The scarps had previously been noted at the moon's equator, but this is the first evidence in other areas, indicating they result from a global process.

Dr Francisco Diego, a fellow of the Royal Astronomical Society in London, agreed with his US colleagues, saying the shrinking was "almost negligible".

"It is an interesting fact to see how these objects in the solar system develop and evolve for us to know ... the origins of these bodies and how they are behaving," he told Al Jazeera.

No large craters

The Science study calls the scarps "evidence of recent thrust faulting on the moon". But this is planetary science, where "recent" can mean a billion years ago.

The scarps, or cliffs, extend across some small craters, and small craters tend to be obliterated over time, Watters said.

In addition, there are no large craters imposed on top of the scarps, another indication they are relatively recent, in planetary terms, he said.

"One of the really cool parts of this ... the faults are so young-looking that you can't escape the possibility that this contraction occurred recently, and could indicate that the moon is still active," he said.

The size of the scarps indicates a shrinkage in the size of the moon of about 100 metres, which would not be nearly enough to be noticed with the naked eye. The moon is about one-fourth the size of the earth in diameter.

The scarps range up to 10 metres high and a few kilometers long, according to Watters.

By comparison, the planet Mercury has much larger scarps indicating considerably more shrinkage over time.