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Looking at the planet from afar, we can easily distinguish between bodies with and without atmosphere

Looking at the planet from afar, we can easily distinguish between bodies with and without atmosphereLooking at the planet from afar, we can easily distinguish between bodies with and without atmosphere. The presence of the atmosphere, or rather, the presence of clouds in it, makes the planet's appearance changeable and significantly increases the brightness of its disk. This is clearly seen if you place the planets in a row from completely cloudless (atmosphereless) to completely covered by clouds: Mercury, Mars, Earth, Venus. Stony atmosphereless bodies are similar to each other to almost complete indistinguishability: compare, for example, large-scale images of the Moon and Mercury. Even the experienced eye can hardly distinguish among themselves the surfaces of these dark bodies densely covered with meteor craters. But the atmosphere gives any planet a unique look.

The presence or absence of the atmosphere in the planet is controlled by three factors: temperature and gravitational potential at the surface, as well as the global magnetic field. Only the Earth has such a field, and it significantly protects our atmosphere from the flows of solar plasma. The moon lost the atmosphere (if it had one at all) due to the low critical velocity near the surface, and Mercury due to the high temperature and powerful solar wind.

Mars, with almost the same gravity as that of Mercury, was able to preserve the remains of the atmosphere, because due to its distance from the Sun it is cold and not so intensively blown by the solar wind.

In their physical parameters, Venus and the Earth are almost twins. They are very similar in size, weight, and hence the average density. Their internal structure should also be similar - crust, mantle, iron core - although there is no certainty about this yet, since there are no seismic and other geological data on the bowels of Venus. Of course, we did not penetrate deep into the bowels of the Earth: in most places 3-4 km, at individual points 7-9 km, and only one 12 km. This is less than 0.2% of the radius of the Earth. But seismic, gravimetric and other measurements make it possible to judge the earth's interior in very detail, but for other planets there is almost no such data. Detailed maps of the gravitational field were obtained only for the moon; heat fluxes from the bowels are measured only on the moon; seismometers so far only worked on the moon and (not very sensitive) on Mars.

Geologists still judge the inner life of planets by the features of their hard surface. For example, the absence of signs of lithospheric plates in Venus significantly distinguishes it from the Earth, in the evolution of the surface of which tectonic processes (continental drift, spreading, subduction, etc.) play a decisive role. At the same time, some indirect data indicate the possibility of plate tectonics on Mars in the past, as well as tectonics of ice fields in Europe, the satellite of Jupiter. Thus, the external similarity of the planets (Venus - Earth) does not guarantee the similarity of their internal structure and the processes occurring in their bowels. And planets that are not alike can exhibit similar geological phenomena.

Let us return to what is accessible to astronomers and other specialists for direct study, namely, to the surface of planets or their cloud layer. In principle, atmospheric opacity in the optical range is not an insurmountable obstacle to studying the solid surface of the planet. Radar from Earth and from space probes made it possible to study the surfaces of Venus and Titan through their atmospheres not transparent to light. However, these works are episodic in nature, and systematic planetary studies are still carried out with optical instruments. And even more important: the optical radiation of the sun serves as the main source of energy for most planets. Therefore, the ability of the atmosphere to reflect, scatter and absorb this radiation directly affects the climate at the surface of the planet.

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