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Our twin?
Look at Venus's vital statistics and it could be Earth's twin. Of all the planets in the solar system, Venus is the closest to Earth and the most similar in size and mass (see Graphic). In the late 1700s, scientists studying the planet with telescopes saw that it appeared to be a fuzzy ball and concluded that it has an atmosphere. And since they couldn't see the planet's surface, they assumed it was covered in puffy rain clouds. Maybe these hid steamy jungles teeming with life?
But NASA's Mariner 2 spacecraft put paid to that idea when it flew past Venus in 1962 and beamed back a vision of hell - a world so hot and poisonous that nothing could live on its surface. The average surface temperature is about 470 °C. Venus's atmosphere is mainly carbon dioxide, and the pressure at ground level is a crushing 90 times that on Earth - equivalent to the pressure about 1 kilometre deep in terrestrial oceans. Venus has no surface water, just water vapour in the atmosphere. And though it does have rain clouds, they're made of corrosive sulphuric acid. What's more, the planet's surface is so hot that the rain evaporates before it touches the ground.
Earth and Venus probably started out with similar compositions. So why did Venus end up so different? Astronomers think they can partly explain: because Venus lies closer to the sun, water boiled off its surface long ago. Spacecraft observations show that the abundance of deuterium relative to hydrogen in the planet's atmosphere is 150 times what it is in Earth's oceans, suggesting that most of Venus's water evaporated into the atmosphere and was broken down by the sun's ultraviolet rays. Lightweight hydrogen would have floated into space, while the heavier deuterium escaped the planet's gravity less easily.
On Earth, the oceans have scoured out most of the terrestrial carbon dioxide and laid it down in carbonate rocks. On dry Venus, the carbon dioxide is still free - hence the weighty atmosphere, which has efficiently cooked Venus by trapping the sun's energy.
Backwards spin
Why would the Scots love living on Venus? Because every other day is New Year's Eve and all-night parties would last as long as two Earth months.
Venus rotates just once every 243 Earth days, and that's longer than a Venusian year - the planet takes a little under 225 days to orbit the sun. What's more, Venus rotates in the opposite direction to the Earth and the vast majority of other bodies in the solar system, so the sun rises in the west and sets in the east. The upshot is that the solar day on Venus - the time between noon on one day and noon on the next - is 117 Earth days, much shorter than the rotation period.
Astronomers admit they really have no idea why Venus's rotation pattern bucks the trend. Possibly in its youth it rotated in the same direction as Earth, but a huge chunk of debris crashed into the planet and sent it spinning in the opposite direction. Alternatively, the majority of the boulders that clumped together to form Venus in the first place may have had the opposite spin to the rest.
But that chaos would go against the traditional picture of the newborn solar system as a neat cloud of gas and dust circling the sun in perfect serenity. "There's this idea that the solar system started out more or less perfect, then got bashed around later," says Fred Taylor, a planetary physicist at the University of Oxford. "But Venus is telling us that's too simple."
Heavy metal snow
Radar mapping by NASA's Pioneer Venus and Magellan spacecraft revealed a strange, shiny frost on Venus's mountain tops above about 2.6 kilometres. Except this can't be normal snow - it freezes out of the atmosphere at about 440°C and its shininess hints that it contains a metal. Suggestions have included the element tellurium, or even iron pyrites - fool's gold.
But the most likely compounds are the semiconductors lead sulphide and bismuth sulphide, say Laura Schaefer and Bruce Fegley of Washington University in St Louis, Missouri. They reached that conclusion last year after analysing the abundance of elements on Venus. Fegley hopes a future mission will one day collect some of the metal snow and return it to Earth. If the snow does contain lead sulphide, analysis of the lead isotopes could help pinpoint Venus's age, just as it helped to date Earth.
Stormy weather
For a planet with such a sedate spin, Venus has some pretty wild weather. Observations show the equatorial clouds at altitudes of 50 to 60 kilometres whipping round the planet at more than 350 kilometres per hour - 50 times faster than the rotation rate of the planet's surface. No one has produced a convincing theory to explain how Venus can whip up these winds. And each of Venus's poles has a pair of tornadoes no one can model on a computer.
