An international group of planetary scientists from Russia, Finland and the United States for the first time analyzed the factors that determine the number of boulders on the surface of the nearest planet from the Sun - Mercury.
Boulders are fragments of rock that are formed as a result of meteorite impacts on the planet's surface and are located in areas of fresh impact craters hundreds of meters in diameter and wider. Their exact age is difficult to estimate, but the fact that they are deeper than heavily degraded craters of similar size indicates their relative youth.
Scientists have published a description of the studies and conclusions from them in the journal Iсarus, one of the leading scientific publications in the world, devoted to the study of the solar and other planetary systems. First, the researchers examined all images of the surface of Mercury, obtained in 2015 during the last phase of flights over it by the automatic interplanetary station Messenger.
In three thousand images, scientists noted only 14 boulders up to five meters in size (smaller objects are indistinguishable). Then scientists monitored images of the lunar surface: Mercury is the smallest planet in the solar system, only slightly larger than the moon, and their surfaces are similar.
The resolution of the images of the Moon (they were taken by the automatic station Lunar Reconnaissance Orbiter, launched to the Moon in 2009) is much better. Therefore, in order to make a correct comparison with the Mercurian "portraits" of the Earth's satellite, it was necessary to specially "worsen" - to the quality of the photographs of Mercury.
“A comparison of materials led us to the conclusion that boulders are found on Mercury about 30 times less frequently than on the Moon. Of course, this quantitative relationship is not accurate. Nevertheless, we can confidently conclude that boulders on Mercury are much rarer than on the Moon,”emphasizes Mikhail Kreslavsky, a researcher at the University of California.
The authors of the article attributed the difference they found to three factors. The first is a micrometeorite stream to Mercury. Due to the planet's proximity to the Sun, it is more than 50 times more abundant and 1, 5–5, 5 times faster than the flow to the Moon. Microscopic particles of interplanetary dust, hitting Mercury boulders, grind them like an abrasive material almost 15 times faster than on the Moon.
The second factor is a thicker layer of regolith (loose soil) on the surface of Mercury, a “product” of the same intense micrometeorite stream. Due to the special thickness of the regolith, space objects tens of meters in size, hitting Mercury, “traumatize” the planet less and knock out much fewer boulders from its soil. The erasure of boulders by microparticles in combination with the effect of thick regolith is perhaps the main condition explaining the insignificant number of boulders on Mercury, planetary scientists suggest.
Finally, the third factor is the cyclical changes in temperature that occur on Mercury during a solar day (it is equal to 176 Earth days). As the closest to the Sun, Mercury has the largest temperature drops of all the planets in the solar system: from -180 ° C to + 430 °.
“Large thermal stress, which is 2.5 times stronger than on the Moon, causes rapid material fatigue, numerous large and microcracks and - ultimately - the destruction of boulders. On the moon, their "life" reaches 100 million years, "explains Maria Gritsevich, senior researcher at the Ural Federal University and the Finnish Institute for Geospatial Research, associate professor of planetary research at the University of Helsinki.
Thus, the comparatively small number of boulders on Mercury, scientists determine both the rarer cases of formation, and the faster process of destruction. “Our analysis is limited by the poor quality of most available images of the surface of Mercury and the very limited number of high-resolution images.
Research can be continued on the basis of the next mission to Mercury. The new automatic station of the European Space Agency with Japanese participation, BepiColombo, was launched in October 2018 and is scheduled to reach Mercury in December 2025,”notes Maria Gritsevich.