Scientists believe that the surface of the asteroid Bennu is like a beach with a lot of fine sand and pebbles, which would be the perfect choice for collecting samples. In the past, telescope observations from the earth showed that there is a large area of fine-grained matter less than a few centimeters, called fine sandstone. However, when NASA’s OSIRIS-REx mission probe arrived in Bennu at the end of 2018, the mission saw a surface covered by boulders.
When mission scientists observed evidence that might be able to grind boulders into fine stones, the mysterious lack of fine stones became even more surprising. The new research, published in the journal Nature, led by Saverio Cambioni of the University of Arizona, uses machine learning and surface temperature data to solve this mystery. Cambioni conducted this research at the University’s Lunar and Planetary Laboratory. He and his colleagues eventually discovered that Benu’s highly porous rock was responsible for the lack of fine stones on its surface.
The researchers said that when Bennu’s first batch of images came, they noticed that the resolution in some places was not high enough to see if there were small rocks or tiny thunderstones. They began to use machine learning methods, using thermal emission (infrared) data to distinguish between tiny thunderstones and rocks. The thermal emission of fine thunderstone is different from that of large rocks because the size of its particles controls the former, while the latter is controlled by the porosity of the rock.
When the data analysis was completed, the researchers discovered something surprising. The tiny thunderstones are not randomly distributed on Bennu. On the contrary, in areas where there are very few rocks without porosity, its proportion is as high as several tens of percent, while in places where rock porosity is high, that is, most of the surface, its proportion is systematically low. The research team concluded that Bennu’s high-porosity rocks produced very little fines, because these rocks were compressed rather than broken by meteoroid impact. Like sponges, the gaps in the rocks can cushion the blow of meteoroids. These findings are also consistent with laboratory experiments by other research groups.
Basically, most of the energy of the meteor impact is used to smash the pores, limiting the fragmentation of rocks and the production of new fine rocks. In addition, as the asteroid rotates day and night, the cracks caused by the heating and cooling of the Bennu rock proceed more slowly in the porous rock than in the dense rock, which further limits the formation of fine stones.