In 2020, China’s Chang’e 5 mission sampled more than a kilogram of moon rock and soil and returned it to Earth. The samples contained numerous tiny glass beads, created when asteroids hit the moon and splashed droplets of molten rock around the impact site.
We analyzed these glass beads and the impact craters near where they were found in detail. our results, published in scientific progressreveal new details about the history of asteroids that have hit the moon over the past 2 billion years.
In particular, we found traces of several waves of impacts that occurred at the same time as impacts on Earth, including the Chicxulub impact 66 million years ago that led to the extinction of the dinosaurs.
Billions of years of space rocks
The destructive power of meteorite impacts has been observed throughout human history. Recently notable event from 2013, the spectacular Chelyabinsk meteor that hundreds of people injuredwas a relatively minor event compared to historical effects.
Throughout the Earth’s long geological history, impacts of different scales have occurred. Only about 200 impact craters have been found all over the world, as erosion and geological activity are constantly altering our planet’s surface, erasing evidence of past impacts.
On the moon, where impact craters do not disappear, several hundreds of millions are recognizable. It’s not hard to imagine that Earth experienced a similar dizzying barrage of projectiles early in its life.
As the solar system evolved over the past 4.5 billion years, the number of asteroids declined exponentially over time as space rocks were dragged along by Earth and the other planets.
However, the details of this process remain obscure. Was there a gradual decline in the number of impacts on the Earth, moon and other planets in the solar system over time? Are there periods when collisions were more frequent, against this general background of decline? Is there a possibility that the number of collisions will suddenly increase in the future?
The best available place to look for answers is the moon, and the best available monsters are lunar soils, like the one Chang’e 5 brought home.
Moon soil contains spherical droplets of solidified melt (glass) with sizes ranging from a few millimeters to less than a millimeter. These droplets are formed during high-velocity collisions that melt the target rock.
The molten droplets could splash out 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 an indication of when these impacts on the moon took place.
Each lunar soil sample seems to register multiple effects. The ages of the effects are spread over the past ~4 billion years, with the youngest being only a few million years old.
A simple landing site
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 located in the middle of a vast basalt plateau almost 400 kilometers wide. The plateau is “only” 2 billion years old, which is young relative to the age of the total lunar crust.
This makes the history of the site shorter and easier to unravel. This made it easier to identify droplets from nearby impacts, and to interpret chemical and chronological data from satellite images of the surrounding lunar surface.
We combined this interpretation with modeling how the droplets would have formed and ejected on impacts of different sizes.
It turns out that glass droplets can be transported 20 to 100 kilometers from the site of the impact, even if the impact leaves a crater only 100 meters wide. Models also indicate that impacts that form craters more than 1 kilometer in diameter are more efficient at producing the droplets.
All this information together helped to find specific impact craters responsible for the production of glasses extracted from the sample.
The basalt plateau surrounding the Chang’e 5 landing site contains more than 100,000 craters over 100 meters in size. Matching glass drops 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. However, this matching led to another important result.
Previous studies had shown that the age distribution of glass droplets in the individual soil samples is uneven. There are periods in the timeline with large numbers of drops and periods with little to none.
Our analysis of glass in the Chang’e 5 samples and our attempts to link them to specific craters confirm a variation in impact velocity over time.
In addition, the ages of the periods identified from these droplets appear to be similar to those visible in a number of extant meteorite groups originating in the asteroid belt. These meteorite groups may be the result of ancient collisions in the asteroid belt.
One of these cluster ages also coincides with the extinction of the dinosaurs. Our study has not examined this in detail, but this coincidence may indicate that, for reasons as yet unknown, there are periods when regular orbits of small bodies in the solar system destabilize and enter orbits where they can hit the Earth or the moon.
Taken together, these ages suggest that there may have been periods in Earth’s history when collisions increased in the inner solar system. This means that Earth could also have experienced periods when the impact was higher than normal — and similar increases are possible in the future.
How would such an increase affect the evolution of life on the planet? That remains a mystery.
Moon glass shows lunar asteroid impacts mirrored on Earth
Quote: Glass beads in lunar soil reveal ancient asteroid bombardments on the moon and Earth (2022, October 1), retrieved October 1, 2022 from https://phys.org/news/2022-09-glass-beads-lunar-soil-reveal . html
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