Venus
Early ideas about Venus
Because Venus is between us and the Sun, (mean distance from Sun 108 million Kilometres), we only see it near the Sun, either just before Sunrise, or just after Sunset.
Although we can only see part of the daylight side lit up by the sun - through a telescope it appears as a crescent, it is still extremely bright. This is not only because the planet is our near neighbour, it has a high albedo (reflected light) because it is covered by a thick cloudy atmosphere which reflects the sunlight. These clouds also make it impossible to see the surface of Venus through a telescope. Radar has to be used.
Venus was seen as the Morning Star and the Evening Star. The careful records by astronomers showed that Venus was one and the same planet, whether it appeared at dawn or sunset.
In Northern latitudes, Venus is the most frequently visible of the planets, in Siberia it was the only planet to have a name. It is called Cholbon, and was identified as both the Morning Star and the Evening Star and sometimes any another bright planet which happened to be visible such as Jupiter or Mars.
It was observers at more southern latitudes who were able to trace the paths of the planets around the year, and who built up cosmologies, beliefs and methods of divination (astrology), around their observed passages through the apparently fixed stars.
On astronomical tables Venus has this symbol which is also used for "female".
Venus is named after the Roman goddess of love and peace. To the Greeks this was the goddess Aphrodite. The planet was associated with the metal copper (from Cyprus, which was Aphrodite's birthplace), a flattish triangle, the number five, the colour blue, and the day Friday.
The Saxons used the name of their fertility goddess, Fria which gives the English name of Friday.
Greek astronomy derived many of its ideas from the long tradition of astronomy in what is now present day Iraq. This, and its system of making records, derived originally from the Sumerians - of south Siberian origin who conquered what is now south Iraq in the Bronze Age. Inanna, also Ishtar or Eshtar was the morning star and the goddess of love - particularly sex and erotic love,(redlight district in cities, etc) and also of war and battles. She had wings, rode on a lion and was armed with bow and arrows.She is usually depicted near the sun god, since she is always seen near the sun. She had a sister - her evening counterpart who was the guardian of the gateway to the world of the dead.
To the Chinese the planet was the Great White One and associated with the element metal - more specifically gold, the direction - west, and the same day in the week. The idea of the seven day week and the ruling planets of the hours and days may have derived from India. Astronomers in central America had a similar concept.
Different associations with this planet were thought of by the astronomers in America. The Pawnee used to sacrifice a maiden to the planet Venus until the 1920s when the US government banned the custom.
To the Maya in Central America, this planet was the god of war, and it was believed to beam down evil rays bringing death and destruction. The astronomers therefore made very careful records of this planet, and a ritual calendar was based on this.
About Venus
Venus is similar to Earth in size, density and mass - only a little smaller (0.815 of Earth's mass). Its radius is 6052 kilometres. It would weigh 4.87 x 1024 kilogrammes.
The rotation of Venus was discovered by radar by a collaboration between Jodrell Bank and a Russian installation on the Black Sea Coast. The results on 9th January 1966, showed that Venus rotated backwards (retrograde) ie the opposite way to Earth and most other planets, with a day 243.09 Earth days long.
Venus seems now to be upside down - since its rotation is in the opposite direction of Earth and most of the other planets, with just a 3 degree tilt in its axis.
Venus also has a much slower rotation than that of Earth. Its day is longer that its year which is 224-5 Earth days. The rotation of Venus appears to have been slowed by its proximity to Earth - which with its Moon makes a larger gravitational pull than Venus does on Earth. When Venus is nearest the Earth on its orbit, the same side always faces the Earth. And the rotation period is close to the resonance period of the Earth-Venus system, that is 243.16 Earth days. So Venus is almost phase-locked to the Earth.
Probing Venus
The first probe to Venus was launched by Russia in 1959, only two years after they had launched the first space satellite, Sputnik. After many not very successful attempts, Venera 3 landed a capsule on the surface in 1965, but communication ceased immediately. Venera 4 in 1967, also landed a capsule by parachute through the atmosphere, which transmitted for a short time, but did not survive on the surface which has a temperature hot enough to melt lead and an atmospheric pressure 90 times that on Earth.
