The Moon Is Getting Slammed Way More Than We Thought

Animation of a temporal pair of the new 39-foot (12-meter) impact crater on the moon photographed by NASA’s Lunar Reconnaissance Orbiter Credit: NASA/GSFC/Arizona State University

We often hear how the Moon’s appearance hasn’t changed in millions or even billions of years. While micrometeorites, cosmic rays and the solar wind slowly grind down lunar rocks, the Moon lacks erosional processes such as water, wind and lurching tectonic plates that can get the job done in a hurry.

One of a series of photos Apollo 11 astronaut Edwin Aldrin made of his bootprint in the dusty, sandy lunar soil, called regolith. Based on a newy study, the impression may disappear in a few tens of thousands of years instead a few million. Credit: NASA

Remember Buzz Aldrin’s photo of his boot print in the lunar regolith? It was thought the impression would last up to 2 million years. Now it seems that estimate may have to be revised based on photos taken by the Lunar Reconnaissance Orbiter (LRO) that reveal that impacts are transforming the surface much faster than previously thought.

This map shows the distribution of new impact craters (yellow dots) discovered by analyzing 14,000 narrow-angle camera (NAC) temporal pairs. The two red dots mark the location of the March 17, 2013 and September 11, 2013 impacts that were recorded by Earth-based video monitoring. LRO’s mission was recently extended an addition two years through September 2018. Credit: NASA/GSFC/ASU
The LRO’s high resolution camera, which can resolve features down to about 3 feet (1-meter) across, has been peering down at the Moon from orbit since 2009. Taking before and after images, called temporal pairs, scientists have identified 222 impact craters that formed over the past 7 years. The new craters range from 10 feet up to 141 feet (3-43 meters) in diameter.

By analyzing the number of new craters and their size, and the time between each temporal pair, a team of scientists from Arizona State University and Cornell estimated the current cratering rate on the Moon. The result, published in Nature this week, was unexpected: 33% more new craters with diameters of at least 30 feet (10 meters) were found than anticipated by previous cratering models.

LRO before and after images of an impact event on March 17, 2013. The newly formed crater is 59 feet (18 meters) in diameter. Subsurface regolith not exposed to sunlight forms a bright halo around the new crater. There also appears to be a larger nimbus of darker reflectance material visible much further beyond but centered on the impact. Credit: NASA/GSFC/Arizona State University

Similar to the crater that appeared on March 17, 2013 (above), the team also found that new impacts are surrounded by light and dark reflectance patterns related to material ejected during crater formation. Many of the larger impact craters show up to four distinct bright or dark reflectance zones. Nearest to the impact site, there are usually zone of both high and low reflectance.  These two zones likely formed as a layer of material that was ejected from the crater during the impact shot outward to about 2½ crater diameters from the rim.

An artist’s illustration of a meteoroid impact on the Moon. Impacts dig up fresh material from below as well as send waves of hot rock vapor and molten rock across the lunar landscape, causing a much faster turnover of the moon soil than previously thought. Credit: NASA

From analyzing multiple impact sites, far flung ejecta patterns wrap around small obstacles like hills and crater rims, indicating the material was traveling nearly parallel to the ground. This kind of path is only possible if the material was ejected at very high speed around 10 miles per second or 36,000 miles per hour! The jet contains vaporized and molten rock that disturb the upper layer of lunar regolith, modifying its reflectance properties.

How LRO creates temporal pairs and scientists use them to discover changes on the moon’s surface.

In addition to discovering impact craters and their fascinating ejecta patterns, the scientists also observed a large number of small surface changes they call ‘splotches’ most likely caused by small, secondary impacts. Dense clusters of these splotches are found around new impact sites suggesting they may be secondary surface changes caused by material thrown out from a nearby primary impact. From 14,000 temporal pairs, the group identified over 47,000 splotches so far.

Here are two examples of a low reflectance (top) and high reflectance (bottom) splotch created either by a small impactor or more likely from secondary ejecta. In either case, the top few inches of the regolith (soil) was churned Credit: NASA/GSFC/Arizona State University
Based on estimates of size, depth and frequency of formation, the group estimated that the relentless churning caused by meteoroid impacts will turn over 99% of the lunar surface after about 81,000 years. Keep in mind, we’re talking about the upper regolith, not whole craters and mountain ranges. That’s more than 100 times faster than previous models that only took micrometeorites into account. Instead of millions of years for those astronaut boot prints and rover tracks to disappear, it now appears that they’ll be wiped clean in just tens of thousands!

Bob King

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. I also write a daily astronomy blog called Astro Bob. My new book, "Wonders of the Night Sky You Must See Before You Die", a bucket list of essential sky sights, will publish in April. It's currently available for pre-order at Amazon and BN.

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