Nearer the surface of Venus, scientists assumed that conditions would be very calm because the atmosphere is very dense. But a tantalising image of the clouds at altitudes of about 30 kilometres, beamed back from the Galileo spacecraft in 1991, shows that it is actually in turmoil. "It looks like there are massive great storms," says Taylor. "This is the biggest mystery of all and without more data, we can't begin to understand it."
Life in the clouds?
Is there life on Venus? It's a controversial idea, but a few scientists are convinced that microbes can live in the Venusian clouds.
Mars has traditionally been thought the most likely site for extraterrestrial life today or in the past - especially now that there is firm evidence the planet was once a warm world with water flowing on its surface. As Venus also had more water than it does now, there is speculation that it too could have harboured life, before the runaway greenhouse effect desiccated the planet.
A small group of scientists has argued that if Venus was once cool and then heated up slowly enough for life to adapt to the hot new environment, microbes could still live there today in the acidic clouds at altitudes of about 50 kilometres. The temperature there is a tolerable 50 to 70 °C, and this region also has the highest concentration of residual water, mainly tied up in hydrated sulphur compounds.
Dirk Schulze-Makuch of the University of Texas in El Paso and his colleagues have argued that spacecraft observations of the Venusian atmosphere reveal chemical clues that microbes may be subtly influencing its composition. Schulze-Makuch says there are even hints that bugs are slurping up sulphur dioxide and carbon monoxide then converting them to carbonyl sulphide, or using sunlight as an energy source.
Critics argue that there's not enough water in the Venusian atmosphere to sustain microbes, and that the intense ultraviolet radiation from the sun would damage proteins and DNA. But Schulze-Makuch thinks the microbes could suck water out of hydrated sulphur compounds, and that certain compounds in the atmosphere might act as a sunscreen, converting dangerous ultraviolet light into harmless visible wavelengths.
The team is pressing for a spacecraft mission specifically to look for life on Venus. The researchers have already suggested a plan to the European Space Agency. A mother ship orbiting Venus would release a canister to sample the atmosphere at an altitude of about 50 kilometres. The mother ship would then recapture the canister and return it to Earth or the International Space Station for analysis.
Hint of a flash
When the Soviet Union's Venera spacecraft spotted faint flashes coming from the dusky side of Venus in 1975, astronomers assumed that they were signs of lightning. But frustratingly, there have been no clear follow-up sightings of electric storms lighting up the night sky.
Lightning storms certainly occur on Jupiter and Saturn. And in 1993, observations by a telescope at Mount Bigelow in Arizona saw similar short flashes from the dusky side of Venus on six or seven occasions over the course of a year. However, the same telescope failed to see lightning when it looked a few years later, and other Venus probes found at best only borderline evidence for lightning. The Cassini spacecraft, which detected the radio hiss of lightning storms when it swung past Earth on its way to Jupiter, failed to hear anything near Venus.
Joseph Grebowsky of NASA's Goddard Space Flight Center in Greenbelt, Maryland, says spectacular cloud-to-ground lightning is unlikely on Venus because the clouds are much higher than on Earth, and electric fields would not discharge easily in the Venusian atmosphere. It is possible, however, that charges build up inside the clouds and lightning crackles between them.
The Venus Express spacecraft has instruments that should detect lightning if it does occur. Grebowsky says that a positive result would be intriguing, because all other lightning in the solar system seems to require water, something that's in short supply on Venus. "If there is lightning, it might require some new mechanism that we don't see anywhere else in the solar system," he says.
Venus in transit
The transit on 8 June will last about 6 hours and will be visible from just about everywhere, except for the west of North America and a few regions in and around the Pacific. With proper eye protection such as eclipse viewing glasses, Venus will look like a round black dot crossing the sun's bright face.
Venus's disc will only appear 3 per cent as wide as the sun, so the skies won't darken as they do during a total eclipse. And there's a chance you could mistake the planet for a darkened sunspot. To be sure it is Venus, look for its slow but steady motion from east to west across the sun's southern hemisphere (see Graphic).
Because Venus doesn't orbit the sun in exactly the same plane as the Earth, transits are rare and occur in pairs eight years apart. After this year's transit, there will be another one in 2012. The next will not be until 11 December 2117.
