VENUS

Image of Venus taken by the Galileo spacecraft. (Calvin J. Hamilton)

Venus is the second planet of the Solar System.   It has been known since prehistory, in fact it is very bright and it can be easily seen even with the naked eye. At the peak of its luminosity, the planet is 12 times brighter than Sirius, the brightest star in the sky.  Due to its orbit, Venus can be seen in the sky near the Sun, sometimes at dawn, sometimes at dusk. For this reason, in ancient times people thought they were two different stars: Lucifer in the morning, Vesper in the evening.  Due to its brightness, this planet was dedicated to the goddess of beauty and love.

Venus owes its luminosity not only to the closeness to the Sun, but also to the fact that it is the closest planet to the Earth, therefore the most visible. Besides, the planet reflects 70% of the light it receives from the Sun: its albedo is the highest in the whole Solar System. In fact, it is covered by a thick layer of clouds, which prevent the light of the Sun from penetrating and reflect it outwards.
The mass of the planet is 4,869 10²⁷ g, that is about 80% that of the Earth. Its diameter measures 12,103 Km, 95% that of the Earth. Its mean density is 5.18. 

Different views of Venus, centred: A) on the North Pole, B) at a longitude of 0° E, C) at a longitude of 90° E, D) at a longitude of 180°, E) at a longitude of 270°E.

Venus has no satellites.   Its mean distance from the Sun is 108.2 million kilometres, which equals 0.72 A.U. The distance at the aphelion is 109 million kilometres, that at the perihelion is 107.46 million kilometres.  That of Venus is the less eccentric of the planetary orbits, in other words the one that is closest to a circle. Its orbital plane has an inclination of 3.39 degrees on the ecliptic.

 

The phases of Venus 
 

Being an inner planet, when it is observed from the Earth is presents phases: that is the surface of the planet appears totally of partially illuminated or completely darkened, according to the relative Earth-Sun-Venus position.
The observations of the phases of Venus at the telescope by Galileo contributed to validate the Copernican theory of the Solar System.
 

The rotation
 

The question of the rotation of Venus has always been a matter of controversy. The planet is surrounded by a thick layer of clouds that darken completely its surface. Therefore, it is impossible to determine the rotation period on the basis of a simple visual observation.
The modern radar techniques allowed to study the motion of the planet. The radio waves, in fact, can penetrate beyond the clouds, reaching the venusian soil.
Venus and its clouds have two independent rotation motions, and both rotate in a retrograde manner (that is to say in the opposite direction with respect to the other planets).
Venus rotates around its axis, which has an inclination of 177.36 degrees on the  ecliptic, with a period of 243.16 days. The clouds rotate with a period of 4 days, that is with a speed that is 60 times higher than that of the planet. 

Cylindrical map of the surface of Venus, reconstructed with the radar images of the Magellan probe.  (NASA/JPL)

Radar image of Venus, reprocessed in false colours to highlight the superficial structures, with a resolution of 3 Km. (NASA/JPL)

 

Internal structure 
 

The physical characteristics of Venus (mass, density, presence of an atmosphere, size) are very similar to the terrestrial ones. Even though there are no definite indications, it is very likely that the planets have the same inner structure: an iron nucleus with a diameter measuring 3,000 Km approximately, a rocky mantle and an external crust about 100 Km thick.
The magnetic field of Venus is practically non-existent: it is estimated less than one thousandth of that of the Earth. Therefore there is no magnetosphere. On the other hand, there is a ionosphere, at an altitude between 120 and 180 Km. 
 

The atmosphere 

Ultraviolet image of Venus of 1995, in which the structures of the clouds are evident; in particular, you can notice a horizontal Y near the equator. (HST)

As we already know, venus has a very dense and very hot atmosphere. A thick layer of clouds is present in its upper part.  The atmosphere is composed by carbon dioxide (96%), nitrogen (4%), with traces of sulphur dioxide, argon and vapour.  At the ground level, the pressure is approximately 92 atmospheres: on our planet, a pressure that high is present in the sea, at the depth of 1 Km. The temperature at the ground level ranges between 446 and 482 °C, this means that on Venus metals such as lead and tin would melt: definitely, Venus appears to be an inhospitable planet...

