Venus: the hot spot
This week a European spacecraft will arrive for a date with Venus, our closest planetary neighbour. Scientists hope the mission, made on a shoestring budget, will reveal vital lessons on how unchecked greenhouse gases can turn a world into a blistering Hades. Robin McKie reports on a journey to the Forgotten Planet
Sunday April 9, 2006
On Tuesday morning, mission controllers in the European Space Agency’s operations centre in Darmstadt will put the finishing touches to an international bid to study the ultimate neighbour from hell.
They will transmit a series of radio commands to a robot spacecraft currently hurtling towards the Sun. Its rocket engine will fire for 50 minutes as it passes Venus, slowing the craft down so that it can be captured by the planet’s gravitational field. Once in orbit, the wardrobe-sized probe – Venus Express – will then study the planet’s acid clouds, searing heat, crushingly dense atmosphere and hurricanes to find out why Earth’s nearest neighbour has become a place of insufferable heat and poison.
‘Venus is very like Earth in that it is the same size and has an orbit round the Sun close to ours,’ said David Southwood, head of science at the ESA. ‘Yet Venus went wrong. We did not. We want to find out why Venus became our evil twin.’
Venus and Earth are almost identical in size. In addition, both orbit the Sun in ‘the Goldilocks zone’, a swath of space in which conditions are considered by astronomers as being not too hot and not too cold to prevent the evolution of life. Venus should make ideal planetary real estate, in other words. Yet it is the solar system’s most inhospitable planet.
‘It’s very disturbing that we do not understand the climate on a planet that is so much like the Earth,’ said Professor Fred Taylor, a planetary scientist based at Oxford University and one of the ESA’s chief advisers for the Venus Express mission. ‘It is telling us that we really don’t understand the Earth. We have ended up with a lot of mysteries.’
Such puzzles are recent, however. Throughout history, Venus has simply been seen as the heavenly embodiment of a deity. Intriguingly, this was invariably a female one. For example, the Babylonians, Ancient Greeks and Romans all linked it with their goddesses of love.
Venus was later revealed to be a planet, one that was assumed to be more or less the same as Earth. Only its permanent cloud covering prevented astronomers from working out the details of these similarities. Even in the Fifties, popular science books depicted a mist-shrouded world either of deserts or of swamps and ferns. A few more fanciful versions had dinosaur-like creatures lumbering about in the background.
Then the first robot spacecraft – built by Russia and the US – reached Venus and sent back data that astounded astro-nomers. The planet was unbelievably hot, dense, and had virtually no oxygen. Russia tried landing probes on the surface. All were crushed flat by the atmosphere’s incredible pressure. ‘On Earth, atmospheric pressure is one ton per square foot,’ said Taylor. ‘On Venus, it is 100 tons.’
Earth’s sister was also found to have a surface temperature of 450C and a covering of thick clouds of sulphuric acid. As a vision of Hades, it could hardly be beaten. On top of these disturbing discoveries, scientists also found that a day on Venus – the time the planet takes to make one full rotation – is the equivalent of 243 days on Earth. By contrast, a Venusian year – the time it takes to make one revolution of the Sun – is a mere 225 days. Thus, on Venus a day is longer than a year. The planet also rotates on its axis in the opposite direction to the Earth, so the Sun – if it could be seen through the Stygian gloom beneath its thick cloud – would appear to rise in the west and set in the east.
However, the planet’s principal problem – from a human point of view – lies with its greenhouse effect, scientists now realise. Venus’s thick atmosphere traps solar radiation and heats the world to boiling point. Prospects of finding life here have since been rated – not surprisingly – as vanishingly low, and astro-nomers’ keenness to study Venus has waned.
Yet several tantalising questions re-main unanswered about our strange planetary neighbour and, as technology has progressed, instruments that can probe the planet through its thick cloud veil have been developed. So the ESA decided to send Europe’s first probe to the planet. The decision followed the agency’s triumph – in 2003 – with Mars Express, a spacecraft that is still returning reams of key data about the Red Planet. The probe showed that Mars was once awash with water and has raised hopes, shared by many scientists, that primitive life forms could still survive deep below the surface of Mars in buried lakes and springs.
The success of Mars Express has been matched by other European missions. It currently has a probe, Smart-1, in orbit around the Moon; it was responsible for building the Huygens probe that landed on Saturn’s giant moon, Titan, last year; and it has launched a probe which will land on a comet and study its structure. Adding Venus to its list of destinations proved irresistible for its scientists. ‘We had enough spare parts to go back to Mars with a second probe, but our scientists decided the time was ripe to investigate Venus,’ said Don McCoy, project manager for Venus Express.
The result was the ultimate low-cost space mission. Venus Express was put together from old instruments left over from previous missions and in the end cost a mere £150 million. ‘It was cheaper to produce than Kevin Costner’s Waterworld,’ said Taylor. ‘Ours is the better investment, however.’
