(ORDO NEWS) — A couple of relatively recent missions – the Japanese Hayabusa-2 and the American OSIRIS-REx – have given scientists a valuable piece of previously impossible data: photographs of asteroids from as close as possible.
And humanity was in for a surprise, because small cosmic bodies looked somewhat different than previously imagined.
Their surface was devoid of dust and was covered with boulders and cobblestones. 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 simulation results was published in the journal Nature Astronomy. It was prepared by specialists from the University of Colorado at Boulder (UCB) under the guidance of Hsiang-Wen Hsu.
The idea for the study came about after the publication in 2020 of images of the asteroid (101955) Bennu, in which it was possible to distinguish the smallest details of its surface.
They were made by the OSIRIS-REx apparatus during the closest approach to the object of their study for soil sampling.
According to the point of view, widespread both in mass consciousness and in scientific circles, asteroids are lumps of large solid fragments of rock and fine dust. Thus, their surface should be covered with sandy “lakes” with occasional inclusions of protruding stones.
It sounds logical: the smallest particles of regolith are more easily attracted by even weak gravity and fill the space between larger cobblestones, gravel and pebbles. However, the reality 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 uneven grain size. Or rather, it looks like a close-up of a dump of unnecessary rock from a quarry – no dust and almost no sand, only pebbles of various colors.
A similar picture was shown by the Hayabusa-2 apparatus, which sent the first images of another relatively small asteroid – (162173) Ryugu – two years earlier.
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, this institution has been studying the behavior of regolith particles in a vacuum.
By repeating some particularly revealing experiments, Hsu’s team hypothesized the following mechanism that determines the appearance of the surface of asteroids.
The smallest particles of dust under the action of sunlight acquire a negative charge. At some point, its value becomes sufficient for two nearby dust particles to repel each other. Looking closely at the samples in the vacuum chamber, it may look like very small popcorn is being fried in a frying pan.
In some cases, the speed obtained as a result of repulsion reaches noticeable values - up to eight meters per second. For small asteroids like Ryugu and Bennu, it exceeds the first space one.
Given the experimental data, the researchers built a model of such an object in interplanetary space. In just a few hundred thousand years, a virtual asteroid with a diameter of about 800 meters (larger than Bennu, but slightly smaller than Ryugu) completely lost all dust from its surface.
In addition, such objects rotate, and the stones of which they are composed are constantly going through cycles of heating by sunlight and cooling.
As a result, sooner or later they crack, throwing out a new portion of dust, which eventually escapes into space. It turns out that without any noticeable impacts, small asteroids gradually lose mass.
The same effect can affect the trajectory of small celestial bodies – escaping dust transfers momentum to the “parent” object. If the regolith is thrown unevenly in different directions, then the asteroid’s orbit will also change.
True, it is not yet possible to notice such small fluctuations; much longer observations are needed than have been carried out so far.
But large objects, the gravity of which can keep grains of sand flying at a speed of several meters per second, retain dust on the surface.
And thus they are more tenacious than their smaller counterparts. The fact is that a layer of dust partially insulates the underlying rocks from temperature effects and slows down their erosion.
In addition to modeling, the theory of American scientists will be tested in practice in less than three months. This fall, the DART mission will approach the asteroid (65803) Didymus and its satellite Dimorph. The latter is a very small object with a diameter of 160 meters.
If the calculations of Hsu and his colleagues are correct, the surface of both bodies will be practically devoid of dust (Didim is comparable in size to Ryugu). Otherwise, the theory will have to be modified.
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