As the second planet outward from the sun and the one that comes closest to earth, Venus superficially appears to be the twin of Earth. It has a similar mass and a similar radius to the earth. But that is the full extent of any similarity; Venus must be one of the most hostile environments of the whole of the solar system.
The planet’s highly reflective atmosphere, together with its relative proximity to the sun, causes Venus to be the second brightest object in the night sky (next to the moon), with a magnitude of —4.4. Furthermore, because it is closer to the sun than is the earth, when viewed from the earth it exhibits a series of phases (like those of the moon), being “full” when it is farthest from us.
The mysterious planet
Venus has been known since ancient times, but at first it was identified with two different objects, the Morning Star (Phosphorus) and the Evening Star (Hesperus). Later, it was recognized as a planet and named after the Roman goddess of love and beauty, equivalent to the Greek goddess Aphrodite. But only since the explorations by Soviet and American space probes has much been revealed about the surface of the planet itself. The reason for this quickly becomes apparent when Venus is viewed through a low-powered telescope: it is completely shrouded in a dense, pale yellow layer of cloud. In an attempt to study the hidden structure of Venus, astronomers have used radio and radar observations and sent space probes to fly by, orbit, and even land on the planet. Because of its close approach to the earth, it was the first to which unmanned spacecraft were sent, the first fly-by being made by the American Mariner 2 probe in 1962.
Surface and atmosphere
Radio observations revealed that the surface of Venus must be very hot, a fact confirmed by the Soviet Venera probes. It is now thought that it has the hottest surface of any planet, at 864° F. (462° C). Early probes that attempted to land on the planet’s surface were both scorched by the heat and crushed by the enormous atmospheric pressure of about 1,323 pounds per square inch (93 kilograms per square centimeter), 90 times that of Earth’s atmosphere. In addition, Venus’s atmosphere absorbs about 99 per cent of the sunlight that strikes the top of the clouds, 50 miles (80 kilometers) above the surface. Any sunlight that does reach the surface, however, is absorbed and re-emitted, heating up the clouds from below and producing a “greenhouse” effect, which leads to opacity of the atmosphere.
The atmosphere is made up almost entirely of carbon dioxide, with nitrogen and argon forming most of the remainder. Water vapor, hydrogen chloride, and hydrogen fluoride have also been detected. The clouds themselves are composed of droplets of concentrated sulfuric acid which, together with the high temperatures at the surface, seems to be the principal cause of erosion on the planet. Surface wind velocities are only a few miles per hour on average, although high in the atmosphere the velocities are much greater, and they follow the retrograde (clockwise) direction of the planet’s rotation as a whole.
Radar observations have revealed important information about the general form of the planet’s surface. There seems to be fairly extensive cratering, some of which may have been caused by meteors that have penetrated the atmosphere of Venus, and some by volcanic activity. Indeed, there is a large volcano, 185 miles (300 kilometers) wide and less than one mile high, and this may be one source of the carbon dioxide and nitrogen discovered in the planet’s atmosphere. Mountain ranges and a 900-mile (1,500-kiIometer) trench (near the Venusian equator) have also been found. On a smaller scale, photographs sent back from the surface of Venus by the Soviet probes Venera Pand Venera 10 in 1975 show that it is probably strewn with stones averaging a few feet in size, although general conclusions are difficult to draw from the scant information available. The soil seems to have a chemical composition and density corresponding to basalt. Lava flow has also been detected by radar.
Orbit and structure
Radar observations from earth indicate that Venus rotates very slowly on its axis (as well as in the opposite direction to most planets), completing one “day” every 243 earth-days. As a result, Venus is the only planet whose day is longer than its year (its orbital period being 225 earth-days). It is believed that this long rotation period results from the gravitational effect of the earth.
Such a long period of rotation rules out a “dynamo” mechanism as the source of Venus’s magnetic field, which is about one one-thousandth the strength of the earth’s field. It is possible that the magnetic field, like that of the moon, results from permanent magnetism, although a complete explanation is lacking. But the planet does possess an ionosphere like that of the earth.
Under its harsh surface, Venus becomes similar in structure to the earth. Present models suggest that there is a dense iron and nickel core with a radius of 1,850 miles (2,980 kilometers), surrounded by a mantle. With a size and structure similar to those of the earth, and volcanic features on its surface, it seems that the crustal layer is split up into plates that interact and give rise to the Venusquakes and fold mountains.