The high temperature is partly due to the closeness to the Sun, partly to the green house effect: the atmospheric carbon dioxide, together with the sulphuric acid of the clouds, let the visible radiation out but retain the infrared.
At 30 Km from the ground, the pressure is approximately 1 atmosphere and the temperature 100 °C. The zone of the low atmosphere is swept by winds which do not exceed the speed of 20 Km/h.
Above this level there is another zone, that finishes at about 85 Km from the ground and comprises the layer of clouds that characterizes Venus. 
The clouds are at an altitude between 42 and 59 Km. As we have already said, they are composed of sulphuric acid and move with an average speed of 360 Km/h. They can be preferentially observed in the ultraviolet band of the spectrum.
At higher altitudes, the solar radiation dissociates the sulphuric acid (H₂SO₄) into water (H₂O) and sulphur dioxide (SO₂). These, together with carbon dioxide, form a uniform fog that surrounds the clouds. In this external region, the pressure is approximately 0.2 atmospheres and the temperature is -83 °C.
 

The surface 
 

The probes sent to Venus were all damaged before they could send data to the Earth, due to the high temperatures and to the corrosiveness of its atmosphere.
Nevertheless some missions succeeded. Among these, we should remember Venera 9, which gave us the first photographs of the surface of the planet, and Pioneer 12, and, of course, the more recent Magellan.

Image taken in 1982 by the Russian probe Venera 13, which landed on the planet.

Topographical projection of the surface of Venus. The uplands are in yellow and orange, the depressions in blue. (NASA/JPL)

The radar present on Pioneer 12 allowed to trace topographical maps of almost the entire surface of Venus. The same type of analysis was carried out also by the Magellan probe.
Venus has no water. The planet probably had, in the past, seas and oceans, like the Earth, but its very high temperature caused their evaporation and now the soil is dry and rocky. Most part of it is occupied by desert plains.
Vast depressions are also present on its surface, two very large uplands and some mountainous regions, some of them reach the height of 10 Km.
These mountains have a volcanic origin, and most of the surface of Venus is covered with solidified lava. A small part of these volcanos are still active.
 

Instead, there are no craters on the venusian surface: the meteorites are probably disintegrated by the dense atmosphere of the planet. Besides, an intense volcanic activity, which took place approximately 800 million years ago, cancelled any crater previously produced by meteorites.

Images of the Alpha Region; the light bands are faults and ridges, with the size of 10-50 Km, and an altitude of up to 4 Km.  (NASA/JPL)

Image of the Alpha Region: the light spots are the vents of volcanos. (NASA/JPL)

 
 

The Eistla Region at the radar of the Magellan probe. You can see the Gula volcano, 3 Km high. (NASA/JPL)

The Lakshmi upland, with a height between 2.5 and 4 Km, and its slope. (NASA/JPL)

 
 

The Eistla Region, with the Gula volcano and the Cunitz impact crater. (NASA/JPL)

The Eistla region photographed by the Magellan probe. On the horizon you can see the Gula volcano and the Sif volcano, the latter with a diameter measuring 300 Km and 2 Km high.  (NASA/JPL)

 
 

Two groups of parallel lines that intersect forming a right angle, maybe faults or fractures, associated to volcanic phenomena. (NASA/JPL)

Structures called arachnoids, revealed by the Magellan probe; they are circular or elliptic, with concentric rings with diameters measuring 50-230 Km; they may have a volcanic origin. (NASA/JPL)

 
 
 

ANIMATIONS

The Magellan probe over the Artemis Region,  AVI, 11 Mb (NASA/JPL)

Rotation of Venus and of the Earth,  AVI, 1 Mb (NASA/JPL)

The Magellan probe over the Eistla Region,   AVI, 3.3 Mb (NASA/JPL)

Rotation of Venus,  MPEG, 296 Kb (Calvin J. Hamilton)