Venus

Venus is the brightest object in the sky, after the Sun and the Moon. Like Mercury, Venus can be seen only in the morning or in the evening sky. (It is sometimes possible to see Venus even if the Sun is above the horizon, if its exact position is known.) In antiquity, Venus was thought to be two different planets, Hesperos and Phosphorus, evening star and morning star.

The maximum elongation of Venus is about 47°. Venus is a remarkable object when shining in the dark sky at its brightest, 35 days before or after the inferior

Fig. 7.24. The phases of Venus were discovered by Galileo Galilei in 1610. This drawing illustrates how the apparent size of Venus changes with phase. The planet is far behind the Sun when the illuminated side faces the Earth conjunction, when one-third of the surface is seen lit (Fig. 7.24). At the inferior conjunction, the Earth-Venus distance is only 42 million km. The diameter of Venus is about 12,000 km, which means that the apparent diameter can be as large as one arc minute. Under favourable conditions it is even possible to see the shape of the crescent Venus with binoculars. At the superior conjunction, the apparent diameter is only 10 arc seconds.

Venus is covered by clouds. Its surface is nowhere visible; only featureless yellowish cloud tops can be seen (Fig. 7.25). The rotation period was long unknown, and the measured 4-day period was the rotation time of the clouds. Finally, in 1962, radar measurements revealed that the rotation period is 243 days in a retrograde direction, i. e. opposite to the rotation of other planets. The axis of rotation is almost perpendicular to the orbital plane; the inclination is 177°.

The temperature at the cloud tops is about 250 K. Because the Bond albedo is as high as 75%, the surface temperature was believed to be moderate, even suitable for life. Opinions changed dramatically when thermal radio emission was measured at the end of the 1950's. This emission originates on the surface of the planet and can penetrate the clouds. The surface temperature turned out to be 750 K, well above the melting point of lead. The reason for this is the greenhouse effect. The outgoing infrared radiation is blocked by atmospheric carbon dioxide, the main component of the atmosphere.

The chemical composition of the Venusian atmosphere was known prior to the space age. Spectroscopic observations revealed CO2, and some clues to the cloud composition were obtained from polarimetric observations. The famous French planetary astronomer Bernard Lyot made polarimetric observations in the 1920's, but not until decades later was it realised that his observations could be explained by assuming that light was

Fig. 7.25. Left: Venus in visible light imaged by the Galileo orbiter in February 1990. The cloud features are caused by winds that blow from east to west at about 100 m/s. Right: The northern hemisphere of Venus in a computer-generated picture of the radar observations. The north pole is at the centre of the image of the Magellan synthetic aperture radar mosaic. (NASA/JPL)

scattered by liquid spherical particles whose index of refraction is 1.44. This is significantly higher than the index of refraction of water, 1.33. Moreover, water is not liquid at that temperature. A good candidate was sulphuric acid H2SO4. Later, spacecraft confirmed this interpretation.

Venus' atmosphere is very dry: the amount of water vapour present is only 1 /1,000,000 of that in the Earth's atmosphere. One possible explanation is that, due to solar UV radiation, the water has dissociated to hydrogen and oxygen in the upper layers of the atmosphere, the former escaping into interplanetary space.

About 1% of the incident light reaches the surface of Venus; this light is deep red after travelling through clouds and the thick atmosphere. Most of the incident light, about 75%, is reflected back from the upper layers of clouds. The absorbed light is emitted back in infrared. The carbon dioxide atmosphere very effectively prevents the infrared radiation from escaping, and the temperature had not reached the equilibrium until at 750 K.

The pressure of the atmosphere at the surface is 90 atm. The visibility is several kilometres, and even in the clouds, a few hundred metres. The densest clouds are at a height of 50 km, but their thickness is only 2-3 km. Above this, there are haze-like layers which form the visible "surface" of the planet. The uppermost clouds move rapidly; they rotate around the planet in about 4 days, pushed by strong winds powered by the Sun. The sulphuric acid droplets do not rain on the Venu-sian surface but they evaporate in the lower atmosphere before reaching the surface.