In 1969 Venera 5 and Venera 6 also sent back data but did not survive long on the surface. Venera 7 in 1970 did survive on the surface for a short time and transmitted data. Venera 8 aimed at the day side of Venus and sent back data for 50 minutes after landing. It had to survive temperatures of more than 10,000 degrees as it entered Venus's atmosphere. The surface temperature was recorded by Venera 7 as 748K ± 20 degrees.
In 1975, Venera 9 and 10 were sent to Venus and landed at a distance of 2000 kilometres from each other. The capsule of Venera 9 had been built to withstand the pressure of the atmosphere and temperatures up to 2,000 degrees, but still only survived 65 minutes after landing. It sent back the first pictures of the surface of Venus which was sufficiently light despite the permanent dense clouds, no extra lighting was needed. The rocks had shadows. Venera 10 was also successful in sending back pictures of the surface of Venus.
Venera 11 and Venera 12, were launched to Venus in 1978. They sent back lots of data but not pictures. They found storms with lightning, volcanic activity and that only a very small percentage of light actually reached the surface through the clouds.
Venera 13, and Venera 14, launched 1981, took soil samples. They found rocks were similar to Earth's, including basalt. There were high levels of uranium, thorium and potassium in the rocks. They transmitted colour television pictures back. The scenery looked yellow as the light looked yellow. Venera 15 and Venera 16 in 1983, mapped the surface of Venus with radar. This enabled an atlas to be published of Venus. They found two-thirds of the surface is hilly, about a quarter is flat lowland and about one tenth, mountains.
Vega 1 and Vega 2, passed Halley's comet in 1985 and took the first pictures from space of the comet before going on to Venus. There they dropped probes with balloons into the atmosphere to land onto the surface.
The USA sent Mariner probes past Venus in the 1960s which sent back some data about the atmosphere. In 1978 they sent a Pioneer to orbit Venus with several small probes to collect data on the atmosphere of Venus.
In 1989, the USA launched the Magellan probe to Venus via the shuttle Atlantis. It mapped Venus by radar and also collected data on the magnetic field.
The surface features on Venus are named after goddesses, heroines and other famous women, and suggestions for names are welcomed. They must not be political.
Venus was thought to have been similar to the Earth in its earliest history. The primordial crust formed. Was heavily bombarded and cratered. Convection through the mantle caused by internal heating within the centre (from gravitational squeezing, radio-active decay....) deformed the outer crust which is thinner in low-lying areas (might be seas on Earth) and thicker in highlands (might have been continents on Earth). High mountain regions developed by uplifting and outflowing. The Ishtar Tierra mountains formed as a result of the uplift of the plateau and formation of Maxwell mountains. Low lying areas and craters became filled with lava. The shield in the Beta Regis area was formed before the mountain ridges along the fault which broke the surface of the shield. Tectonic activity with volcanic eruptions continues today.
The mass of Venus's atmosphere is about one hundred times as much as the mass of the Earth's atmosphere and the density of the atmosphere at the surface is at least seventy times higher that on Earth. Being even higher in some places. Local temperatures and air pressures vary as they do on Earth except the temperature on Venus remains the same night and day, and there is little difference in the temperature from the equator to the poles.
Venus's atmosphere is chiefly carbon dioxide (97%) and 2-3% Nitrogen. There are clouds of smelly hydrogen sulphide. Sulphur seems to play the same role in Venus's meteorology as water does on Earth. These smelly clouds are high in the sky and low in density - like mist or fog. Only a small proportion of solar energy manages to reach the planet's surface but it is enough to maintain high temperatures because of the "greenhouse" effect.
The Nasa pictures created from the Magellan radar data, have been criticised as giving a very false image of the surface of Venus. Although it was coloured in accordance with the colour photos from Venera, the sky which should be a yellowish fog, has been made completely black, and the surface features which are flattened under the pressure of Venus's heavy atmosphere, rather as features are under the sea on Earth, have been exaggerated in height, 20 times. A much more realistic reconstruction should have been made.
It was thought that Venus may have been covered in water like the Earth, but being nearer the Sun this evaporated off, forming clouds which trapped the heat and made a greenhouse effect. Something like this could so easily have happened on Earth, so it was a cause for concern. The thick atmosphere is the reason very small impact craters are not found on the surface, and many craters are elongated with black 'skid' marks, where the meteorite has been slowed by the atmosphere.
The radar mapping by Magellan was completed in August 1993, when Magellan's orbit was changed to collect gravity data before it crashed onto the surface.The data received has shown that Venus, although apparently a twin of the Earth, has in fact had a very different history.
