Venus is named after the Roman goddess of love and beauty. Yet it is anything but that. Instead Venus presents us with a nasty toxic atmosphere and scorching heat. Its surface is permanently veiled in clouds of poisonous gases and sulphuric acid, which are swirled and mixed by the high winds. The atmosphere is about 80 kilometres thick and crushes down on the surface of Venus with a force 90 times greater than that of Earth.
Particles from the sun and space dust arrive and cause lightning in the upper atmosphere, which is so dense that lightning is prevented from striking the surface. Venus is the most reflective planet. Its clouds of sulphuric acid reflect 80% of the sun’s light.
Its surface temperature is 464°C, the hottest of all the planets, and hot enough to melt away many metals. On the lower plains, where temperatures can reach as high as 480°C, reflective pyrite minerals are vaporised, entering the atmosphere as a kind of metallic dust, and leaving only the dark volcanic rocks like basalt behind. Atmospheric mist condenses onto the mountain tops forming shiny, metallic frost. These snowcaps are made of lead sulphide and bismuth sulphide. Overall, the ground features of Venus comprise: 70% rolling plains; 10% highlands; and, 20% lowlands. The Maxwell Montes mountain range, rising to a peak of 10 600 metres, is the highest point on Venus.
Volcanoes and greenhouse gases
Venus has at least 1 600 major volcanoes, and many smaller ones with upwards of fifty thousand suggested. Venus does not have tectonic plates, so it has a fairly uniform, but thin crust. Regular volcanic activity randomly punches its way through the crust to the surface. In this respect some sources suggest there may be as many as 100 000 mini volcanoes—even a million spread widely across the globe! The 8-kilometre high Maat Mons is the highest volcano on the planet.
The concept of greenhouse gases was actually discovered on Venus. Its atmosphere is mostly carbon dioxide (96%). The volcanoes pump out between 8 000 to 30 000 tonnes of material into the atmosphere each day. This includes sulphur dioxide and heavy doses of carbon dioxide. Heat is trapped in the atmosphere by this process.
Water can absorb carbon dioxide, reducing it to solid carbonates. But Venus suffered rising temperatures in its formative years, and this caused its oceans to evaporate into the atmosphere. The sulphuric acid probably formed this way (sulphur dioxide plus water). In turn, the sun’s ultraviolet radiation split the water molecules into hydrogen and oxygen, which then dispersed into space. Venus eventually lost all its water, leaving the atmosphere to absorb the carbon dioxide; which is why it now reaches such murderous temperatures of around 464°C.
Given the dense cloud cover, how do we get such dramatic pictures of the surface of the planet? The answer lies in radar. Radio signals can penetrate the thick atmosphere and bounce back radar soundings to help create images of the planet’s surface. This was achieved by NASA’s Magellan Spacecraft, which arrived in 1990 for a four-year orbital survey. It mapped the whole of the planet’s surface. There have been many missions to Venus. NASA’s Mariner 2 was the first spacecraft to successfully fly by Venus in 1962. It reported surface temperatures of as high as 428°C; that there was no water vapour in the atmosphere; and there was no evidence of a magnetic field. NASA’s Pioneer Venus Orbiter grabbed ultraviolet images, and released several scientific probes into the atmosphere. It arrived in Venus orbit in December 1978 and continued to transmit data up until 1992. The Soviets dispatched a series of Venera spacecraft. Venera 13 landed on the planet’s surface in March 1982.
At just 42 million kilometres distant Venus is the nearest planet to Earth, and the brightest in our sky. It is visible just before sunrise and just after sunset.
Venus is a real slowcoach. It takes 243 days to rotate on its axis and 225 days to orbit the sun. It also rotates backwards (retrograde)—from the point of view of the earth’s north pole. It is thought this slow retrograde motion was caused by a massive meteor impact. On Venus the sun rises in the west and sets in the east. It’s as if the planet is upside down, which is why we give its axial tilt as 177.3° instead of 2.7° (180 – 177.3). In effect, like Mercury, its axis is almost vertical, so it does not experience any change of seasons.
The gravity of Venus is close to that of earth. At 68kg here, you would weigh 61kg on Venus. Venus is the sixth largest of the planets with a diameter of 12 104 kilometres (very close to Earth’s 12 756 kilometres).
The transit of Venus
Venus passes across the front of the sun four times every 243 years. These transits occur in pairs, separated by about eight years. In turn, these pairs are separated by around 120 years. In the telescopic age Venus passed across the face of the sun in 1632 and 1639; 1761 and 1769; 1874 and 1882; 2004 and 2012. Its next pairing performance will be in 2117 and 2125.
The reason for the long intervals between sightings lies in the fact that the orbits of Venus and Earth do not lie in the same plane. A transit can only be witnessed if both planets and the Sun are directly aligned. The first person to predict a transit of Venus was Johannes Kepler. This was in December 1631. He died before it occurred. The phenomenon has particular historical significance. The 17th- and 18th-Century transits were used by the astronomers of the day to work out fundamental facts about the Solar System. Employing a method of triangulation (parallax), they were able to calculate the distance between the Earth and the Sun—the so-called astronomical unit (AU)—which we know today to be about 149.6 million kilometres. This allowed scientists to get their first real handle on the scale of things beyond Earth.
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By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.
Last updated: Tuesday, 28th January 2020