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The Moon is shrinking
Posted: 19 August 2010

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Evidence that the Moon is shrinking has been found by NASA’s Lunar Reconnaissance Orbiter (LRO), which has discovered thrust faults all over the Moon where the surface has crumpled upwards as the Moon has contracted.

Thrust faults are formed when the core of the Moon contracts, and the surface crust is pushed together. Image: Arizona State University.

“The cooling of the Moon’s interior is causing it to shrink,” says Mike Wargo, NASA’s chief lunar scientist. “Not by a lot, but enough to change the surface.”

Although thrust faults – which create features called lobate scarps – were first seen on the Moon by the panoramic cameras on the Apollo 15, 16 and 17 missions, LRO’s Narrow Angle Cameras have now found them all over the Moon, including at the poles, proving they are a global rather than localised phenomenon, and as such require a global explanation.

One of the lobate scarps on the Moon formed from a thrust fault, inside Gregory Crater on the lunar farside. The arrows point to crustal material pushed up the side of the crater. Image: NASA/Goddard/Arizona State University/Smithsonian.

When the Moon was born, it was born hot. The leading theory behind the birth of the Moon is that it came together after a small protoplanet crashed into a very young Earth, splattering molten debris into space that eventually coalesced into our Moon. It has been gradually cooling ever since. Thrust faults are created as the interior of the Moon cools and contracts, and the surface ‘wrinkles’ up.

What is most surprising is the age of the faults – they are some of the freshest features ever seen on the Moon, as ‘young’ as 800 million years old, and possibly even younger (by comparison, most features on the Moon, such as craters and maria, formed over 3.5 billion years ago). Some are seen cutting across small craters less than 400 metres wide. These craters can be relatively easily eroded or destroyed by other impacts, so they cannot be that old – the fact that they are seen with a scarp running through them implies that the scarp is even younger than the craters. Similarly, no large craters, which would be much older, are seen on top of the scarps. The total contraction of the Moon over the last 800 million years, as measured by the scarps, is as much as 100 metres. “The Moon may still be geologically and tectonically active, and still shrinking today,” says Thomas Watters from the Center for Earth and Planetary Studies at the Smithsonian National Air and Space Museum in Washington DC, who is lead author on a paper describing the results published in the 20 August edition of the journalScience.

A close up of a lobate scarp cutting across young, small craters on the Moon, showing that the scarp must be even younger than the craters. Image: NASA/Goddard/Arizona State University/Smithsonian.

We see similar features on Mercury, where lobate scarps hundreds of kilometres long riddle the innermost planet’s surface. The fact that the Moon’s scarps are tiny in comparison indicates that the Moon did not start off as hot as Mercury did – perhaps only the interior of the Moon was molten.

It may be possible to re-analyse some unique archival data to learn more about the scarps and the rate of contraction. Seismometers left behind on the Moon by the crews of Apollo 12, 14, 15 and 16 recorded various shallow moonquakes. Some of these tremors will have been caused by meteorite impacts, Earth’s gravitational tides, and day/night temperature fluctuations, but some could resulted from instability caused by the slow contraction.

“That is one of the really tantalising possibilities,” says Watters. “We are looking into if there is any correlation between the locations of the moonquakes and the scarps, although at this stage we have not made any definite connection.” In addition, Watters wants to take a look at imagery taken of the Moon during the Apollo missions, and compare them with modern images to see if there have been any changes to surface features between then and now as a result of the contraction.

Since it launched last June, LRO has dramatically changed the way we view the Moon. “Before LRO launched, it was common to think of the Moon as dead, but after a year of observations we are finding that the Moon is a dynamic world,” adds Wargo. “It is not your grandfather’s moon any more. This only marks the beginning of our understanding of its dynamic nature, so stay tuned!”

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