Venus, which in the early epoch was almost a twin of the Earth, has gone a different way in its further evolution. Therefore, Venus, like no other planet, allows us to see what could (or could) be the evolution of our planet under the influence of not yet fully understood external or internal causes.
In the early epoch, physical conditions on Mars were favorable for the emergence and development of the simplest forms of life. One of the main tasks of future research is to determine whether life on Mars ever existed, and if not, why not.
Venus
Venus is the second planet from the sun. It occupies a special position among other planets of the Earth group. Until recently, it was called the doppelganger of the Earth. Similarity between Venus and the Earth in size and weight (and hence in density and gravity) allows to assume that the internal structure of the two planets is similar. However, the further advanced was the study of Venus, the less it had the features of the "double" of the Earth.
Venus - the planet nearest to the Earth, in the maximum rapprochement it separates from the Earth only 40 million km. Light passes this distance in 2 minutes 12 seconds. But in this period we see only the night strings of the planet. Fully its stronghold we see at the greatest distance of Venus from the Earth (260 million km). In its orbital motion, it sometimes appears on the line of the Sun-Earth, and then it can be seen as a small black dot crossing the solar disk.
What does she look like?
M.V.Lomonosov, observing in 1791 such a "passage of Venus on the Sun", found that at the moment of visible contact with the Sun's disk of one side of the planet around the opposite appeared bright bezel. "This nothing else shows as refraction of beams of solar in the Venus atmosphere", - wrote Lomonosov; he called this atmosphere "noble", but in reality, it was much denser than it was possible to assume then.
In the telescope, Venus looks very bright incomplete disk, which changes the phase, like the moon. Studying Venus, astronomers gradually realized that they see a solid cloud cover of the planet. In 1927 on ultra-violet photos of the cloud layer of the planet it was possible to distinguish a characteristic combination of stripes and spots which, as it was found out further, repeats every 4 days, moving aside, opposite to a direction of rotation of the Earth and other planets. At the same time, many astronomers assumed that the planets rotate synchronously, ie always turned to the Sun by the same side, and on the other side of the reigns eternal night.
Research
Since 1961, the U.S. and Soviet Union began radar research on Venus. At first, a weak reflected impulse allowed to determine only the distance to the planet. In the 1970s, the difference between the signals reflected by the right and left sides of the disk was determined by the period of Venus rotation: 243.0185 ± 0.0001 days in a direction not typical of other planets - in a clockwise direction when viewed from the north pole of the ecliptic. Two bright (in the radio range) areas that were temporarily named Alpha and Beta helped to determine the rotation. But, as is often the case, the temporary names are fixed. It later became clear that Alpha and Beta were giant geological formations, apparently of volcanic origin. Because of the slow rotation of dawn and dusk on Venus, several earthly days last. There is no change of seasons there, as the axis tilt does not exceed 3˚.
The combination of two rotations - around the axis and around the Sun - occurring in different directions, leads to the fact that the sunny day on Venus lasts 116.8 earthly days. The period of repetition of Venus's lower compounds (its maximum approach to the Earth) is 584 earthly days; it is easy to see that during this time there are exactly 5 solar days on the planet. Therefore, in each lower connection, Venus is turned to the Earth by one and the same side. The reason for such synchronization is not clear, because the tidal interaction between the planets is very weak.
Mars
Mars has attracted the attention of scientists and astronomers since ancient times. During the great confrontation of Mars in 1877, the Italian astronomer Giovanni Schiaparelli (1835-1910) drew a detailed map of the planet, which depicted the many fine dark lines connecting the Martian "seas". During the following confrontations, Schiaparelli drew more and more new lines on the map. He wasn't the first to notice them, but it was his observations that became very popular and assigned the name Canali to the lines. Schiaparelli did not claim that the "channels" were of artificial origin and contained any water at all. The Italian word canali should have been translated as "straits": if "seas" are connected by something, they are "straits". But the talented and very energetic American astronomer Percival Lovell (1855-1916) took this term literally and believed in the artificiality of the Martian canals.
Fantasies about Mars
Levell took over in 1894 and the number of channels he had fixed was increasing. Unlike the Schiaparelli canals, which connected the dark areas of Mars, the Lovell canals could also cross these areas, because Lovell did not consider them seas, but the canals themselves, as strips of vegetation stretched along the waterways. He was able to infect his fellow scientists and many astronomers with his enthusiasm. Newspapers and magazines of those times are full of the most amazing reports about Mars. They wrote that the Martians suffered from thirst on an anhydrous planet; that they were struggling to create global irrigation structures and save the last drops of codes ... Even organized a fundraiser to build a missile that allegedly was to deliver water to Mars (and this is in the XIX century!), after which both the collectors and the collection or the funds mysteriously disappeared.
A wave of fantasies about Mars also captured the beginning of the 20th century. "The War of the Worlds" by G.Wells, "Aelita" by A.N.Tolstoy and many other works are devoted to Martians - kind or ruthless, wise and dying out. The advent of radio has increased the effect of "Martian fantasies": in the 1930s in the U.S. panic broke out when the radio broadcast a well-performed play based on a novel by G. Wells. The audience was prepared for the visit of Martians. Interestingly, the Martians remained in the literature of the second half of the XX century. These are the lyrical "Martian Chronicles" by R. Bradbury, the adventures of Lacca Starr among the densely populated Martians with A. Azimov, and the mysterious Martian predators with A. and B. Stragatsky, movie fighters ... But this is a different kind of literature, rather, standard scenery, populated by earthly problems.
The general interest in Mars stimulated his study; as a result, the means of ground astronomy here were exhausted earlier than for other platens, and very timely appeared spacecraft. When the first space probe rushed to the Moon in 1959, it became clear that Mars did not have long to wait either.
In 1965, Mariner 4 (USA), during the rapprochement with Mars, transmitted several images, which showed a lot of craters like the lunar, which did not fit the previous ideas about Mars. But four years later, the Mariner-6 and -7 fly-by-wire probes transmitted new images of the surface of Mars, many areas of which are not similar to the moon. For example, images of areas near the server and south poles undoubtedly indicate precipitation there.