Venus in some respects is very similar to Earth. However, these two planets have significant differences due to the peculiarities of the formation and evolution of each of them, and scientists reveal more and more of these features. We will examine here in more detail one of the distinguishing features - the special nature of the magnetic field of Venus, but first we turn to the general characteristics of the planet and some hypotheses that affect its evolution.
Venus in the Solar System
Venus is the second planet closest to the Sun, a neighbor of Mercury and the Earth. Relative to our luminary, it moves in an almost circular orbit (the eccentricity of the Venusian orbit is less than the Earth's) at an average distance of 108.2 million km. It should be noted that eccentricity is a variable quantity, and in the distant past it could be different due to the gravitational interactions of the planet with other bodies of the solar system.
Venus has no natural satellites . There are hypotheses according to which the planet once had a large satellite, subsequently destroyed by the action of tidal forces or lost.
Some scientists believe that Venus experienced a tangential collision with Mercury, as a result of which the latter was thrown into a lower orbit. Venus has changed the nature of rotation. It is known that the planet rotates extremely slowly (as, incidentally, Mercury) - with a period of about 243 Earth days. In addition, the direction of its rotation is opposite to that of other planets. We can say that it rotates, as if turning upside down.
The main physical features of Venus
Along with Mars, Earth and Mercury, Venus belongs to the planets of the terrestrial group, that is , it is a relatively small rocky body of predominantly silicate composition. It is similar to the Earth in size (diameter 94.9% of the earth) and mass (81.5% of the earth). The runaway speed on the surface of the planet is 10.36 km / s (on Earth - about 11.19 km / s).
Of all the planets on Earth, Venus has the most dense atmosphere. The pressure on the surface exceeds 90 atmospheres, the average temperature is about 470 ° C.
To the question whether Venus has a magnetic field, the following answer exists: the planet has practically no field of its own, but due to the interaction of the solar wind with the atmosphere, a “false” induced field arises.
A bit about the geology of Venus
The vast majority of the planet’s surface is formed by the products of basaltic volcanism and is a combination of lava fields, stratovolcanoes, shield volcanoes and other volcanic structures. There were few impact craters, and based on counting their number, it was concluded that the surface of Venus cannot be older than half a billion years. Signs of plate tectonics on the planet are not traced.
On Earth, plate tectonics, together with mantle convection processes, serves as the main mechanism of heat transfer, but this requires a sufficient amount of water. Presumably, on Venus, due to lack of water, plate tectonics either stopped at an early stage or did not take place at all. So the planet could get rid of excess internal heat only by means of the global arrival of superheated mantle material on the surface, possibly with the complete destruction of the crust.
Such an event could take place about 500 million years ago. It is possible that in the history of Venus it was not the only one.
The core and magnetic field of Venus
On Earth, a global geomagnetic field is generated due to the dynamo effect created by the special structure of the nucleus. The outer layer of the core is molten and characterized by the presence of convective currents, which, together with the fast rotation of the Earth, create a fairly powerful magnetic field. In addition, convection promotes active heat transfer from the internal solid core, which contains many heavy, including radioactive elements, the main source of heating.
Apparently, on the neighbor of our planet this whole mechanism does not work due to the lack of convection in the liquid outer core - this is why Venus does not have a magnetic field.
Why is Venus and Earth so different?
The reasons for the serious structural difference between two planets similar in physical characteristics are not yet clear. According to one of the recently constructed models, the internal structure of rocky planets is formed in layers, as mass increases, and the rigid stratification of the core prevents convection. On Earth, the multilayer core, presumably, was destroyed at the dawn of its history as a result of a collision with a fairly large object - Teia. In addition, the result of this collision is the appearance of the moon. The tidal effects of a large satellite on the earth's mantle and core can also play a significant role in convective processes.
Another hypothesis suggests that Venus initially had a magnetic field, but the planet lost it due to a tectonic catastrophe or a series of catastrophes, which were discussed above. In addition, in the absence of a magnetic field, many researchers “blame” the too slow rotation of Venus and the small precession of the axis of rotation.
Features of the Venusian atmosphere
Venus has an extremely dense atmosphere, consisting mainly of carbon dioxide with a small admixture of nitrogen, sulfur dioxide, argon and some other gases. Such an atmosphere serves as a source of irreversible greenhouse effect, not allowing the surface of the planet to cool slightly. Perhaps the above-described “catastrophic” tectonic regime of its bowels is also responsible for the state of the atmosphere of the “morning star”.
The largest part of the gas shell of Venus lies in the lower layer - the troposphere, which extends to heights of about 50 km. Above it lies the tropopause, and above it is the mesosphere. The upper boundary of the clouds, consisting of sulfur dioxide and drops of sulfuric acid, is located at an altitude of 60–70 km.
In the upper atmosphere, the gas is strongly ionized by solar ultraviolet. This layer of rarefied plasma is called the ionosphere. On Venus, it is located at altitudes of 120–250 km.
Induced magnetosphere
It is the interaction of the charged particles of the solar wind and the plasma of the upper atmosphere that determines whether Venus has a magnetic field. The lines of force of the magnetic field carried by the solar wind bend around the Venusian ionosphere and form a structure called the induced (induced) magnetosphere.
This structure has the following elements:
- A head shock wave located at about a third of the radius of the planet. At the peak of solar activity, the region where the solar wind meets the ionized atmosphere significantly approaches the surface of Venus.
- Magnet layer.
- Magnitopause is actually the boundary of the magnetosphere, located at an altitude of about 300 km.
- The tail of the magnetosphere, where the extended magnetic lines of force of the solar wind are straightened. The length of the magnetospheric tail of Venus is from one to several tens of radii of the planet.
The tail is characterized by special activity - magnetic reconnection processes leading to the acceleration of charged particles. In the polar regions, magnetic harnesses similar to terrestrial ones can form as a result of reconnection. On our planet, reconnection of magnetic lines of force underlies the phenomenon of auroras.
That is, Venus has a magnetic field, formed not by internal processes in the bowels of the planet, but by the influence of the Sun on the atmosphere. This field is very weak - its intensity is on average a thousand times weaker than that of the Earth's geomagnetic field, but it plays a role in the processes taking place in the upper atmosphere.
The magnetosphere and the stability of the gas shell of the planet
The magnetosphere shields the surface of the planet from the effects of energetic charged particles of the solar wind. It is believed that the presence of a sufficiently powerful magnetosphere made possible the emergence and development of life on Earth. In addition, the magnetic barrier to some extent prevents the "blowing" of the atmosphere by the solar wind.
The ionizing ultraviolet, which is not held back by a magnetic field, also penetrates into the atmosphere. On the one hand, this creates the ionosphere and forms a magnetic screen. But ionized atoms can leave the atmosphere, falling into the magnetic tail and accelerating there. This phenomenon is called runaway ions. If the speed acquired by the ions exceeds the runaway speed, the planet intensively loses the gas shell. Such a phenomenon is observed on Mars, characterized by weak gravity and, accordingly, low runaway speed.
Venus, with its more powerful gravity, more effectively holds the ions of its atmosphere, since they need to gain more speed to leave the planet. The induced magnetic field of the planet Venus is not powerful enough for a substantial acceleration of ions. Therefore, the loss of atmosphere here is far from being as significant as on Mars, despite the fact that the intensity of ultraviolet radiation is much higher due to proximity to the Sun.
Thus, the induced magnetic field of Venus is one example of the complex interaction of the upper atmosphere with various types of solar radiation. Together with the gravitational field, it is a factor in the stability of the gas shell of the planet.