A pair of relatively recent missions - Japan's Hayabusa-2 and the US' OSIRIS-REx - gave scientists a precious batch of previously impossible data: photographs of asteroids at the closest possible distance. And mankind was in for a surprise, because the small space bodies looked somewhat different than previously imagined. Their surface appeared to be devoid of dust and covered with boulders and cobbles. A new study has proposed a natural mechanism to explain this difference between predictions and reality.
A scientific paper describing a series of experiments and modelling results has been published in the journal Nature Astronomy. It has been written by researchers at the University of Colorado at Boulder (UCB), led by Hsiang-Wen Hsu. The idea for the study came after images of the asteroid (101955) Bennu, which can distinguish the smallest details of its surface, were published in 2020. They were taken by the OSIRIS-REx spacecraft during its maximum approach to the object of its study for soil sampling.
The popular and scientific view is that asteroids are lumps of large, hard rock fragments and fine dust. Thus, their surface must be covered with sand "lakes" with occasional inclusions of protruding rocks. It sounds logical: tiny particles of regolith are more easily attracted even by weak gravity and fill the space between larger cobbles, gravel and pebbles. The reality, however, turned out to be different.
In the images sent by the OSIRIS-REx probe, the surface (101955) of Bennu looks like poor-quality sandpaper with a very irregular grain size. Or rather, it looks like a close-up of a junk heap from a quarry - no dust and almost no sand, just random pebbles. Hayabusa-2, which sent its first images of another relatively small asteroid - (162173) Ryugu - two years earlier, showed a similar picture.
The authors of the new study drew attention to the work of their colleagues at the Laboratory of Atmospheric and Space Physics (LASP) at UCB. For more than 30 years, that institution has been studying the behaviour of regolith particles in a vacuum. By repeating some particularly revealing experiments, Hsu's team hypothesised the following mechanism, which determines the shape of asteroid surfaces.
Tiny dust particles are negatively charged by the action of sunlight. At some point, it becomes large enough to cause two nearby dust particles to bounce off each other. A close look at the samples in the vacuum chamber can make it look like very small popcorn is being fried in a pan. In some cases, the resulting repulsive velocity reaches notable values, as much as eight metres per second. For small asteroids like Ryugu and Bennu, it exceeds the first cosmic velocity.
Given the experimental data, the researchers have built a model of such an object in interplanetary space. Just a few hundred thousand years a virtual asteroid with a diameter of about 800 meters (larger than Bennu, but a little smaller than Ryugu) completely lost all the dust from its surface. In addition such objects rotate and the rocks of which they are composed constantly go through cycles of heating up by sunlight and cooling down.
As a result, sooner or later they crack, spewing out new portions of dust, which eventually fly off into space. So, without any noticeable effect, small asteroids slowly lose mass.
