Science News describes how the MASCOT asteroid lander has found the near-Earth asteroid Ryugu oddly devoid of dust:
The asteroid, thought to have formed from the breakup of a larger body around 700 million years ago, has no atmosphere to protect it from interplanetary dust streaming through the solar system. These miniature missiles pummel exposed space rocks at high speed, breaking down their surfaces into thin layers of dust and dirt, such as those found on the moon and the asteroid Vesta.
But when MASCOT bounced across Ryugu in October 2018, the lander took high-resolution photos that show no sign of any dust-sized particles, down to a resolution of about 100 micrometers, about the thickness of a sheet of paper, researchers report in the Aug. 23 Science.
Or Ryugu could spray dust into space when sunlight heats patches of trapped ice and releases volatile gases. A similar asteroid, Bennu, seems to spew plumes of small rocks into space, according to NASA’s OSIRIS-REx spacecraft. But [German Aerospace Center scientist Ralf] Jaumann thinks that explanation is less likely for Ryugu. Observations from the Japanese Hayabusa2 craft, which has been orbiting Ryugu since June 2018 and brought MASCOT along, suggest that Ryugu has less water in its minerals than Bennu.
An answer to the mystery may not come until after Hayabusa2 returns to Earth with samples of Ryugu’s surface and subsurface in late 2020.