With giant telescopes and a raft of spacecraft already probing Venus's atmosphere and surface, Tuesday's transit is of little scientific interest to astronomers. But for millions of enthusiasts it is the chance of a lifetime, and some are planning to travel half way round the world to ensure the best view. Aram Kaprielian who works for tour company Travelquest International in Prescott, Arizona, says interest the transit has been "phenomenal", with travellers paying up to $2500 each to view the spectacle in Rome, Venice and Florence. Meanwhile, Explorer Tours based in Middlesex, UK, has filled all 280 places on its trip to see the transit from the Red Sea resort of Sharm El Sheikh in Egypt.
Ashen glow
In 1643, a Jesuit monk from Bologna called Giovanni Riccioli reported seeing a faint glow from the night side of Venus through his telescope. Other reports followed, and wide-eyed observers suggested citizens of Venus were lighting their cities with bonfires, or celebrating feast-days with firework displays. Many later astronomers dismissed the ashen light as pure fantasy. But modern observations show that it does exist. It is patchy, and has a dull reddish-brown colour. It moves around and sometimes is not visible from Earth at all.
Popular explanations have included lightning or auroras. But now there's a better explanation for this mystery - the ashen light is the faint glow of the red-hot planet seeping through the clouds. This idea gradually emerged in the mid-1980s, when a team of astronomers who were studying cool stars using the Anglo-Australian Telescope in New South Wales turned the telescope on Venus. Surprisingly, they found that the side of Venus facing away from the sun glowed brightly at near-infrared wavelengths.
Astronomers conclude we're seeing the glow of the hot planet itself. And calculations have confirmed that you should even be able to see the optical glow through small telescopes on Earth.
A million volcanoes
Radar observations of Venus have revealed some 1600 major volcanoes, and when you include all the smaller ones the total may top a million. That makes Venus the most volcano-packed body in the solar system. But are the volcanoes still active? While radar observations reveal lava all over the planet's surface, they don't tell us whether it is new or millions of years old. But astronomers suspect volcanoes do still erupt on Venus.
One clue comes from sulphur dioxide gas thought to have been burped into the Venusian atmosphere by volcanoes. NASA's Pioneer Venus orbiter, which arrived there in 1978 and circled the planet for a decade, noticed that levels of sulphur dioxide varied by a factor of 10. That suggests volcanic eruptions replenish the gas from time to time.
Scientists hope the European Space Agency's Venus Express will find better evidence for volcanic activity. It will monitor changes in levels of sulphur compounds and also look out for pressure waves in the atmosphere that might come from eruptions or quakes.
Missing magnetism
According to spacecraft measurements, Venus has no magnetic field to speak of. And that's strange because Venus is similar in size to Earth and probably has an almost identical dense molten iron core. Circulation in the Earth's core sets up electrical currents that generate the terrestrial magnetic field, so why does the core of Venus not do the same?
"It's a mystery, because the planet's interior is very Earth-like, as far as we can tell," says Taylor. "It's probably related to the slow spin rate of the planet and could tell us something about how planetary magnetic fields are produced." An instrument on board the European Space Agency's Venus Express spacecraft will make sensitive measurements of the planet's possible magnetic field when it arrives there in 2006.
Solar yardstick
British astronomers first observed a transit of Venus in 1639, and there have only been four transits since then, in 1761, 1769, 1874 and 1882. These transits caused a big stir. Edmund Halley realised that by timing a transit seen from different parts of the world, it should be possible to calculate the distance from the Earth to the sun by simple geometry. That in turn would allow astronomers to calibrate the distances to all the planets.
Scientists already knew from Kepler's laws of planetary motion that the distance between Venus and the sun is 0.72 times the Earth-sun distance. By measuring the position of Venus on the sun's face during a transit from two different latitudes on a given meridian, they could work out the distance to the sun by triangulation (see Graphic).
Calculating the distance using transit observations from lots of different countries round the globe is more complicated, but scientists achieved this after observing the transits of Venus in 1761 and 1769 from many remote places. It gave astronomers their first accurate value for the sun's distance, within 1 per cent of today's accepted figure of 149,598,000 kilometres - which is deduced by bouncing radar off the surface of Venus.