Mariner 2 (1962) was the first spacecraft to encounter the planet. Five years later, the Soviet Venera 4 sent the first data from below the clouds, and the first pictures of the surface were sent by Venera 9 and 10 in 1975. The first radar map was completed in 1980, after 18 months of mapping by the US Pioneer Venus 1. The best and the most complete maps (about 98% of the planet's surface) were made using the synthetic aperture radar observations of the Magellan spacecraft in 1990-1994. The resolution of the maps is as high as 100 m and the elevation was measured with a resolution of 30 metres.

Radar mapping revealed canyons, mountains, craters, volcanoes and other volcanic formations (Fig. 7.26).

Fig. 7.26. Surface features of Venus. (Top left): A Magellan image of a 50 km peak-ring crater Barton at 27.4° N and 337.5° E. (Top right): A Magellan radar image of a region 300 km across, located in a vast plain to the south of Aphrodite Terra. The large circular structure near the centre of the image is a corona, approximately 200 km in diameter. North of the corona is a 35 km flat-topped volcanic construct known as a pancake dome. Complex fracture patterns like in the upper right of the image are often observed in association with coronas and various volcanic features. (NASA/JPL). (Bottom): The surface of Venus photographed by the Venera 14 lander in March 1982

Fig. 7.26. Surface features of Venus. (Top left): A Magellan image of a 50 km peak-ring crater Barton at 27.4° N and 337.5° E. (Top right): A Magellan radar image of a region 300 km across, located in a vast plain to the south of Aphrodite Terra. The large circular structure near the centre of the image is a corona, approximately 200 km in diameter. North of the corona is a 35 km flat-topped volcanic construct known as a pancake dome. Complex fracture patterns like in the upper right of the image are often observed in association with coronas and various volcanic features. (NASA/JPL). (Bottom): The surface of Venus photographed by the Venera 14 lander in March 1982

The surface of Venus is covered by about 20% of lowland plains, 70% of gently rolling uplands and lava flows, and 10% of highlands. There are only two major highland areas. The largest continent, Aphrodite Terra, is close to the equator of Venus; its size is similar to South America. Another large continent at the latitude 70° N is called Ishtar Terra, where the highest mountain on Venus, the 12 km high Maxwell Montes is situated. (IAU has decided that the Venusian nomenclature has to be feminine. Maxwell Montes, after the famous physicist James Clerk Maxwell, is an exception.)

Unlike the Earth, volcanic features are quite evenly distributed all over the surface of Venus. There is no evidence of massive tectonic movement although local deformations can exist. Almost all volcanism on Venus seems to involve fluid lava flows without any explosive eruptions. Due to the high air pressure, Venusian lavas need a much higher gas content than the Earth lavas to erupt explosively. The main gas driving lava explosions on the Earth is water, which does not exist on Venus.

Venus has more volcanoes than any other planet in the solar system. Over 1500 major volcanoes or volcanic features are known, and there may even be one million smaller ones. Most are shield volcanoes, but there are also many complex features. None are known to be active at present, although large variations of sulphur dioxide in the atmosphere may indicate that some volcanoes are active.

Flat-topped volcanic constructs known as pancake domes are probably formed by the eruption of an extremely viscous lava. A corona is a circular trench surrounding an elevated plain, the diameter of which can be as big as several hundreds of kilometres. They are possibly examples of local hot spots, mantle up-wellings that have expanded and formed bulges. When the flow has stopped, the bulge has sunk and formed a set of ring mountains.

In other places fluid lava flows have produced long, sinuous channels extending for hundreds of kilometres.

Most of the Venusian impact craters are undeformed. This indicates that the Venusian surface must be young because erosion, volcanism and tectonic forces should affect the craters, too. Resurfacing processes may frequently cover the old craters, and all craters visible are therefore young, presumably less than 500 million years. There are no impact crates smaller than about 1.5-2 km because smaller meteoroids are burned in the thick atmosphere.

The Earth and Venus are almost equal in size, and their interiors are assumed to be similar. Venus has an iron core about 3000 km in radius and a molten rocky mantle covering the majority of the planet. Probably due to its slow rotation, however, Venus has no magnetic field. The analyses made by the Venera landers have shown that the surface material of Venus is similar to terrestrial granite and basalt (Fig. 7.26).

Venus has no satellites.

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