Tuesday, October 18, 2005
Venus...from A to Z
B is for burn-up. The thick atmosphere on Venus ensures that small objects and meteors from space burn up quickly so that nothing very small manages to hit the Venusian ground - which is why only big impact craters exist on Venus. The planet has the hottest surface of any in the solar system, with temperatures hot enough to melt lead.
C is for craters because the surface of Venus is littered with them. Astronomers have so far identified some 900 impact craters ranging in size from 1.5km to 280km in diameter. Most of them appear as pristine as the day they were formed, with sharply defined edges surrounded by ejected material.
D is for distance of the Earth from the Sun. Early astronomers estimated the distance between the Earth and the Sun by carefully observing the movement or "transit" of Venus across the solar disc and by measuring a feature known as the solar parallax.
E is for express. The next space probe to be sent to the planet is the Venus Express, modelled on the successful Mars Express mission carried out by the European Space Agency, pictured above. Its launch is scheduled for 26 October and the probe should arrive at Venus next April.
F is for fantasy. Like other planets that were visible to ancient eyes, Venus attracted its own fan club of astrologers and mystics. The Greeks called it Aphrodite - although they originally thought it was two stars, a morning one and an evening one. The Babylonians named it Ishtar. Astrologers today continue to attribute all kinds of fantasy to the influence of Venus.
G is for greenhouse effect. Venus suffers from a runaway greenhouse effect with a very dense atmosphere which traps most of the sunlight that is reflected from its surface. Average temperatures on Venus hover around 464C, about 400C higher than they would be if the planet lacked an atmosphere that creates such a strong greenhouse effect.
H is for highlands. Venus has some impressive volcanic mountains such as the highland region of Aphrodite Terra which includes the volcano of Maat Mons, pictured below, which rises 9km from the surrounding landscape and measures 200km across.
I is for inferior planet because Venus orbits closer to the Sun than the Earth. When Venus is at its inferior conjunction it lies directly between the Earth and the Sun and its position is lost to the glare of our own star. At the point in its orbit known as its eastern elongation Venus is visible in the east after sunset, at its western elongation it is visible in the west before sunrise.
J is for Jeremiah. The transit of Venus - when the planet passes in front of the Sun and becomes visible as a small black dot - was first predicted and recorded by Jeremiah Horrocks, on 24 November 1639. He died two years later aged 22 before his observation was published.
K is for Kepler, the great German astronomer. Although Horrocks was the first to witness a transit of Venus it was Johannes Kepler who first made the mathematical calculations to predict one. Unfortunately Kepler did not live long enough to see his prediction come true. He died in 1630.
L is for love. Venus was the Roman goddess of love and beauty, pictured top left, probably because it was the brightest planetary object that could be seen with the naked eye. Nudity was the natural state of Venus and it became socially acceptable for artists throughout history to depict her as the erotic goddess of sexual healing. If men were from Mars, then women were definitely Venusians.
M is for moons, or rather lack of them. Venus is a planet with no natural satellites. M is also for Nasa's Mariner 2, which in 1962 was the first space probe to pass close to Venus. The mission found carbon dioxide in the planet's atmosphere. M is also for the Mekon, the Venusian arch enemy of Dan Dare, Pilot of the Future.
N is for names. Many of the features of Venus are named after famous women, from Eve to Cleopatra, pictured top right. It is also a girl's name, one famous example being the tennis player Venus Williams.
O is for the circular orbit of Venus around the Sun. Most of the other planets follow elliptical orbits but the track of Venus as it passes around its star forms an almost perfect "O" with an anomaly of just 1 per cent.
P is for plains. Vast areas of Venus - about 75 per cent of its surface - are covered by flat plains formed by volcanic activity. One of the main plains on the planet is Lavinia Planitia which, like many of the flat areas of Venus, is pock-marked by volcanic eruptions or impact craters formed by collisions with meteors.
Q is for queen of the evening and morning sky. Venus has phases like the Moon, which means that we see differing amounts of its sunlit side depending on where it is in relation to the Sun and the Earth. We never see the full Venus when the whole of the sunlit side points towards Earth because it becomes obscured by the Sun.
R is for rocky planet. Like the Earth, Venus is composed of a sphere of rock. Some planets, such Jupiter, are composed of gas and are unsuitable for life. R is also for radar mapping, the method used by the American Magellan and Russian Venera probes to map the surface of Venus.
S is for second planet from the Sun. Venus is about 30 per cent closer to the Sun than the Earth. Only Mercury orbits nearer to our fiery star. Venus's other neighbour is Earth, the third rock from the Sun. S is also for spin because Venus spins slowly in the opposite direction to most other planets with one Venusian day lasting 243 Earth days.
T is for twin of Earth because of its similar size, mass, density and volume. If Venus had started out a little further from the Sun it might have formed oceans of water where life could have evolved and be a true twin of Earth. But the extra heat generated by the planet's position meant that any water on the planet boiled off and ended up as water vapour in the atmosphere.
U is for ultraviolet, the wavelength of light used by the Pioneer space probe to take some of the first stunning images of the planet. The pictures showed the cloud patterns at the top of the Venusian atmosphere which helped scientists to understand its composition.
V is for volcanoes which dominate the Venusian landscape. Lava flows and craters are seen all over the planet which is pockmarked by volcanic activity arising from some 156 large volcanoes measuring more than 100 km across. Venus has another 300 "smaller" volcanoes between 20km and 100 km in diameter. Whether these volcanoes are still active is an unsolved mystery.
W is for weather. Hot gases on Venus spiral up from the equator to the polar regions, moving the thick clouds of sulphuric acid around the planet. Heat from the Sun drives the hot, superfast winds, moving gases to high altitudes as well as towards the cooler poles, where the gases lose heat and sink back towards the equator.
X is for the X-rays emitted by Venus. The Chandra space telescope first captured an image of these rays in 2001. The X-rays from Venus are not reflected but are produced by fluorescence. Radiation from the Sun knocks electrons from the gases in its atmosphere causing the atoms to emit fluorescent radiation in the form of X-rays - much like how a strip light works.
Y is for the Y-shaped clouds of sulphuric acid photographed by the Hubble space telescope in 1995. They were also seen by the Pioneer Venus, Galileo and Mariner 10 space probes. The unusual cloud formation near the equator, where swirls of vapour converge like two tributaries joining a river, could indicate the presence of atmospheric waves similar to the high and low-pressure weather systems seen on Earth.
Z is for Zontar: Thing from Venus, probably the worst sci-fi movie ever. Plot involves a small town invaded by Venusian aliens with weird eyes. The DVD is going cheap on E-bay.