The closest to the luminary and the smallest planet in the solar system is still a mystery. Like Earth and the four gas giants - Jupiter, Saturn, Uranus, and Neptune, Mercury has its own magnetosphere. After investigations of the MESSENGER station (MErcury Surface, Space Environment, GEochemistry), the nature of this magnetic layer began to become clear. The main results of the mission are already included in monographs and textbooks. How a small planet managed to preserve the magnetosphere.
For a celestial body to have its own magnetosphere, a source of magnetic field is needed. According to most scientists, the dynamo effect is triggered here. In the case of the Earth, it looks like this. In the bowels of the planet there is a metal core with a solid center and a liquid shell. Due to the decay of radioactive elements, heat is released, leading to the formation of convective flows of a conductive fluid. These currents generate the planet's magnetic field.
The field interacts with the solar wind - streams of charged particles from the star. This cosmic plasma carries with it its own magnetic field. If the planet's magnetic field withstands the pressure of solar radiation, that is, deflects it at a considerable distance from the surface, then they say that the planet has its own magnetosphere. In addition to Mercury, Earth and the four gas giants, Ganymede, the largest satellite of Jupiter, also has a magnetosphere.
In the rest of the planets and moons of the solar system, the stellar wind encounters practically no resistance. This happens, for example, on Venus and, most likely, on Mars. The nature of the Earth's magnetic field is still considered the main mystery of geophysics. Albert Einstein considered it one of the five most important tasks of science.
This is due to the fact that although the geodynamo theory is practically uncontested, it causes great difficulties. According to classical magnetohydrodynamics, the dynamo effect should decay, and the planet's core should cool down and harden. There is still no precise understanding of the mechanisms by which the Earth maintains the effect of self-generation of the dynamo together with the observed features of the magnetic field, primarily geomagnetic anomalies, migration, and pole reversal.
The difficulty of a quantitative description is most likely due to the essentially nonlinear nature of the problem. In the case of Mercury, the dynamo problem is even more acute than for the Earth. How did such a small planet keep its own magnetosphere? Does this mean that its core is still in a liquid state and generates enough heat? Or are there some special mechanisms that allow the celestial body to protect itself from the solar wind?
Mercury is about 20 times lighter and smaller than Earth. The average density is comparable to that of the earth. The year lasts 88 days, but the celestial body is not in tidal capture with the Sun, but rotates around its own axis with a period of about 59 days. Mercury is distinguished from other planets of the solar system by a relatively large metal core - it accounts for about 80 percent of the radius of a celestial body. For comparison, the Earth's core only takes up about half of its radius.
The magnetic field of Mercury was discovered in 1974 by the American station Mariner 10, which recorded bursts of high-energy particles. The magnetic field of the celestial body closest to the Sun is about a hundred times weaker than the earthly one, it would completely fit into a sphere the size of the Earth and, like our planet, is formed by a dipole, that is, it has two, not four, like gas giants, magnetic poles.
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Photo: Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington / NASA
The first theories to explain the nature of Mercury's magnetosphere were proposed in the 1970s. Most of them are based on dynamo effect. These models were verified from 2011 to 2015, when the MESSENGER station studied the planet. The data obtained from the device revealed the unusual geometry of the magnetosphere of Mercury. In particular, in the vicinity of the planet, magnetic reconnection - the mutual rearrangement of the intrinsic and external lines of force of the magnetic field - occurs about ten times more often.
This leads to the formation of many voids in the magnetosphere of Mercury, allowing the solar wind to reach the planet's surface almost unhindered. In addition, MESSENGER discovered remanence in the crust of a celestial body. Using this data, scientists have estimated the lower bound for the average age of Mercury's magnetic field at 3.7-3.9 billion years. This, as the scientists noted, confirms the validity of the dynamo effect for the formation of the planet's global magnetic field, as well as the presence of a liquid outer core in it.
Meanwhile, the question of the structure of Mercury remains open. It is possible that the outer layer of its core contains metal flakes - iron snow. This hypothesis is very popular because, explaining Mercury's own magnetosphere by the same dynamo effect, it allows low temperatures and a quasi-solid (or quasi-liquid) core inside the planet.
Photo: Carnegie Institution of Washington / JHUAPL / NASA
It is known that the cores of the terrestrial planets are formed mainly by iron and sulfur. Sulfur inclusions are also known to lower the melting point of core matter, leaving it liquid. This means that less heat is required to maintain the dynamo effect, which Mercury already produces too little. Almost ten years ago, geophysicists, conducting a series of experiments, demonstrated that under high pressure conditions, iron snow can fall towards the center of the planet, and a liquid mixture of iron and sulfur can rise towards it from the inner core. This can create a dynamo effect in the bowels of Mercury.
The MESSENGER data confirmed these findings. The spectrometer installed at the station showed an extremely low content of iron and other heavy elements in the volcanic rocks of the planet. There is almost no iron in the thin layer of Mercury's mantle, and it is formed mainly by silicates. The solid center accounts for about half (about 900 kilometers) of the radius of the core, the rest is occupied by the molten layer. Between them, most likely, there is a layer in which metal flakes move from top to bottom. The density of the core is about twice that of the mantle, and is estimated at seven tons per cubic meter. Sulfur, scientists believe, accounts for about 4.5 percent of the mass of the nucleus.
MESSENGER discovered numerous folds, bends and faults on the surface of Mercury, which makes it possible to draw an unambiguous conclusion about the tectonic activity of the planet in the recent past. The structure of the outer crust and tectonics, according to scientists, are associated with the processes taking place in the bowels of the planet. MESSENGER showed that the planet's magnetic field is stronger in the northern hemisphere than in the southern. Judging by the gravity map compiled by the apparatus, the thickness of the crust near the equator is on average 50 kilometers higher than at the pole. This means that the silicate mantle in the northern latitudes of the planet is heated more strongly than in its equatorial part. These data are in excellent agreement with the discovery of relatively young traps in northern latitudes. Although volcanic activity on Mercury ceased about 3.5 billion years ago, the current picture of thermal diffusion in the planet's mantle is largelymost likely determined by her past.
In particular, convective flows can still exist in the layers adjacent to the core of the planet. Then the temperature of the mantle under the north pole of the planet will be 100-200 degrees Celsius higher than under the equatorial regions of the planet. Moreover, MESSENGER discovered that the residual magnetic field of one of the sections of the northern crust is directed in the opposite direction relative to the planet's global magnetic field. This means that in the past, an inversion occurred on Mercury at least once - a change in the polarity of the magnetic field.
Only two stations have explored Mercury in detail - Mariner 10 and MESSENGER. And this planet, primarily because of its own magnetic field, is of great interest to science. By explaining the nature of its magnetosphere, we can almost certainly do this for the Earth. In 2018, Japan and the EU plan to send a third mission to Mercury. Two stations will fly. 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 start takes place in 2018, the station's destination will be reached only in 2025.
Yuri Sukhov