Magellan showed long rivers of lava, flattened volcanos (like Sapa Mons in this picture) like mushroom tops, collapsed domes with cobweb-like cracks (graben), but no tectonics and volcanism like we have on Earth.
What it did have, which has not been seen on any other world so far, was craters scattered all over the surface in the same way. Normally a world shows surfaces that craters reveal to be of different ages, as parts of the surface are changed and obliterated by tectonic and volcanic processes. But on Venus the surface was all the same age all over. This means that Venus completely resurfaced in one giant upheaval, in effect turning itself inside out, then went relatively quiet. From the craters and other evidence this catastrophic event was dated to about 700 million years ago.
If there had been water and life on Venus - and there may have been - see also - it might have been destroyed and it may have been then if not earlier, that Venus became the apparently lifeless, steamy, hot smelly (sulphurous) planet it is now.
About 650 million years ago, there was a massive extinction event on Earth. It appears to have been associated with a massive movement of tectonic plates sending continents hurtling around the globe in the space of a few million years. 70 percent of the single celled life forms which then existed were destroyed, the climate became cold and Earth was covered by ice.
Some life on Earth survived the extinction event 650 million years ago, evolved, and the first muli-cellular life forms appeared. The Vendian age now renamed Ediacarian.
It is possible that the events on Earth and Venus are related. And that at one time Venus and Earth were very similar planets with seas, continents and perhaps Venus too had single celled life as on Earth.
Around 700 million years ago the Earth had just one large continent over the south pole - "Rodinia" - its breaking up was at least partly due to massive volcanic eruptions over vast areas (traps) -which was to lead to the glacial age.
Venus is now very different from Earth. Earth has not swung upside down as Venus seems to have done at some time in the past, so it now appears to rotate backwards. Venus also rotates so slowly that its day is longer than its year - and is affected by the gravitational pull of Earth and Moon together, as the same side always faces Earth when they are close to each other on their orbits.
Earth may have been saved from the fate of Venus by the presence of its large Moon - making a double body. The Moon affects the axial rotation of the Earth - it produces a wobble in the axial rotation, but at the same time prevents it tilting much further than the present 21.5 degrees. The Moon also speeded up the rotation of the Earth. This effect lessens as the Moon gradually drifts away from the Earth, but 650 million years ago the day on Earth was about a third shorter than now. The regular tidal pull of the Moon also keeps the Earth's geology active - so the Earth's crust is in constant movement.
Earth has a thin crust, continuously shifting and breaking up pushed by the hot convection currents deep in the Earth's fluid mantle. Information from Magellan suggested Venus is encased in a thick solid shell of crust and mantle perhaps about 50 km. thick. There are signs of upwelling and downwelling in vertical heat currents from the planet's centre.
Hot plumes rising up are marked by volcanos, all over the planet. Small domes a few km. across are dotted all over the planet and also in dense swarms, and huge volcanos several kilometres across are usually found at the tops of broad rises.
Another volcanic feature is unique to Venus. It is the corona. These are hundreds of kilometres across, heavily fractures and sometimes surrounded by a trench. They may have been caused by a hot volcanic plume rising from deep in the planet, lifting a circular region. This then cracks as it cools, relaxes, and drops back. Another peculiar feature are the steep-sided hills, like mushroom tops or "pancake domes". They may be formed from thick lava that oozed onto the surface slowly through a central vent.
There are also features that look just like rivers. One of these which is called Hildr, is longer than the River Nile, nearly 7,000 km. long.
Life on Venus?
Using data from the Russian Venera space missions and also the US Pioneer Venus and Magellan probes, researchers studying the high concentration of water droplets in the Venusian clouds found hydrogen sulphide and sulphur dioxide. These two gases react with each other, and are not seen in the same place unless something is producing them. Despite solar radiation and lightning - the atmosphere contains hardly any carbon monoxide, suggesting that something is removing the gas. "Bugs living in the Venusian clouds could be combining sulphur dioxide with carbon monoxide and possibly hydrogen sulphide or carbonyl sulphide in a metabolism similar to that of some early Earth bugs".
The Transit of Venus and the Astronomical Unit
When we look at Venus through a telescope we can only see a partially lit crescent. That is because when its fully lit side is facing us it is behind the Sun, and when its dark side is facing us, it is between us and the Sun.
