Venus is very similar to Earth in some characteristics. However, these two planets also have significant differences due to the peculiarities of the formation and evolution of each of them, and scientists are identifying more and more such features. We will consider 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 affecting the issues of its evolution.
Venus in the solar system
Venus is the second closest planet 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 that of the earth) at an average distance of 108.2 million km. It should be noted that the eccentricity is a variable value, 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, which was subsequently destroyed by the action of tidal forces orlost.
Some scientists believe that Venus experienced a tangent collision with Mercury, causing the latter to be thrown into a lower orbit. Venus changed the nature of rotation. It is known that the planet rotates extremely slowly (as does Mercury, by the way) - with a period of about 243 Earth days. In addition, the direction of its rotation is opposite to that of other planets. It can be said that it rotates, as if turning upside down.
Main physical features of Venus
Along with Mars, Earth and Mercury, Venus belongs to the terrestrial planets, 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 escape velocity on the surface of the planet is 10.36 km/s (on Earth it is approximately 11.19 km/s).
Of all the terrestrial planets, Venus has the most dense atmosphere. The pressure on the surface exceeds 90 atmospheres, the average temperature is about 470 °C.
To the question of whether Venus has a magnetic field, there is the following answer: the planet has practically no own field, 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 products of bas altic volcanism and is a combination of lava fields, stratovolcanoes, shield volcanoes and other volcanic structures. Few impact craters have been found, andon the basis of counting their number, it was concluded that the surface of Venus cannot be older than half a billion years. There are no signs of plate tectonics on the planet.
On Earth, plate tectonics, together with mantle convection processes, is the main mechanism for heat transfer, but this requires a sufficient amount of water. One must think that 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 through the global supply of superheated mantle matter to the surface, possibly with the complete destruction of the crust.
Just such an event could have taken place about 500 million years ago. It is possible that it was not the only one in the history of Venus.
The core and magnetic field of Venus
On Earth, the global geomagnetic field is generated due to the dynamo effect created by the special structure of the core. The outer layer of the core is melted and is characterized by the presence of convective currents, which, together with the rapid rotation of the Earth, create a fairly powerful magnetic field. In addition, convection contributes to active heat transfer from the inner solid core, which contains many heavy, including radioactive elements, the main source of heating.
Apparently, on our planet's neighbor, all this mechanism does not work due to the lack of convection in the liquid outer core - this is why Venus has no magnetic field.
Why are Venus and Earth so different?
The reasons for the serious structural difference between two planets similar in physical characteristics are not yet completely clear. According to one recently constructed model, 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 multi-layered core, presumably, was destroyed at the dawn of its history as a result of a collision with a fairly large object - Theia. In addition, the emergence of the Moon is considered the result of this collision. The tidal effect 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 originally had a magnetic field, but the planet lost it due to a tectonic catastrophe or a series of catastrophes mentioned above. In addition, in the absence of a magnetic field, many researchers "blame" the too slow rotation of Venus and the small amount of precession of the rotation axis.
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 an irreversible greenhouse effect, preventing the surface of the planet from cooling in any way. Perhaps the above-described "catastrophic" tectonic regime of its interior is also responsible for the state of the atmosphere of the "morning star".
The largest part of the gas envelopeVenus is enclosed in the lower layer - the troposphere, extending to altitudes of about 50 km. Above is the tropopause, and above it is the mesosphere. The upper boundary of the clouds, consisting of sulfur dioxide and droplets of sulfuric acid, is located at an altitude of 60–70 km.
In the upper atmosphere, gas is strongly ionized by solar ultraviolet radiation. 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 bow shock wave located at a height of about a third of the radius of the planet. At the peak of solar activity, the region where the solar wind meets the ionized layer of the atmosphere is much closer to the surface of Venus.
- Magnetic layer.
- Magnetopause is actually the boundary of the magnetosphere, located at an altitude of about 300 km.
- The tail of the magnetosphere, where the solar wind's stretched magnetic field lines straighten out. The length of the magnetospheric tail of Venus is from one to several tens of planetary radii.
The tail is characterized by a special activity - the processes of magnetic reconnection, leading to the acceleration of charged particles. In the polar regions, as a result of reconnection, magnetic bundles can be formed,similar to earth. On our planet, the reconnection of magnetic field lines 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 certain role in the processes occurring in the upper atmosphere.
The magnetosphere and the stability of the gas shell of the planet
The magnetosphere shields the planet's surface from the impact 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 solar wind from blowing away the atmosphere.
The ionizing ultraviolet, which is not delayed by the magnetic field, also penetrates into the atmosphere. On the one hand, due to this, the ionosphere arises and a magnetic screen is formed. But ionized atoms can leave the atmosphere by entering the magnetic tail and accelerating there. This phenomenon is called ion runaway. If the velocity acquired by the ions exceeds the escape velocity, the planet rapidly loses its gas envelope. Such a phenomenon is observed on Mars, which is characterized by weak gravity and, accordingly, a low escape velocity.
Venus, with its stronger gravity, holds the ions of its atmosphere more effectively, as they needpick up more speed to leave the planet. The induced magnetic field of the planet Venus is not powerful enough to significantly accelerate the ions. Therefore, the loss of the atmosphere here is nowhere near as significant as on Mars, despite the fact that the intensity of ultraviolet radiation is much higher due to the 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 gaseous shell of the planet.
