Glass beads in lunar soil reveal ancient asteroid bombardments of Moon and Earth

In 2020, China’s Chang’e 5 mission extracted more than a kilogram of lunar rock and soil and returned it to Earth. The samples contain myriad tiny glass beads, formed when asteroids struck the moon and sprayed droplets of molten rock around the impact site.

We have analyzed these glass beads and the impact craters near where they were found in great detail. Our results, published in scientific advancesreveal new details about the history of asteroid impacts on the moon over the past 2 billion years.

In particular, we found evidence of multiple impact waves occurring simultaneously with impacts on Earth – including the Chicxulub impact 66 million years ago that led to the extinction of the dinosaurs.

Billion years space rocks

The destructive power of meteorite impacts has been demonstrated throughout human history. The most recent notable event of 2013, the spectacular Chelyabinsk meteor that injured hundreds of people, was a relatively minor event compared to historic impacts.

Impacts of varying magnitude have occurred over the long geological history of the Earth. Only about 200 impact craters have been found worldwide as erosion and geological activity continually alters our planet’s surface, erasing evidence of past impacts.

Hundreds of millions can be seen on the moon, where impact craters persist. It’s not hard to imagine that Earth experienced a similarly overwhelming barrage of projectiles early in its life.

As the solar system has evolved over the past 4.5 billion years, the number of asteroids has decreased exponentially over time as space rock has been swept away from Earth and the other planets.

However, the details of this process remain unclear. Has there been a gentle decline in the number of impacts on the Earth, Moon, and other planets in the solar system over time? Against this general backdrop of decline, are there times when collisions became more common? Is there a possibility that collisions will suddenly increase in the future?

Splattered Glass

The best available place to look for answers is on the moon, and the best available samples are lunar soils – like the ones Chang’e 5 brought home.

Moon soil contains spherical droplets of solidified melt (glass) ranging in size from a few millimeters to less than a millimeter. These droplets are formed during high-velocity blasts that melt the target rock.

The molten droplets can splatter tens or possibly hundreds of kilometers around the impact crater.

By analyzing the chemical composition and radioactivity of these droplets, we can determine how old they are. The age of the droplets then gives us a clue as to when these impacts on the moon occurred.

Each lunar soil sample appears to record multiple impacts. The ages of the impacts are spread over the last ~4 billion years, with the youngest being only a few million years old.

A simple landing spot

Chang’e 5 landed in a location with a relatively simple geological history compared to other locations on the moon where samples have been collected.

The landing site is in the middle of a huge basalt plateau with a diameter of almost 400 kilometers. The plateau is “only” 2 billion years old, which is young compared to the age of the lunar crust as a whole.

This means the site’s history is shorter and easier to unravel. This facilitated the identification of droplets originating from nearby impacts and the interpretation of chemical and chronological data using satellite imagery of the surrounding lunar surface.

We combined this interpretation with modeling of how the droplets would have formed and splattered for different sized impacts.

It appears that glass droplets can be transported 20 to 100 kilometers from the impact site, even if the impact leaves a crater only 100 meters in diameter. Models also show that impacts that form craters larger than 1 kilometer in diameter are more efficient at producing the droplets.

All of this information combined helped initiate the search for specific impact craters responsible for producing glasses extracted from the sample.

Crater Hunt

The basalt plateau surrounding the Chang’e 5 landing site contains more than 100,000 craters over 100 meters in size. Matching glass droplets to their crater of origin is a game of chance, although the odds are slightly better than winning the lottery.

We can say that some of the craters are probably the source of some of the glass droplets in the sample. Nevertheless, this matching led to another important result.

Previous studies had found that the age distribution of the glass droplets in the individual soil samples was uneven. There are periods in the timeline with a large number of droplets and periods with few to none.

Our analysis of the glass in the Chang’e 5 samples and our attempts to associate it with specific craters confirm a variation in the impact rate over time.

In addition, the ages of the periods identified from these droplets appear to be similar to those visible in a number of existing meteorite groups originating from the asteroid belt. These meteorite clusters could be the result of previous collisions within the asteroid belt.

One of these cluster ages also coincides with the extinction of the dinosaurs. Our study did not examine this in detail, but this coincidence may indicate that, for reasons still unknown, there are periods when regular orbits of small bodies in the Solar System destabilize and swing into orbits where they can hit Earth or the Moon.

Taken together, these ages suggest that there may have been periods in Earth’s history when collisions increased throughout the inner Solar System. This means that the Earth may also have experienced periods when the rate of impacts was higher than usual – and that similar increases are possible in the future.

How would such an increase affect the evolution of life on the planet? That remains a mystery.

This article was republished by The Conversation under a Creative Commons license. Read the original article.