It can only be seen then when it is directly in line of sight between us and the Sun, and appears as a dark spot traversing the Sun's surface. This is called a transit. Observations of the transit of Venus have been used to determine, not only the size of the Sun, but the distance of Venus, and the distance between us and the Sun, which is the astronomical unit.
The Astronomical Unit (AU) is the mean radius of the Earth's orbit about the Sun. This is 150 million kilometres. 1 AU = 1.49578 x 10 metres.
Edmund Halley recognized the importance of observing the transits of Mercury and Venus - the rare occasions when one of these planets gets in line of sight between the Earth and the Sun, and appeared to the observer, using a telescope as the lens of a camera obscura, (never look directly at the Sun through a telescope) as a black dot crossing the surface of the Sun.
In 1639, Jeremiah Horrocks (1618-41), and William Crabtree (1610-44) who were penfriends, were the first to use telescopes to observe the transit of Venus, which they found predicted in the astronomical tables of Philip von Lansberg (1561-1632).
Horrocks, had recently graduated from university and was working as a private tutor. He set up his camera obscura type device in an upstairs bedroom. The eyepiece of the camera was shrouded by the curtained window, and the image was to project on marked out white card, which he had prepared and set up ready. But the day was cloudy and Horrocks was busy looking after the children. Luckily the sky cleared just in time. And Horrocks was able to see and record the transit. Crabtree was also able to observe the transit. In 1641, Horrocks died, aged 23, the day before he was to have visited Crabtree for the first time. Crabtree died in the Civil War.
By timing the exact moments that the planet entered and left the Sun's disk, from different places on Earth, it was thought possible, by using parallax, to determine the distance of the Earth from the Sun, and use that as a baseline, the Astronomical Unit, to find the distance of other objects in the Solar System, and the nearest stars.
Once the distance of the Earth to the Sun was known, the comparative distances of other bodies in the solar System and nearby stars could be calculated.
It would also be possible to determine the longitude of the position of the observer. For example observations of the transit of Mercury in 1677, were used to determine the longitude of Port Royal, Jamaica. This is because the transit would be seen differently from different places on Earth.
Major international preparations were made for scientists (regardless of political differences or even war - there was war between Britain and France) to travel to remote parts of the world to observe the transits of Venus in 1761 and 1769. They knew a similar opportunity would not reoccur again until 1874 and 1882 (the next transits were 2004 and still to come 2012).
The astronomers had many adventures on their travels to their destinations in Canada, Tahiti, South Africa, Central Siberia, India, Mexico, and other places.
The adventures of the astronomers who travelled to remote places in the 18th century to meaure the transit of Venus can be found in these books (and others have emerged in time for the next transits): Fernie: J. Donald, The Whisper and the Vision, the Voyages of the Astronomers, Toronto, 1976, his articles about the transits of Venus in the 18th century is online, and Ferris, Timothy, Coming of Age in the Milky Way, 1988, ISBN 0 09 9800500.
The method was to time the moment Venus entered in front of the Sun's disk and the moment it left. These times will be different for each observer in each place. The results were then collected together and from then the distance from the Earth to the Sun would be calculated.
The results were disappointing. One reason was the time-keepers. Pendulum clocks were the most accurate at the time, and they had to be reassembled after arduous journeys.
The other reason was that it was difficult to see the precise moment the transit began, there was a "black drop" effect.
Astronomers rightly concluded this meant Venus had an atmosphere.
In September 1959, there was the first attempt to find the AU by using radar. Radar signals were bounced off the surface of Venus. The radio telescope at Jodrell Bank was used first as the transmitter and then as the receiver for the echoes. This attempt failed.
In April 1961 came a second international attempt. This used as well as Jodrell Bank (and R.V.Jones), equipment in the USSR, MIT near Boston USA, the JPL, Pasadena, California. By measuring the time delay of the echo they found the distance of Venus and also that they had got the mean distance of the Earth to the Sun too small, and this was why they failed before to detect the echo, they had been looking in the wrong place. They now had more precise clocks - hydrogen masers. There was still a discrepancy of 60,000 km between the radar value of the AU and the best of the conventional values. Eventually the average radar value of 149,600,000 kilometres was accepted.
Here is my picture of it - taken 7.30 am from our back garden.
ESA project launched from a 