Moon has been `actively growing and shrinking` in recent past

Moon is often considered as a geologically dead fixture in our sky.

Updated: May 15, 2012, 08:26 AM IST

Washington: Moon is often considered as a geologically dead fixture in our sky.
However, new images returned from NASA’s Lunar Reconnaissance Orbiter (LRO) suggest that our natural satellite isn’t dead at all. It’s actually pretty active having both shrank and grown fairly recently in its history, Discovery News reported.

The first evidence of an active moon came in 2010 when LRO’s camera returned high-resolution images of landforms called lobate scarps.

Earlier found only in the equatorial regions in images from Apollo missions 15, 16, and 17, these lobe-shaped cliffs have now been found scattered across the lunar surface.

Less than 10 meters high and several kilometres long, lobate scarps form along thrust faults - inclined fractures where blocks of a body’s crust rise vertically.

Most likely they formed when the moon’s interior cooled and the rock contracted to force slices upwards.

These are common on the geologically active Earth, but finding them on the moon was a surprise.

Also, lunar scarps are quite young; many have sliced ancient impact craters. As the moon’s core has continued to cool in the comparatively recent past, the crust has buckled under compression.

Based on the sizes of the scarps, scientists have estimated that the distance between the centre of the moon and its surface shrank about 300 feet in the process. It’s a very different moon than the dead one scientists have imagined since long.

Now add features called graben to the picture. Graben are linear valleys normally much longer than they are wide, and finding these features on the moon was a complete shock to mission scientists.

They formed, like the lobate scarps, when the moon cooled. But instead of forming when the moon’s crust buckled, they formed when the crust was stretched, broke, and dropped down between two bounding faults. In a nutshell, it’s growing.

Around the same time the moon was shrinking around its cooling core, forces at some places were acting to pull it apart.

Taken together, these discoveries paint a very different picture of our moon, and it all is linked to the moon’s evolution, how it formed, and how it lost its heat.

Most terrestrial planets in our solar system were so hot when they first formed that they totally melted. This put them in a general state of contraction - the inside stayed hot while the outer layer cooled.

The mix of scarps and graben found on the moon imply that our satellite never completely melted in its earliest stages of formation. Instead, only the outer portion melted, covering the surface with an expansive layer of molten rock dubbed magma ocean.

The balance of stresses acting on that early moon creates the right conditions for the somewhat contradictory surface features we can see presently.

These graben add to the proof of recent geologic activity on the moon.

Scientists believe that these features are only around 50 million years old compared to the moon’s 4.5 billion year life.