Shrinking moon starts to crack up

  • Cliffs or scarps suggest Moon has shrunk in the recent past
  • It could still be slowly shrinking today
  • Observations made by NASA’s Lunar Reconnaissance Orbiter spacecraft

Newly discovered cliffs in the lunar crust indicate the Moon shrank globally in the geologically recent past and might still be shrinking today, according to a team analysing new images from NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft. The results provide important clues to the Moon’s recent geologic and tectonic evolution.

The Moon formed in a chaotic environment of intense bombardment by asteroids and meteors. These collisions, along with the decay of radioactive elements, made the Moon hot. The Moon cooled off as it aged, and scientists have long thought the Moon shrank over time as it cooled, especially in its early history.

Gregory Scarp on the Moon

A prime example of a lunar scarp is this one, called Gregory Scarp. The scale is in metres.

The new research reveals relatively recent tectonic activity connected to the long-lived cooling and associated contraction of the lunar interior.

“We estimate these cliffs, called lobate scarps, formed less than a billion years ago, and they could be as young as a hundred million years,” said Dr. Thomas Watters of the Centre for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum, Washington.

While ancient in human terms, it is less than 25 percent of the Moon’s current age of more than four billion years.

“Based on the size of the scarps, we estimate the distance between the Moon’s centre and its surface shrank by about 300 feet [100 metres],” said Watters, lead author of a paper on this research appearing in the August 20 issue of the journal Science.

The scarps are relatively small; the largest is about 100 metres high and extends for several kilometres or so, but typical lengths are shorter and heights are more in the tens of metres range.

Earth not to blame

The team believes they are among the freshest features on the Moon, in part because they cut across small craters.

Since the Moon is constantly bombarded by meteors, features like small craters (those less than about 400 metres across) are likely to be young because they are quickly destroyed by other impacts and don’t last long.

Diagram of a lunar scarp

Scarps on the Moon could be produced as the rocky world cools and contracts.

So, if a small crater has been disrupted by a scarp, the scarp formed after the crater and is even younger. Even more compelling evidence is that large craters, which are likely to be old, don’t appear on top any of the scarps, and the scarps look crisp and relatively un-degraded.

Because the scarps are so young, the Moon could have been cooling and shrinking very recently, according to the team.

Seismometers emplaced by the Apollo missions have recorded moonquakes. While most can be attributed to things like meteorite strikes, the Earth’s gravitational tides, and day/night temperature changes, it’s remotely possible that some moonquakes might be associated with ongoing scarp formation, according to Watters.

The team plans to compare photographs of scarps by the Apollo Panoramic Cameras to new images from LRO to see if any have changed over the decades, possibly indicating recent activity.

While Earth’s tides are most likely not strong enough to create the scarps, they could contribute to their appearance, perhaps influencing their orientation, according to Watters. During the next few years, the team hopes to use LRO’s high-resolution Narrow Angle Cameras (NACs) to build up a global, highly detailed map of the Moon.

This could identify additional scarps and allow the team to see if some have a preferred orientation or other features that might be associated with Earth’s gravitational pull.

Adapted from information issued by NASA.

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