In 2020, China's Chang'e 5 mission sampled more than a kilogram of moon rock and soil and brought it back to Earth. The samples contain countless tiny beads of glass, created when asteroids hit the moon and splashed out droplets of molten rock around the impact site.Get more news about
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We have analyzed these glass beads and the impact craters near where they were found in great detail. Our results, published in Science Advances, reveal new details about the history of asteroids hitting the moon over the past 2 billion years.
In particular, we found traces of several waves of impacts occurring at the same times 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 seen throughout human history. Recently notable event from 2013, the spectacular Chelyabinsk meteor that injured hundreds of people, was a relatively minor occurrence compared to historical impacts.
Impacts of various scales have happened throughout Earth's long geological history. Only about 200 impact craters have been found around the world, because erosion and geological activity are constantly modifying our planet's surface and erasing evidence of past impacts.
On the moon, where impact craters don't go away, several hundred million are recognizable. It is not difficult to imagine Earth experienced a similar staggering barrage of projectiles early in its life.
As the solar system evolved over the last 4.5 billions of years, the number of asteroids declined exponentially over time as space rocks were swept up by Earth and the other planets.
However, the details of this process remain murky. Was there a smooth decay over time in the number of impacts on Earth, moon and other planets in the solar system? Are there periods when collisions became more frequent, against this general background of decline? Is there a possibility that collisions may suddenly increase in the future?
Splattered glass
The best available place to search for answers is the moon, and the best available samples are lunar soils—like the ones Chang'e 5 brought home.Lunar 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-speed impacts that melt the target rock.
The melted droplets can splash out for tens or possibly hundreds of kilometers around the impact crater.
By analyzing the chemical makeup and radioactivity of these droplets, we can determine how old they are. The ages of the droplets then gives us an indication of when these impacts happened on the moon.
Each lunar soil sample appears to record multiple impacts. The ages of the impacts are spread over the past ~4 billion years, with the youngest being only a few million years old.