During Tuesday's transit, you can join thousands of amateur astronomers to follow in these pioneers' footsteps and work out the distance to the sun once again. This time, the European Southern Observatory is online to help. ESO will ask you to estimate your geographical location as accurately as possible using a Global Positioning System device or an online database of city coordinates. Then you need to measure the timing of four stages of the transit - when Venus first touches the solar disc, when Venus is first fully on the solar disc, when Venus starts to leave the disc and when Venus leaves the disc completely.
After everyone has submitted their results, ESO will do the number-crunching and publish the result for the distance to the sun online. For further information, see www.vt-2004.org/ and www.transit-of-venus.org.uk.
Missions to Venus
When the Soviet Union and the US both dreamed of sending spacecraft to other planets at the height of the cold war, the Earth's nearest neighbour was the obvious target. Both superpowers launched missions in the early 1960s and quickly discovered that interplanetary travel was no picnic. Most probes never reached Venus, and if they attempted to land, they were usually destroyed by Venus's crushing, baking atmosphere.
NASA scored the first success in 1962, when its Mariner 2 spacecraft became the first to successfully fly past an alien world. Then, after a string of false starts, the Soviet Union's Venera programme got into full swing. Between 1967 and 1983 it sent a dozen successful missions to Venus and landed the first spacecraft on the surface. In 1975, Venera 9 became the first lander to beam back spectacular panoramas of the surface.
Other US missions to Venus included Mariner 5, Mariner 10 and two Pioneer Venus missions. In 1985, the Soviet Union's two Vega missions dropped landers and balloons into Venus's atmosphere on their way to an encounter with Halley's comet. The last spacecraft devoted to Venus was Magellan, which NASA launched back in 1989. But all these missions have raised more questions than they answered, says Taylor. "The assumptions we made about Venus all turned out be wrong," he says. "Every time we looked at it, it got more puzzling."
The European Space Agency hopes to solve some of those puzzles when it launches the first European Venus mission next year. The mission will reuse the design of ESA's Mars Express spacecraft, currently in orbit around the Red Planet. And it will recycle some of the spare instruments developed for both Mars Express and the Rosetta comet mission that launched in March.
If all goes according to plan, Venus Express will blast off from Baikonur Cosmodrome in Kazakhstan in November 2005, then spend 150 days en route to Venus. Once in orbit, it will loop over the poles of the planet for about 16 months. The spacecraft cameras and spectrometers will scan the planet at ultraviolet, optical and infrared wavelengths, and send back crucial information about the atmosphere's composition, circulation and evolution.
The probe will also look for signs of active volcanoes on Venus's surface, and could help resolve other big mysteries about the planet - how it sustains hurricane-force winds, for instance, and why its atmosphere has evolved so radically differently from that of its sister planet, the Earth. "I'm convinced we'll get very exciting science out of this," says Håkan Svedhem, ESA project scientist for Venus Express from Noordwijk in the Netherlands.
Not long after Venus Express arrives at Venus, the planet will also receive a flying visit from NASA's Messenger spacecraft, scheduled for launch this July or August. The spacecraft will fly past Venus twice, in October 2006 and June 2007, before heading to Mercury, where it will go into orbit in 2011. Messenger will beam back observations of Venus's upper cloud layers and measure charged particles from the sun. It will also hunt for a Venusian beast as elusive as the Loch Ness monster - lightning on the planet's night side.
And the future?
If volcanoes are still active on Venus today, they are likely to be a driving force behind the greenhouse effect, belching out gases that help the planet sustain its hellish heat. But the internal heat driving the volcanism will gradually wind down, so at some stage, possibly in the next few million years, the volcanic activity will stop. The atmosphere would cool and possibly become almost Earth-like. Liquid water might even fall to the surface as rain. "Venus could become suitable for plant life, at least," says Taylor.
But if Venus's climate does mellow, it won't be permanent. The sun is gradually brightening and will cook Venus back to a parched, scorching ball. Eventually, 7 or 8 billion years from now, the sun will balloon into a red giant star big enough to swallow the planet. Venus will end its days ablaze in the solar atmosphere, hotter than ever before.
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