Scientists explain the rocky surface of the asteroid Bennu


At the end of 2018, the American interplanetary station OSRIS-REx entered orbit near Bennu, a small near-Earth B-class asteroid less than 600 meters in diameter. The main task of the spacecraft was to take a soil sample from the surface of Bennu for subsequent sending it back to Earth. However, even during the approach to the asteroid, experts planning the mission, faced a problem: the surface of Bennu was covered with large amounts of large debris rocks, making it very difficult to land. This was completely contrary to what the astronomers had hoped for. At the start of the mission, it was assumed that the asteroid would be covered with fine regolith.

Subsequently, NASA experts were able to adjust the landing program and find a suitable place for it, and now scientists from France and the United States have published a study explaining this kind of surface Bennu.

Flying close to the asteroid, OSIRIS-REx captured high-resolution images of its entire surface in the visible and infrared ranges. The first thing the scientists noted was the extreme irregularity of the fine regolith distribution.

Infrared images allow us to determine the temperature of rocks, and having many images at different times of the day we can determine their thermal conductivity. The temperature properties of regolith and large rock fragments are different: in the first case they depend on particle size, and in the second – on the density and, therefore, porosity of rocks.

A study using machine learning technology showed that several tens of percent of all the regolith found on the surface of Bennu is concentrated in a few places, composed of dense rocks. The bulk of the asteroid is covered with porous rocks, from which the regolith was not formed.

Planetary scientists believe that when meteorites strike porous rocks, their energy is spent on destroying the pores, rather than crushing the clastic material. And porous rocks are more compressed on impact than destroyed. This statement was confirmed in a laboratory experiment. In addition, the destruction due to temperature fluctuations during the day in porous rocks is slower than in high-density rocks.

Indirectly on a similar pattern indicate and other studies of asteroids. Thus, the Japanese interplanetary station Hayabusa 2 (Hayabusa 2) found that the asteroid Ryugu is entirely covered with large debris. Like Bennu, it is a carbon asteroid. But the S-class asteroid Itokawa, studied by the first Hayabusa, is covered with small regolith. Observations from Earth indicate that it is less porous.

Scientists now believe that a large amount of regolith is in principle atypical for carbonaceous asteroids. Conversely, slightly less common silicon asteroids will have few large pieces of raw material debris on the surface and lots of fine or dusty regolith.

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