Geologists Kelsey Crane and Christian Klimkzak from the University of Georgia (USA) estimated the rate of cooling of Mercury and the time during which the smallest and closest planet to the Sun in the solar system acquired its present size. The study is published in the journal Geophysical Research Letters and is briefly reported on the American Geophysical Society blogs.
Mercury is lighter and smaller than Earth by about 20 times, the average density is about the same. The year on Mercury lasts 88 days.
Mercury differs from other planets in the solar system by its large metal core - it accounts for 85 percent of the radius of this celestial body. By comparison, the Earth's core is only half of its radius. Unlike Venus and Mars, Mercury, like Earth, has its own magnetosphere, not an induced one.
The space station MESSENGER (MErcury Surface, Space Environment, GEochemistry) has discovered numerous folds, bends and faults on the surface of Mercury, which allows an unambiguous conclusion about the tectonic activity of the planet, at least in the past. The structure of the outer crust, according to scientists, is determined by the physical processes occurring in the interior of the planet, in particular, thermal diffusion of the mantle and, probably, the generation of a magnetic field.
A compiled image of Mercury from Mariner 10 imagery. Image: NASA
The first data that the size of Mercury was changing was received by the space station Mariner 10. On the planet's surface, escarps were found - high and extended cliffs. Scientists have suggested that they arose from the cooling of Mercury, as a result of which the crust of a small planet, shrinking in size, was deformed. However, only now have geologists been able to estimate when and at what speed these processes took place.
The data on craters obtained by the MESSENGER station helped. Geologists believe that the global contraction of the planet began more than 3.85 billion years ago. Since then, the surface of Mercury has been approaching its center at a rate of 0.1-0.4 millimeters per year.
The reduction of the planet is gradually slowing down and is now almost imperceptible. In total, the radius of Mercury has decreased by more than five kilometers.
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The researchers believe that Mercury began to contract after the meteorite bombardment, which ended 3.8 billion years ago and lasted 400 million years. During this time, many impact craters appeared on Mercury, Venus, Earth, Moon and Mars. The reasons for the cataclysm are unclear. Probably, it was caused by a change in orbits by gas giants or some kind of gravitational disturbance on the outskirts of the solar system, as a result of which many comets and asteroids rushed to its center. Their blows warmed up Mercury.
The age of craters on Mercury was estimated using the method used to determine the time of formation of geological formations on the Moon. The more the crater degrades, and the darker it is due to the dust that has covered it, the older it is. This visual method has proven itself in the dating of craters on the Moon, confirmed by the results of radioisotope analysis of soil samples that were delivered to Earth as part of the American Apollo manned lunar program.
The craters of Mercury, studied by specialists, exceed 20 kilometers in diameter. In total, more than six thousand features of geological formations were analyzed, many of which had not been previously paid attention to. Most of the features, though not all, turned out to be associated with the global contraction of Mercury. Old craters, as a rule, intersect faults, which means that these craters arose even before the planet began to contract. Young craters are most often not affected by faults.
Scientists agree that Mercury is still an excellent platform for testing the models of formation and evolution of the terrestrial planets. The celestial body is still changing, although tectonic activity there has almost stopped and the magnetic field is weakening more and more. Venus and Mars have not had their own magnetic field for a long time, tectonic activity on Venus has not yet had time to emerge, and Mars has probably already ended.
Apollodorus crater and Pantheon furrows. Image: NASA
Moreover, one of the latest simulations of the formation of celestial bodies of the terrestrial group from the protoplanetary disk around the Sun showed that Mercury should not have arisen at all. Astronomers have run the model 110 times within the framework of the N-body problem, for which more than one hundred large planetary embryos and about six thousand planetesimals were used. Most launches were able to reproduce the birth of Venus and Earth, while Mercury and Mars were formed in only nine cases.
As a rule, the planet closest to the luminary was formed at a distance of 0.27-0.34 astronomical units from the star, with a small eccentricity (the parameter describing the elongation of the orbit), and was lighter than the Earth by about five times. The planet was formed mainly from the matter of embryos, and it took ten million years.
Only two stations explored Mercury in detail - Mariner 10 and MESSENGER. In 2018, Japan and the EU plan to send a third mission to Mercury, BepiColombo, from two stations. First, MPO (Mercury Planet Orbiter) will compile a multi-wavelength map of the surface of a celestial body. The second, an MMO (Mercury Magnetospheric Orbiter), will explore the magnetosphere. It will take a long time to wait for the first results of the mission - even if the launch takes place in 2018, the station will reach Mercury only in 2025.
Andrey Borisov