Venus Express was blasted into space from Baikonur in Kazakhstan in November on a Soyuz launcher and has been hurtling on its 260,000,000-mile journey towards Venus ever since. Tuesday’s rocket firing will be its most crucial manoeuvre, however. Nearly all the fuel on the 1.3-tonne probe will be used up to decelerate as it hurtles towards the Sun. Should something go wrong, Venus Express will sweep past its target. ‘Without doubt this is the hardest part of the mission,’ added Taylor. ‘Everything depends on a successful burn of Venus Express’s rocket engine on Tuesday.’
After the probe has reached Venus, it will swing into a highly elliptical orbit, ranging from 200,000 miles at its highest to 250 at its lowest point. Further firing of its rocket engine will stabilise this into a regular orbit over the poles. By June the craft will be in position, with its instruments ready to begin its survey of Venus.
‘You can think of this mission as the Return to the Forgotten Planet,’ added McCoy. ‘We are going back to find answers to questions that are a lot more important to Earth today than they were 30 years ago.’
In particular, scientists want to understand how Venus became the victim of its greenhouse effect. ‘Venus is the queen of the greenhouse,’ said Dimitri Titov, a mission scientist for Venus Express. ‘On Earth our atmosphere traps a little heat, and keeps us nice and warm. Morning on on Earth would be freezing cold if it was not for our greenhouse warming, which adds about 40C to average temperatures. But on Venus it adds several hundred degrees.’
It is not simply that our wayward sister gets more solar radiation than Earth, scientists stress. Yes, it is closer to the Sun, but the energy differential is not that great. Something else is involved – and the obvious candidate is carbon dioxide. Venus’s thick atmosphere is almost entirely made of CO2, which is known to be highly effectively at trapping and holding the Sun’s heat. Hence Earth’s impending climate crisis as man-made emissions build up in our atmosphere.
But why has Venus got so much carbon dioxide? ‘The answer may be that it lost its water some time in the remote past,’ said Taylor. ‘On Earth, carbon dioxide is absorbed by the oceans, where it forms carbonate minerals and over the millennia is deposited as rock. That process was arrested early on Venus when it lost its oceans.’
In other words, it was Earth – not Venus – that changed. Billions of years ago both had thick atmospheres of carbon dioxide but, thanks to our oceans, which continue to absorb the gas, we lost ours. Venus – with no oceans – kept its carbon dioxide. ‘We should not be too complacent,’ added Taylor. ‘As temperatures rise, seas become less and less able to hold on to carbon dioxide. Soon they will absorb less of the gas and may eventually start to give it off. That will have a very serious impact on our planet.’
As to the cause of the disappearance of Venus’s water, a key theory – to be tested by Venus Express – centres on the idea that the planet’s upper atmosphere is battered by solar storms. Without a magnetic field like Earth’s to protect it from these solar particles, water vapour was lost to space. Essentially the planet’s oceans boiled dry.
And there is the question of those sulphuric acid clouds. Accounting for these takes more effort, though again scientists believe they have answers. Venus is assumed to be highly volcanic and is frequently racked by massive eruptions that vent vast amounts of material into the atmosphere, with sulphur a key component. Mixed with other gases, this falls as gentle sulphuric acid drizzle.
‘We can see volcanoes on Venus from the radar images sent back by previous probes,’ said Taylor. ‘But these do not show if there are plumes of ash coming out or if molten lava is streaming down the sides of their calderas, so we don’t know if the volcanoes of Venus are active. However, the infra-red detectors on Venus Express will show up features like that. Then we can start to understand Venusian volcanoes and the planet’s internal structure.’ In the end, however, it will be Venus Express’s studies of the planet’s runaway greenhouse effect that will dominate the probe’s research activities.
‘The Apollo mission had a huge impact on people in the Sixties,’ said Taylor. ‘For the first time, we could see Earth from distant space. You could see how small and finite it was. That affected people’s thinking about the world.
‘Venus should now have a similar impact on the public imagination,’ he added. ‘We are going to see – graphically – what happens when greenhouse heating runs out of control on a planet. That should concentrate a lot of minds.’
The power of a planet
· Venus is the third brightest object in the sky, after the Sun and Moon. Venus will even cast a shadow during nights when there is a new Moon. The planet can also been seen during the day.
· Napoleon said he saw Venus at noon while addressing his troops at Luxembourg and interpreted the sight as a portent of victory in a forthcoming battle – which he won.
· All features on Venus are named after women with the exception of the Maxwell mountains (after the Scottish physicist James Clerk Maxwell). The rest are female, including Mead crater, named after anthropologist Margaret Mead, and highland regions, Ishtar Terra and Aphrodite Terra, named after the Babylonian and Greek goddesses of love.
· Hurricanes blowing at 200mph and over continually sweep round Venus several miles above ground.
· Venus is at its brightest as it approaches Earth, when it appears as a crescent. Many cultures around the world describe it as the ‘horned star’, which suggests that early astronomers, although lacking telescopes, could somehow make out its crescent shape.