Samples brought back by the Hayabusa2 mission show that the asteroid Ryugu was hit by a barrage of micrometeorites only 1,000 years ago. The evidence? A thin film of sodium, a few nanometers thick, that tells the story of this cosmic event.
This accumulation is surprising because volatile elements like sodium are normally blown away by the solar wind or scattered into space. Yet researchers detected this layer on the surface of fragments brought back to Earth. According to their estimates, Ryugu passed through a dense cloud of micrometeorites about 1,000 years ago.
Artist's rendition of the Hayabusa probe skimming the asteroid in 2018 / Japan Aerospace Exploration Agency (JAXA)
Scientists used analytical techniques capable of studying layers just nanometers thick. By examining two fragments of Ryugu, they discovered not only sodium but also microscopic impact craters and glassy structures. These clues confirm that the asteroid underwent an intense micrometeorite bombardment.
Ernesto Palomba, of the Italian National Institute for Astrophysics, explains that sodium should have disappeared under the effect of the solar wind. The presence of this element therefore indicates that the impact is very recent on a geological timescale. The team also observed an enrichment in iron, a sign of additional alteration by the solar wind.
These observations show that near-Earth asteroids are regularly struck by swarms of meteoroids. For planets like our own, the atmosphere provides protection and turns these encounters into luminous displays such as the Perseids. For an asteroid without an atmosphere, the consequences are quite different.
Researchers now plan to reproduce Ryugu's chemical observations in the lab. These experiments will help better understand how micrometeorites alter the surface of small celestial bodies. The findings were published in the journal
The Astrophysical Journal Letters.