (LRO), launched in June 2009, have revealed 14 steeply sloping cliffs. These cliffs, known as lobate scarps because of their semi-circular shape, have a maximum height of 130 metres and lengths no more than tens of kilometres. They are tectonic landforms produced by thrust faulting. The significance of the landforms, previously identified only near the equator by Apollo missions-15, 16, and 17, was not understood and they were dismissed as local phenomena of little significance. But according to a paper published online in Science lobate scarps were identified on the nearside and farside at both low- and mid-latitudes, and also in the higher latitudes, even very near the poles. Their global distribution tells a different story: the Moon is shrinking. However, it is just a 100-metre reduction in the Moon’s 3,500 km diameter. Shrinking induces stress on the crust, which in turn results in thrust faulting, where one edge of the scarp moves upward relative to the other. The faults are pristine in nature and have undergone little degradation. The scarps should be as young as a hundred million years and not older than a billion years. The age is inferred based on the scarps cutting across smaller craters, which are quickly destroyed by other impacts.

Contrary to the long-held notion that the Moon had shrunk in its early history as it cooled, the presence of recently formed faults indicates that it may still be cooling and shrinking. This in turn indicates that it is not a cold mass of rock that is geologically dead. Unlike the large-scale scarps (nearly 2 km high and about 600 km long) seen on Mercury, the small-sized scarps seen on the Moon signify that it has not shrunk significantly. Small-scale scarps, which signify low compressional stress acting on the Moon, along with only a small change in its radius, are consistent with the thermal history model of our natural satellite. But more detailed studies are required to rule out the role of regolith on the Moon’s surface and the underlying fractured zone in reducing the compressional stress that prevents large-scale thrust faults from forming.